100-year Flood
A statistic that indicates the magnitude of flood which can be expected to occur on average with a frequency of once every 100 years at a given point or reach on a river. The 100-year flood is usually developed from a statistical distribution that is based on historical floods. This is also called a base flood.
100-year Flood Plain
The flood plain that would be inundated in the event of a 100-year flood.
500 hPa
Pressure surface (geopotential height) in the troposphere equivalent to about 18,000 feet above sea level. Level of the atmosphere at which half the mass of the atmosphere lies above and half below, as measured in pressure units. This area is important for understanding surface weather, upper air storms tend to be steered in the direction of the winds at this level and are highly correlated with surface weather.
A
1. Abbrevation for hail in weather observations.

2. Symbol used on long-term climate outlooks issued by CPC to indicate areas that are likely to be above normal for the specified parameter (temperature, precipitation, etc.).
A AMS
Arctic Air Mass
A Index
A daily index of geomagnetic activity derived as the average of the eight 3-hourly a indices.
AAAS
American Association for the Advancement of Science
AAWU
Alaskan Aviation Weather Unit
Ablation
Depletion of snow and ice by melting and evaporation.
ABNDT
Abundant
Absolutely Stable Air
An atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.
Absolutely Unstable Air
An atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.
Absorption
The process in which incident radiant energy is retained by a substance by conversion to some other form of energy.
ABT
About
Abutment
The part of a valley or canyon wall against which a dam is constructed. Right and left abutments are those on respective sides of an observer looking downstream.
Abutment Seeping
Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.
ABV
Above
AC
1. Abbreviation for Altocumulus - a cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).

2. Convective outlook issued by the Storm Prediction Center. Abbreviation for Anticipated Convection; the term originates from the header coding [ACUS1] of the transmitted product.
ACCAS
(usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show cumulus-type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.
Accessory Cloud
A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.
Accretion
The growth of a precipitation particle by the collision of a frozen particle with a supercooled liquid water droplet which freezes upon impact.
ACCUMS
accumulation
Accuracy
Degree of conformity of a measure to a standard or true value; in other words, how close a predicted or measured value is to the true value.
Acid Precipitation
Precipitation, such as rain, snow or sleet, containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
Acid Rain
Rain containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
ACLD
Above Cloud Level
ACPY
Accompany
Acre-foot
The amount of water required to cover one acre to a depth of one foot. An acre-foot equals 326,851 gallons, or 43,560 cubic feet.
ACRS
Across
Action Stage
The stage which, when reached by a rising stream, represents the level where the NWS or a partner/user needs to take some type of mitigation action in preparation for possible significant hydrologic activity. The appropriate action is usually defined in a weather forecast office (WFO) hydrologic services manual. Action stage can be the same as forecast issuance stage (see / forecast issuance stage/).
Active
(abbrev. ACTV). In solar-terrestrial terms, solar activity levels with at least one geophysical event or several larger radio events (10cm) per day (Class M Flares)
Active Conservation Storage
In hydrologic terms, the portion of water stored in a reservoir that can be released for all useful purposes such as municipal water supply, power, irrigation, recreation, fish, wildlife, etc. Conservation storage is the volume of water stored between the inactive pool elevation and flood control stage.
Active Dark Filament (ADF)
In solar-terrestrial terms, an Active Prominence seen on the Disk.
Active Longitude
In solar-terrestrial terms, the approximate center of a range of heliographic longitudes in which Active Regions are more numerous and more flare-active than the average.
Active Prominence
In solar-terrestrial terms, a prominence displaying material motion and changes in appearance over a few minutes of time.
Active Prominence Region (APR)
In solar-terrestrial terms, a portion of the solar limb displaying active prominences.
Active Region (AR)
In solar-terrestrial terms, a localized, transient volume of the solar atmosphere in which plages, sunspots, faculae, flares, etc. may be observed.
Active Storage Capacity
In hydrologic terms, the total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.
Active Surge Region (ASR)
In solar-terrestrial terms, an Active Region that exhibits a group or series of spike-like surges that rise above the limb.
ACTV
Active. In solar-terrestrial terms, solar activity levels with at least one geophysical event or several larger radio events (10cm) per day (Class M Flares)
ACYC
Anticyclone - A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.
ADAPTATION (ADAPTABLE) PARAMETER
Generally, data related to a specific WSR-88D unit. These data may consist of meteorological or hydrological parameters or of geographic boundaries, political boundaries, system configuration, telephone numbers (auto dial), or other like data. Such data may be generated at either a centralized location or locally at the WSR-88D unit.
ADAS
Automated Data Acquisition System
Additive Data
A group of coded remarks that includes pressure tendency, amount of precipitation, and maximum/minimum temperature during specified periods of time
ADDS
Aviation Digital Data Service
Adiabat
A line on a thermodynamic chart relating the pressure and temperature of a substance (such as air) that is undergoing a transformation in which no heat is exchanged with its environment.
Adiabatic
Changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere, with no exchange of heat with the surrounding air.
Adiabatic Lapse Rate
The rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. For parcels that remain unsaturated during lifting, the (dry adiabatic) lapse rate is 9.8°C per kilometer.
Adiabatic Process
A process which occurs with no exchange of heat between a system and its environment.
Adirondack Type Snow Sampling Set
In hydrologic terms, a snow sampler consisting of a 5-foot fiberglass tube, 3 inches in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch snow depth capacity.
ADJ
Adjacent
ADPC
Acoustic Doppler Current Profiler
Advanced Baseline Imager (ABI)
A moderate resolution imaging system on the GOES and Himawari family of satellites.
Advanced Hydrologic Prediction Service (AHPS)
A web-based suite of accurate and information-rich forecast products. They display the magnitude and uncertainty of occurrence of floods or droughts, from hours to days and months in advance. These graphical products are useful information and planning tools for many economic and emergency managers.
ADVCTN
Advection- Transport of an atmospheric property by the wind.
Advection
(Abbrev. ADVCTN)- Transport of an atmospheric property by the wind.
Advection Fog
A fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.
ADVIS
In hydrologic terms, a program which combines the Antecedent Precipitation Index (API) method of estimating runoff with unit hydrograph theory to estimate streamflow for a headwater basin.
Advisory
(Abbrev. ADVY)- Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.
ADVN
Advance
ADVY
Advisory - Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.
Aeration Zone
A portion of the lithosphere in which the functional interstices of permeable rock or earth are not filled with water under hydrostatic pressure. The interstices either are not filled with water or are filled with water that is no held by capillarity.
Aeroallergens
Any of a variety of allergens such as pollens, grasses, or dust carried by winds.
Aerosol
A system of colloidal particles dispersed in a gas, such as smoke or fog.
AFB
Air Force Base
AFCT
Affect
AFD
Area Forecast Discussion - This National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product. The forecaster will try to focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The term non-convective refers to weather that is not caused by thunderstorms. An intermediate Area Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur.
AFOS
Automation of Field Operations and Services. Computer system linking NWS offices for the transmission of weather data. This system was installed in the early to mid 1980s and it is being replaced by Advanced Weather Interactive Processing System (AWIPS).
AFRED
Abbreviation for the A Index for Fredericksburg.
AFSS
Automated Flight Service Station
AFT
After
Afterbay
In hydrologic terms, the tail race of a hydroelectric power plant at the outlet of the turbines. The term may be applied to a short stretch of stream or conduit, or to a pond or reservoir.
AFTN
Afternoon
AFTR
after
AFWA
Air Force Weather Agency
AGC
Automatic Gain Control
AGDISP
A particular atmospheric disperison model used for treating the transport and diffusion of aerially sprayed pest control agents in agricultural applications.
AGFS
Aviation Gridded Forecast System
Agglomerate
An ice cover of floe formed by the freezing together of various forms of ice.
AGL
Above Ground Level
AGL
Above Ground Level
AGN
Again
AHD
Ahead
AHOS
Automatic Hydrologic Observing System
AHOS-S
Automatic Hydrologic Observing System - Satellite
AHOS-T
Automatic Hydrologic Observing System - Telephone
Air
The mixture of gases comprising the earth's atmosphere.
Air Mass
A body of air covering a relatively wide area and exhibiting horizontally uniform properties.
Air Mass Thunderstorm
Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.

Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable.
Air Pollutant
Harmful substance or product introduced into the atmosphere.
Air Pollution Potential
The meteorological potential for air pollution problems, considered without regard to the presence or absence of actual pollution sources.
Air Quality Alert
This product is used by the National Weather Service to relay non-routine air quality alert messages issued by state and local agencies.
Air Quality Model
Mathematical or conceptual model used to estimate present or future air quality.
Air Stagnation
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
Air Stagnation Advisory
This National Weather Service product is issued when major buildups of air pollution, smoke, dust, or industrial gases are expected near the ground for a period of time. This usually results from a stagnant high pressure system with weak winds being unable to bring in fresh air.
Air Toxin
Toxic air pollutant.
Air Transportable Mobile Unit
A modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.
Airborne Snow Survey Program
In hydrologic terms, Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.
AIRMET
Airman's Meteorological advisory (WA)
AIV
Aviation Impact Variables
Alaska Current
A North Pacific Ocean current flowing counterclockwise in the Gulf of Alaska. It is the northward flowing (warm) division of the Aleutian Current
Albedo
Reflectivity; the fraction of radiation striking a surface that is reflected by that surface.
Alberta Clipper
A fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".
ALERT
Automated Local Event Reporting in Real Time. Network of automatic raingauges that transmit via VHF radio link when precipitation occurs. Some sites are also equipped with other sensors such as temperature, wind, pressure, river stage or tide level.
Alert Stage
The stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. (Used if different from action stage, and at the discretion of the WFO or river forecast center [RFC].) The term "alert stage" is to be used instead of warning stage. Monitor stage or caution stage may be used instead of alert stage in some parts of the country.
Aleutian Current
An eastward flowing North Pacific Ocean current which lies north of the North Pacific Current.
Aleutian Low
A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum instensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.
ALF
Aloft
ALG
Along
Algorithm
A computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.
ALIASING
The process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.
Alluvium
Sediments deposited by erosional processes, usually by streams.
Along-slope Wind System
A closed, thermally driven diurnal mountain wind circulation whose lower branch blows up or down the sloping sidewalls of a valley or mountain. The upper branch blows in the opposite direction, thereby closing the circulation.
ALQDS
All Quadrants
ALTHO
although
Altimeter
An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.
Altimeter Setting
A correction of the station pressure to sea level used by aviation. This correction takes into account the standard variation of pressure with height and the influence of temperature variation with height on the pressure. The temperatures used correspond to the standard atmosphere temperatures between the surface and sea level.
Altocumulus
A cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).
Altostratus
A cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).
Ambient
Of the surrounding area or environment.
AMD
Amend
AMOS
Automatic Meteorological Observing System
Amplifier
A device used to increase the strength of an analog signal
Amplitude
The maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.
AMS
1. Air Mass - a body of air covering a relatively wide area and exhibiting horizontally uniform properties.

2. American Meteorological Society
AMT
Amount
AMVER
Automated Mutual Assistance Vessel Rescue System. A system operated by the U.S. Coast Guard which computes the nearest available rescue vessels for vessels in distress using vessel track and position reports supplied by participating vessels.
AMVER/SEAS
A software program created by the National Weather Service intended to efficiently generate AMVER and VOS reports as part of a cooperative effort.
Anabranch
A diverging branch of a river which re-enters the main stream.
Analog
1. Class of measuring devices in which the output varies continuously as a function of the input (non-digital).

2. A historical instance of a given meteorological scenario or feature that is used for comparison with another scenario or feature. For example, a long-range forecaster predicting conditions for the upcoming winter may make comparisons to analog seasons in which meteorological factors were similar to those of the upcoming season.
Analog Signal
A signal, such as voice, that varies in a continuous manner.
ANBURS
Alphanumeric Backup Replacement System
Anchor Ice
In hydrologic terms, submerged frazil ice attached or anchored to the river bottom, irrespective of its formation.
Anchor Ice Dam
An accumulation of anchor ice which acts as a dam and raises the water level.
Anemometer
An instrument used for measuring the speed of the wind.
Aneroid Barometer
An instrument for measuring atmospheric pressure in which a needle, attached to the top of an evacuated box, is deflected as changes in atmospheric pressure cause the top of the box to bend in or out.
Angels
Radar echoes caused by birds, insects, and localized refractive index discontinuities.
Angle of Reflection
The angle at which a reflected ray of energy leaves a reflecting surface. It is measured between the outgoing ray and a perpendicular to the surface at the point of incidence (i.e., where the ray strikes).
Angstrom
A unit of length equal to 10-8 cm.
Annual Exceedance Probability (AEP)
The probability that a stream reach will have a flow of a certain magnitude in any given year.
Annual Flood
In hydrologic terms, the maximum discharge peak during a given water year (October 1 - September 30).
ANOMALOUS PROPAGATION (AP)
Non-standard atmospheric temperature or moisture gradients will cause all or part of the radar beam to propagate along a non-normal path. When non-standard index-of-refraction distributions prevail, "abnormal" or "anomalous" propagation occurs. When abnormal downward bending occurs, it is called "superrefraction." If the beam is refracted downward sufficiently, it will illuminate the ground and return signals to the radar from distances further than is normally associated with ground targets. The term "subrefraction" is applied when there is abnormal upward bending of the radar beam.
Anomaly
The deviation of a measurable unit (e.g., temperature or precipitation) over a period in a given region from the long-term average, often the thirty-year mean, for that region.
Antedecent Precipitation Index
(Abbrev. API) - an index of moisture stored within a drainage basin before a storm.
ANTENNA GAIN
The measure of effectiveness of a directional antenna as compared to an isotropic radiator, maximum value is called antenna gain by convention.
Anthelion
A luminous white spot that appears on the parhelic circle at the same altitude as the sun and 180 degrees from it in azimuth.
Anthropogenic Source
A pollutant source caused or produced by humans.
Anti-wind
The upper or return branch of an along-valley wind system, as confined within a valley, and blowing in a direction opposite to the winds in the lower altitudes of the valley.
Anticyclogenesis
The formation or intensification of an anticyclone or high pressure center.
Anticyclone
A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere
Anticyclonic Rotation
Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.
Antilles Current
A current which originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current.
Anvil
The flat, spreading top of a cumulonimbus cloud, often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind.
Anvil Crawler
[Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS.
Anvil Dome
A large overshooting top or penetrating top.
Anvil Rollover
Slang for a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil.
Anvil Zits
Slang for frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.
AO
Arctic Oscillation - the Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
AOA
At or above
AOB
At or below
AOPA
Aircraft Owners and Pilots Association
AP
Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.
AP Index
In solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific stations.
APD
On a buoy report, the average wave period (seconds) of all waves during the 20-minute period.
Aphelion
The point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion.
API Method
In hydrologic terms, a statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the antecedent precipitation index, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity.
Apogee
The farthest distance between the moon and earth or the earth and sun.
Apparent Temperature
A measure of human discomfort due to combined heat and humidity (e.g., heat index).
Apparent Wind
The speed and true direction from which the wind appears to blow with reference to a moving point. Sometimes called RELATIVE WIND.
APRCH
Approach
APRCHG
approaching
APRNT
apparent
APST
Aviation Products and Services Team
Aquiclude
In hydrologic terms, a formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring.
Aquifer
In hydrologic terms, permeable layers of underground rock, or sand that hold or transmit groundwater below the water table that will yield water to a well in sufficient quantities to produce water for beneficial use.
Aquifuge
In hydrologic terms, a geologic formation which has no interconnected openings and cannot hold or transmit water.
ARAM
Aviation, Range, and Aerospace Meteorology
Arch Dam
A concrete arch dam is used in sites where the ratio of width between abutments to height is not great and where the foundation at the abutments is solid rock capable of resisting great forces. The arch provides resistance to movement. When combined with the weight of concrete (arch-gravity dam), both the weight and shape of the structure provide great resistance to the pressure of water.
Arch Filament System (AFS)
In solar-terrestrial terms, a bright, compact plage crossed by a system of small, arched filaments, which is often a sign of rapid or continued growth in an Active Region.
Arctic
The region within the Arctic Circle, or, loosely, northern regions in general, characterized by very low temperatures.
Arctic front
The boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air.
Arctic Oscillation
(abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
Arctic Sea Smoke
Steam fog, but often specifically applied to steam fog rising from small open water within sea ice.
Arcus
A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds both are types of arcus clouds.
Area Forecast Discussion
This National Weather Service product is intended to provide a well-reasoned discussion of the meteorological thinking which went into the preparation of the Zone Forecast Product. The forecaster will try to focus on the most particular challenges of the forecast. The text will be written in plain language or in proper contractions. At the end of the discussion, there will be a list of all advisories, non-convective watches, and non-convective warnings. The term non-convective refers to weather that is not caused by thunderstorms. An intermediate Area Forecast Discussion will be issued when either significant forecast updates are being made or if interesting weather is expected to occur.
Area Hydrologic Discussion (AHD)
A short range, episodic, discussion and graphic which highlights locations across the nation that may be impacted by rapid-onset flooding, using National Water Model and other guidance.
Area of Influence
In hydrologic terms, the area covered by the drawdown curves of a given pumping well or combination of wells at a particular time.
Area Source
An array of pollutant sources, so widely dispersed and uniform in strength that they can be treated in a dispersion model as an aggregate pollutant release from a defined area at a uniform rate. Compare line source and point source.
Area Wide Hydrologic Prediction System
(Abbrev. AWHPS) - A computer system which automatically ingests areal flash flood guidance values and WSR-88D products and displays this data and other hydrologic information on a map background.
Area-Capacity Curve
In hydrologic terms, a graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
Arid
An adjunctive applied to regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable without irrigation.
ARINC
Aeronautical Radio, Incorporated
ARND
Around
ARR
Arrive/Arrival
Arroyo
In hydrologic terms, a water-carved channel or gully in arid country, usually rather small with steep banks, dry most of the time, due to infrequent rainfall and the shallowness of the cut which does not penetrate below the level of permanent ground water.
ARSI
Atmospheric Research System, Inc.
ART
The Automatic Radiotheodolite. A ground-based radio direction finder that automatically tracks a ballon-borne radiosonde.
ARTCC
Air Route Traffic Control Center
Artesian Well
In hydrologic terms, a well drilled into a confined aquifer with enough hydraulic pressure for the water to flow to the surface without pumping. Also called a flowing well.
Artificial Control
In hydrologic terms, a weir or other man-made structure which serves as the control for a stream-gaging station.
AS
(NOTE: if this appears in an Area Forecast Discussion or other text product in context as the word "as," disregard the technical definition below).

Abbreviation for Altostratus, a cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).
ASAP
1. AHOS SHEF Automatic Processing System

2. As soon as possible (may be used in Area Forecast Discussions)
ASAPTRAN
The software component of ASAP.
ASB
Aviation Support Branch
Ashfall Advisory
This product is issued for a volcano undergoing a minor eruption where the public will be affected by a limited hazard extent, such as less than ¼ inch of ashfall accumulation. Ashfall may originate directly from a volcanic eruption or indirectly by wind suspending the ash.
Ashfall Warning
This product is issued for a volcano undergoing a major eruption where the public will be affected to a significant extent, such as greater than or equal to ¼ inch of ashfall accumulation, significant debris, lava or lahar flows.
ASL
Above Sea Level
ASOS
Automated Surface Observing System. A list of stations currently active and available through the NWS website can be found here.
ASOS IDs
Each Automated Surface Observing System an a four character identifier assigned to it. A list of stations currently active and available through the NWS website can be found here.
ASR-9
Airport Surveillance Radar (FAA)
ASSOCIATED PRINCIPAL USER
A Principal User with dedicated communications to a WSR-88D unit.
Astronomical Dawn
The time at which the sun is 18 degrees below the horizon in the morning. Astronomical dawn is that point in time at which the sun starts lightening the sky. Prior to this time during the morning, the sky is completely dark.
Astronomical Dusk
This is the time at which the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky.
Astronomical Unit
(abbrev. AU)- The mean earth-sun distance, equal to 1.496x1013 cm, or 214.94 solar radii.
ATC
Air Traffic Control
ATDTDCS
Automated Tone Dial Telephone Data Collection System - Data collection system where cooperative observers collect precipitation, stage, and temperature data then transmit the data to the NWS ATDTDCS computer through the telephone lines. The ATDTDCS computer transmits the data to AFOS.
Atmosphere
The air surrounding and bound to the Earth.
Atmospheric Boundary Layer
Same as Boundary Layer - in general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
Atmospheric Circulation Model
A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs.
Atmospheric Pressure
The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.
Atmospheric Radiation
Infrared radiation (energy in the wavelength interval of 3- 80 micrometer) emitted by or being propagated through the atmosphere. It consists of both upwelling and downwelling components. Compare with terrestrial radiation.
ATMP
On a buoy report, the air temperature (Celsius).
Attenuation
It refers to the reduction of the radar beam power due to the reflection or absorption of energy when it strikes a target. The greatest attenuation occurs when the radar beam goes through very heavy rain.
ATTENUATION
Any process in which the flux density (power) of a beam of energy is dissipated.
ATTM
At this time
Augmented report
A meteorological report prepared by an automated surface weather observing system for transmission with certified observers signed on to the system to add information to the report.
Aurora
A faint visual phenomenon associated with geomagnetic activity, which occurs mainly in the high-latitude night sky; typical auroras are 100 to 250 km above the ground
Aurora Australis
Same as Aurora Borealis, but in the Southern Hemisphere. Also known as the southern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
Aurora Borealis
Also known as the northern lights; the luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
Auroral Oval
In solar-terrestrial terms, an oval band around each geomagnetic pole which is the locus of structured aurorae
Automated Event Reporting Gage
(also see Tipping Bucket Rain Gage); for river stage gages, IFLOWS pressure transducer type gages can be programmed to report if water surface rises or falls by a predetermined amount.
Automated Report
A meteorological report prepared by an automated surface weather observing system for transmission, and with no certified weather observers signed on to the system.
Automated Surface Observing System
The ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). Completed in the mid-1990s, the ASOS systems serve as the nation's primary surface weather observing network. ASOS is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, hydrological, and climatological research communities.
AUTOMATIC GAIN CONTROL
Any method of automatically controlling the gain of a receiver, particularly one that holds the output level constant regardless of the input level.
Autumn
The season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice. Meteorological autumn (different from standard/astronomical autumn) begins September 1 and ends November 30.
Autumnal Equinox
The equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately September 22. On that day, daylight is everywhere 12 hours. Compare with vernal equinox, offset by six months.
Avalanche
A mass of snow, rock, and/or ice falling down a mountain or incline. In practice, it usually refers to the snow avalanche. In the United States, the term snow slide is commonly used to mean a snow avalanche.
Avalanche Advisory
A preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.
AVBL
Available
AVG
Average
AVHRR
Advanced Very High Resolution Radiometer. Main sensor on U.S. polar orbiting satellites.
AVN
The Aviation model (120-hour numerical model of the atmosphere). The output from this model is now part of what is known as the GFS model.
AVP
On a buoy report, Average Wave Period is the average period (seconds) of the highest one-third of the wave observed during a 20 minute sampling period.
AWC
Aviation Weather Center
AWHPS
Area Wide Hydrologic Prediction System
AWIPS
Advanced Weather Interactive Processing System. This system replaced the Automation of Field Operations and Services (AFOS). This system allows the operator to overlay meteorological data from a variety of sources.
AWOS
Automated Weather Observation System
Azimuth
A direction in terms of a 360° compass. North is at 0°, east is at 90°, south is at 180°, and west is at 270°.
Azimuth Angle
The direction or bearing toward which a sloping surface faces (e.g., a north-facing slope has an azimuth angle of 360°; a northeast-facing slope, an azimuth angle of 45°).
Azores Current
One of the currents of the North Atlantic subtropical gyre.
Azores High
Alternate term for Bermuda High - a semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High.
Back Door Cold Front
A cold front moving south or southwest along the Atlantic seaboard and Great Lakes; these are especially common during the spring months.
Back-building Thunderstorm
A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.
Back-sheared Anvil
[Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential.
Backfire
A fire started to stop an advancing fire by creating a burned area in its path.
Backflow
In hydrologic terms, the backing up of water through a conduit or channel in the direction opposite to normal flow.
Backing
(abbrev. BCKG)- A counterclockwise shift in wind direction (for example, south winds shifting to the east).
Backing Winds
Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.

In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.
Backscatter
The portion of power scattered back in the incident direction.
Backsight
In hydrologic terms, a rod reading taken on a point of known elevation, a benchmark or a turning point. Backsights are added to the known elevation to arrive at the height of the instrument. With a known height of the instrument, the telescope can be used to determine the elevation of other points in the vicinity.
Backwater Curve
In hydrologic terms, the longitudinal profile of the surface of a liquid in a non-uniform flow in an open channel, when the water surface is not parallel to the invert owing to the depth of water having been increased by the interposition of an obstruction such as a dam or weir. The term is sometimes used in a generic sense to denote all water surface profiles; or for profiles where the water is flowing at depths greater than the critical.
Backwater Effect
In hydrologic terms, the effect which a dam or other obstruction has in raising the surface of the water upstream from it.
Backwater Flooding
Hydrologic terms, upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.
BAM
Binary Angular Measure
BANDPASS FILTER
A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
Bandwidth
The frequency range between the lowest and highest frequencies that are passed through a component, circuit, or system with acceptable attenuation.
Bank
In hydrologic terms, the margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.
Bank Storage
In hydrologic terms, water absorbed and stored in the void in the soil cover in the bed and banks of a stream, lake, or reservoir, and returned in whole or in part as the level of water body surface falls.
Bankfull
The water level, or stage, at which a stream, river or lake is at the top of its banks and any further rise would result in water moving into the flood plain.
Bankfull Stage
An established gage height at a given location along a river or stream, above which a rise in water surface will cause the river or stream to overflow the lowest natural stream bank somewhere in the corresponding reach. The term "lowest bank" is however, not intended to apply to an unusually low place or a break in the natural bank through which the water inundates a small area. Bankfull stage is not necessarily the same as flood stage.
Banner Cloud
A cloud plume often observed to extend downwind behind isolated mountain peaks, even on otherwise cloud-free days.
BAPSU
Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.
Bar
An obstacle formed at the shallow entrance to the mouth of a river or bay.
Barber Pole
[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Baroclinic leaf shield
A cloud pattern on satellite images - frequently noted in advance of formation of a low pressure center.
Baroclinic Zone
A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems; barotropic systems, on the other hand, do not exhibit significant changes in intensity. Also, wind shear is characteristic of a baroclinic zone.
Baroclinity
A measure of the state of stratification in a fluid in which surfaces of constant pressure (isobaric) intersect surfaces of constant density (isosteric).
Barogram
An analog record of pressure produced by a barograph
Barograph
A barometer that records its observations continuously.
Barometer
An instrument that measures atmospheric pressure.
Barometric Pressure
The pressure of the atmosphere as indicated by a barometer.
Barotropic System
A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.

As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.
Barotropy
The state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure; it is the state of zero baroclinity.
Barrage
In hydrologic terms, any artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release.
Barrier Jet
A jet-like wind current that forms when a stably-stratified low-level airflow approaches a mountain barrier and turns to the left to blow parallel to the longitudinal axis of the barrier.
Bartel's Rotation Number
The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartel to begin in January 1833.
BASE DATA
Those digital fields of reflectivity, mean radial velocity, and spectrum width data in spherical coordinates provided at the finest resolution available from the radar.
Base Flood
In hydrologic terms, the national standard for floodplain management is the base, or one percent chance flood. This flood has at least one chance in 100 of occurring in any given year. It is also called a 100 year flood.
BASE PRODUCTS
Those products that present some representation of the base data. This representation may not necessarily be either in full resolution or depict the full area of coverage. Base products can be used to generate a graphic display or further processing.
Base Reflectivity
Base Reflectivity is the default image. Taken from the lowest (½° elevation) slice, it is the primary image used to "see what's out there". There are two versions of Base Reflectivity image; the short range version which extends out to 124 nautical miles (143 statute miles/230 kilometers) and the long range version which extends out to 248 nautical miles (285 statute miles/460 kilometers). This image is available upon completion of the ½° elevation scan during each volume scan
Base Station
In hydrologic terms, a computer which accepts radio signals from ALERT gaging sites, decodes the data, places the data in a database, and makes the data available to other users.
Base Width
In hydrologic terms, the time duration of a unit hydrograph.
Baseflow
In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as ground water flow, or dry-weather flow.
Basin
An area having a common outlet for its surface runoff. Also called a "Drainage Basin."
Basin Boundary
The topographic dividing line around the perimeter of a basin, beyond which overland flow (i.e.; runoff) drains away into another basin.
Basin Lag
In hydrologic terms, the time it takes from the centroid of rainfall for the hydrograph to peak.
Basin Recharge
In hydrologic terms, rainfall that adds to the residual moisture of the basin in order to help recharge the water deficit. i.e; water absorbed into the soil that does not take the form of direct runoff.
Bathymetry
The science of measuring depths of the oceans, lakes, seas, etc.
Beach Erosion
The movement of beach materials by some combination of high waves, currents and tides, or wind.
BEAM FILLING
The measure of variation of hydrometeor density throughout the radar sampling volume. If there is no variation in density, the beam is considered to be filled.
BEAM WIDTH
Angular width of antenna pattern. Usually that width where the power density is one-half that of the axis beam. (Half-Power or 3 dB point)
Bear's Cage
[Slang], a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm. The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.
Beaufort Scale
The Beaufort wind scale is a system used to estimate and report wind speeds when no measuring apparatus is available. It was invented in the early 19th Century by Admiral Sir Francis Beaufort of the British Navy as a way to interpret winds from conditions at sea. Since that time, the scale has been modernized for effects on land.














Beaver('s) Tail
[Slang], a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger.
Bed Load
In hydrologic terms, sand, silt, gravel, or soil and rock detritus carried by a stream on or immediately above its bed. The particles of this material have a density or grain size such as to preclude movement far above or for a long distance out of contact with the stream bed under natural conditions of flow.
Beginning of the Breakup
In hydrologic terms, date of definite breaking, movement, or melting of ice cover or significant rise of water level.
Benchmark
(Abbrev. BM) - In hydrologic terms, a permanent point whose known elevation is tied to a national network. These points are created to serve as a point of reference. Benchmarks have generally been established by the USGS, but may have been established by other Federal or local agencies. Benchmarks can be found on USGS maps.
Bermuda High
A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that migrates east and west with varying central pressure. Depending on the season, it has different names. When it is displaced westward, during the Northern Hemispheric summer and fall, the center is located in the western North Atlantic, near Bermuda. In the winter and early spring, it is primarily centered near the Azores in the eastern part of the North Atlantic. Also known as Azores High.
Best Track
A subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center fix positions.
BIAS
A systematic difference between an estimate of and the true value of a parameter.
Black Ice
1. Slang reference to patchy ice on roadways or other transportation surfaces that cannot easily be seen.

2. In hydrologic terms, transparent ice formed in rivers and lakes.
Blackbody
A hypothetical "body" that absorbs all of the electromagnetic radiation striking it - it does not reflect or transmit any of the incident radiation. A blackbody not only absorbs all wavelengths, but emits at all wavelengths with the maximum possible intensity for any given temperature.
Blackbody Radiation
The electromagnetic radiation emitted by an ideal blackbody adhering to the radiation laws; it is the theoretical maximum amount of electromagnetic radiation of all wavelengths that can be emitted by a body at a given temperature.
Blizzard
(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:
Blizzard Warning
Issued for winter storms with sustained or frequent winds of 35 mph or higher with considerable falling and/or blowing snow that frequently reduces visibility to 1/4 of a mile or less. These conditions are expected to prevail for a minimum of 3 hours.
Blowing Dust Advisory
This product is issued when widespread, synoptically driven blowing dust is reducing visibilities to 1 mile or less, but greater than ¼ mile. Winds of 25 mph or greater are usually required. Generally for a duration of longer than 90 minutes.
Blowing Dust or Sand
Strong winds over dry ground, that has little or no vegetation, can lift particles of dust or sand into the air. These airborne particles can reduce visibility, cause respiratory problems, and have an abrasive affect on machinery. A concentration reducing the visibility to ¼ mile or less often poses hazards for travelers.
Blowing Dust Warning
This product is issued when widespread, synoptically driven blowing dust is reducing visibilities to ¼ mile or less. Winds of 25 mph or greater are usually required. Generally for a duration of longer than 90 minutes.
Blowing Snow Advisory
Issued when wind driven snow reduces surface visibility, possibly, hampering traveling. Blowing snow may be falling snow, or snow that has already accumulated but is picked up and blown by strong winds.
Blue Watch or Blue Box
[Slang], a severe thunderstorm watch.
Bora
A regional downslope wind whose source is so cold that it is experienced as a cold wind, despite compression warming as it descends the lee slope of a mountain range.
Boundary Layer
In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with increasing height, so the "top" of this layer cannot be defined exactly.

There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a portion of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick (from the surface up to 10 m above the ground), but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.
Bounded Weak Echo Region (BWER)
(Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually.
Bowen Ratio
For any moist surface, the ratio of heat energy used for sensible heating (conduction and convection) to the heat energy used for latent heating (evaporation of water or sublimation of snow). The Bowen ratio ranges from about 0.1 for the ocean surface to more than 2.0 for deserts; negative values are also possible. It is named for Ira S. Bowen (1898-1978), an American astrophysicist.
Brackish Ice
In hydrologic terms, ice formed from brackish water.
Braided Stream
In hydrologic terms, characterized by successive division and rejoining of streamflow with accompanying islands. A braided stream is composed of anabranches.
Brash Ice
In hydrologic terms, accumulation of floating ice made up of fragments not more than 2 meters across; the wreckage of other forms of ice.
Breach
In hydrologic terms, the failed opening in a dam.
Breakers
Waves that break, displaying white water. Depends on wave steepness and bottom bathymetry.
Breakup
In hydrologic terms, the time when a river whose surface has been frozen from bank to bank for a significant portion of its length begins to change to an open water flow condition. Breakup is signaled by the breaking of the ice and often associated with ice jams and flooding.
Breakup Date
In hydrologic terms, date on which a body of water is first observed to be entirely clear of ice and remains clear thereafter.
Breakup Jam
In hydrologic terms, an ice jam that occurs as a result of the accumulation of broken ice pieces.
Breakup Period
In hydrologic terms, the period of disintegration of an ice cover.
Bright Band
A distinct feature observed by a radar that denotes the freezing level of the atmosphere. The term originates from a horizontal band of enhanced reflectivity that can result when a radar antenna scans vertically through precipitation. The freezing level in a cloud contains ice particles that are coated with liquid water. These particles reflect significantly more radiation (appearing to the radar as large raindrops) than the portions of the cloud above and below the freezing layer. The bright band can affect the ability of the NEXRAD algorithms to produce accurate rainfall estimates at far ranges because the algorithm may interpret reflectivity from the bright band as an overestimate of precipitation reaching the surface.
Bright Band
The enhanced radar echo of snow as it melts to rain.
Brisk Wind Advisory
A Small Craft Advisory issued by the National Weather Service for ice-covered waters.
Broadband
A method of signaling in which multiple signals share the bandwidth of the transmission by the subdivision of the bandwidth into channels based on frequency.
Bubbler Gage
In hydrologic terms, a water stage recording device that is capable of attaching to a LARC for data automation purposes.
Bulk Richardson Number
A non-dimensional (i.e., no units) number relating vertical stability to vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development.
Buoyancy
The tendency of a body to float or to rise when submerged in a fluid; the power of a fluid to exert an upward force on a body placed in it.
Buttress Dam
Buttress dams are comprised of reinforced masonry or stonework built against concrete. They are usually in the form of flat decks or multiple arches. They require about 60 percent less concrete than gravity dams, but the increased form work and reinforcement steel required usually offset the savings in concrete. Many were built in the 1930's when the ratio of labor cost to materials was comparatively low. However, this type of construction is not competitive with other types of dams when labor costs are high.
C AMS
Continental Air Mass
CA
Cloud-to-Air lightning.
CAA
Cold Air Advection
CAD
Cold Air Damming. The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain. Cold Air Damming often implies that the trapped cold air mass is influencing the dynamics of the overlying air mass, e.g. in an overrunning scenario. Effects on the weather may include cold temperatures, freezing precipitation, and extensive cloud cover
CADAS
Centralized Automated Data Acquisition System - a system of two minicomputers in NWSH.
Calibration
In hydrologic terms, the process of using historical data to estimate parameters in a hydrologic forecast technique such as SACSMA, routings, and unit hydrographs.
Calm
A weather condition when no air motion (wind) is detected.
Canyon Wind
A foehn wind that is channeled through a canyon as it descends the lee side of a mountain barrier.
Cap
(also called "Lid") A layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.
Cap Cloud
A stationary cloud directly above an isolated mountain peak, with cloud base below the elevation of the peak.
CAPE
Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.

However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.) See also CIN.
Capillarity
In hydrologic terms,

1.The degree to which a material or object containing minute openings or passages, when immersed in a liquid, will draw the surface of the liquid above the hydrostatic level. Unless otherwise defined, the liquid is generally assumed to be water.

2. The phenomenon by which water is held in interstices above the normal hydrostatic level, due to attraction between water molecules.
Capillary Fringe
In hydrologic terms, the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials. The capillary fringe is also called the capillary zone.
Capillary Waves
Waves caused by the initial wind stress on the water surface causes what are known as capillary waves. These have a wavelength of less than 1.73 cm, and the force that tries to restore them to equilibrium is the cohesion of the individual molecules. Capillary waves are important in starting the process of energy transfer from the air to the water.
Capillary Zone
Used interchangably with Capillary Fringe; the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials.
Capping
A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent. This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC. The intensity of the cap is measured by its convective inhibition. The term capping inversion is sometimes used, but an inversion is not necessary for the conditions producing convective inhibition to exist.
Capping Inversion
Alternate term for Cap; a layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.
CAPS
Center for Analysis and Prediction of Storms
Carbon Dioxide
CO2; a colorless and odorless gas which is the fourth most abundant constituent of dry air.
Carrington Longitude
A system of fixed longitudes rotating with the sun
Catalina Eddy
A Catalina Eddy (coastal eddy) forms when upper level large-scale flow off Point Conception interacts with the complex topography of the Southern California coastline. As a result, a counter clockwise circulating low pressure area forms with its center in the vicinity of Catalina Island. This formation is accompanied by a southerly shift in coastal winds, a rapid increase in the depth of the marine layer, and a thickening of the coastal stratus. Predominately these eddies occur between April and September with a peak in June. A typical Catalina eddy will allow coastal low clouds and fog to persist into the afternoon. A strong Catalina eddy may extend to 6000 feet and these clouds will move through the inland valleys and reach as far as Palmdale.
Catchment Area
In hydrologic terms, an area having a common outlet for its surface runoff (also see Drainage Area or Basin, Watershed).
Categorical
A National Weather Service precipitation descriptor for a 80, 90, or 100 percent chance of measurable precipitation (0.01 inch). See Precipitation Probability (PoP)
Caution Stage
The stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. Alert stage or caution stage are used instead of caution stage in some parts of the country.
CAVU
Clear or Scattered Clouds (visibility greater than 10 mi.)
CBMAM
Cumulonimbus Mamma
Central Meridian Passage (CMP)
In solar-terrestrial terms, the passage of an Active Region or other feature across the longitude meridian that passes through the apparent center of the solar disk.
Cfs-Day
In hydrologic terms, the volume of water discharged in twenty four hours, with a flow of one cubic foot per second is widely used; 1 cfs-day is 24 x 60 x 60 = 86,400 cubic feet, 1.983474 acre-feet, or 646,317 gallons. The average flow in cubic feet per second for any time period is the volume of flow in cfs-days.
Chance
A National Weather Service precipitation descriptor for 30, 40, or 50 percent chance of measurable precipitation (0.01 inch). When the precipitation is convective in nature, the term scattered is used. See Precipitation Probability (PoP).
Channel
In hydrologic terms, also known as Watercourse; an open conduit either naturally or artificially created which periodically, or continuously contains moving water, or forms a connecting link between two bodies of water. River, creek, run, branch, anabranch, and tributary are some of the terms used to describe natural channels. Natural channels may be single or braided. Canal and floodway are some of the terms used to describe artificial channels.
Channel Inflow
In hydrologic terms, water, which at any instant, is flowing into the channel system form surface flow, subsurface flow, base flow, and rainfall that has directly fallen onto the channel.
Channel Lead
In hydrologic terms, an elongated opening in the ice cover caused by a water current.
Channel Routing
In hydrologic terms, the process of determining progressively timing and shape of the flood wave at successive points along a river.
Channeled High Winds
In mountainous areas or in cities with tall buildings, air may be channeled through constricted passages producing high winds. Santa Ana winds and winds through passes from the cold Alaskan interior to the sea are examples of these winds. Channeled high winds are local in nature but can be extremely strong. These winds generally occur in well-defined areas.
Channelization
In hydrologic terms, the modification of a natural river channel; may include deepening, widening, or straightening.
Chinook Arch
A foehn cloud formation appearing as a bank of altostratus clouds east of the Rocky Mountains, heralding the approach of a chinook. It forms in the rising portion of standing waves on the lee side of the mountains. An observer underneath or east of the cloud sees an arch of clear air between the cloud's leading edge and the mountains below. The cloud appears to converge with the mountains to the north and south due to a perspective effect.
Chlorofluorocarbons
(CFCs) - Manufactured substances used as coolants and computer-chip cleaners. When these products break down they destroy stratospheric ozone, creating the Antarctic Ozone Hole in the Southern Hemisphere spring (Northern Hemisphere autumn). While no longer in use, their long lifetime will lead to a very slow removal from the atmosphere.
Circulation
The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.
Cirrostratus
A cloud of a class characterized by a composition of ice crystals and often by the production of halo phenomena and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Civil Dawn
The time of morning at which the sun is 6 degrees below the horizon. At this time, there is enough light for objects to be distiguishable and that outdoor activities can commence.
Civil Emergency Message
(Abbrev. CEM) - A message issued by the National Weather Service in coordination with Federal, state or local government to warn the general public of a non-weather related time-critical emergency which threatens life or property, e.g. nuclear accident, toxic chemical spill, etc.
Class I Areas
Geographic areas designated by the Clean Air Act where only a small amount or increment of air quality deterioration is permissible.
Clear Air Turbulence
(CAT) - In aviation, sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.
Clear Ice
A thin coating of ice on terrestrial objects, caused by rain that freezes on impact. The ice is relatively transparent, as opposed to rime ice, because of large drop size, rapid accretion of liquid water, or slow dissipation of latent heat of fusion.
Clear Slot
With respect to severe thunderstorms, a local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.
Client Agency
As used in connection with reimbursable National Weather Service (NWS) fire weather services, a public fire service or wildlands management agency, Federal or non-Federal, which requires and uses NWS fire and forestry meteorological services
Climate
The composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.
Climate Change
A non-random change in climate that is measured over several decades or longer. The change may be due to natural or human-induced causes.
Climate Diagnostics Bulletin
(CDB) - The monthly CPC Bulletin reports on the previous months' status of the ocean-atmosphere climate system and provides various seasonal ENSO-related outlooks. It is issued by the fifteenth of the month.
Climate Diagnostics Center
(CDC) - The mission of NOAA's Climate Diagnostics Center is to identify the nature and causes for climate variations on time scales ranging from a month to centuries.
Climate Model
Mathematical model for quantitatively describing, simulating, and analyzing the interactions between the atmosphere and underlying surface (e.g., ocean, land, and ice).
Climate Outlook
A climate outlook issued by the CPC gives probabilities that conditions, averaged over a specified period, will be below-normal, normal, or above-normal.
Climate Prediction Center
This Center is one of several centers under the National Centers for Environmental Prediction (NCEP) part of the National Weather Service (NWS) in the National Oceanic and Atmospheric Administration (NOAA). The Center serves the public by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains.
Climate System
The system consisting of the atmosphere (gases), hydrosphere (water), lithosphere (solid rocky part of the Earth), and biosphere (living) that determine the Earth's climate.
Climatological Outlook
An outlook based upon climatological statistics for a region, abbreviated as CL on seasonal outlook maps. CL indicates that the climate outlook has an equal chance of being above normal, normal, or below normal.
Climatology
The science that deals with the phenomena of climates or climatic conditions.
Closed Basin
A basin draining to some depression or pond within its area, from which water is lost only by evaporation or percolation. A basin without a surface outlet for precipitation falling precipitation.
Closed Basin Lake Flooding
Flooding that occurs on lakes with either no outlet or a relatively small one. Seasonal increases in rainfall cause the lake level to rise faster than it can drain. The water may stay at flood stage for weeks, months, or years.
Cloud Condensation Nuclei
Small particles in the air on which water vapor condenses and forms cloud droplets.
Cloud Layer
An array of clouds whose bases are at approximately the same level.
Cloud Tags
Ragged, detached cloud fragments; fractus or scud.
Coalescence
The process by which water droplets in a cloud collide and come together to form raindrops.
Coastal Flooding
Flooding which occurs when water is driven onto land from an adjacent body of water. This generally occurs when there are significant storms, such as tropical and extratropical cyclones.
Coastal Waters
Includes the area from a line approximating the mean high water along the mainland or island as far out as 100 nautical miles including the bays, harbors and sounds.
Coastal Waters Forecast (CWF)
The marine forecast for areas, including bays, harbors, and sounds, from a line approximating the mean high water mark (average height of high water over a 19-year period) along the mainland or near shore islands extending out to as much as 100 NM.
Coastal/Lakeshore Flood Advisory
Minor flooding is possible (i.e., over and above normal high tide levels. Coastal/Lakeshore Flood Advisories are issued using the Coastal/Lakeshore Hazard Message (CFW) product.
Coastal/Lakeshore Flood Warning
Flooding that will pose a serious threat to life and property is occurring, imminent or highly likely. Coastal/Lakeshore Flood Warnings are issued using the Coastal/Lakeshore Hazard Message (CFW) product.
Coastal/Lakeshore Flood Watch
Flooding with significant impacts is possible. Coastal/Lakeshore Flood Watches are issued using the Coastal/Lakeshore Hazard Message (CFW) product.
Coastal/Lakeshore Flooding
(i) (Oceanic) Coastal Flooding is the inundation of land areas caused by sea waters over and above normal tidal action. This flooding may impact the immediate oceanfront, gulfs, bays, back bays, sounds, and tidal portions of river mouths and inland tidal waterways. (ii) Lakeshore Flooding is the inundation of land areas adjacent to one of the Great Lakes caused by lake water exceeding normal levels. Lakeshore flooding impacts the immediate lakefront, bays, and the interfaces of lakes and connecting waterways, such as rivers.
Coherent Radar
A radar that utilizes both signal phase and amplitude to determine target characteristics.
Cold Advection
Transport of cold air into a region by horizontal winds.
Cold Air Avalanche
Downslope flow pulsations that occur at more or less regular intervals as cold air builds up on a peak or plateau, reaches a critical mass, and then cascades down the slopes.
Cold Air Dam
A shallow cold air mass which is carried up the slope of a mountain barrier, but with insufficient strength to surmount the barrier. The cold air, trapped upwind of the barrier alters the effective terrain configuration of the barrier to larger-scale approaching flows.
Cold Air Damming (CAD)
The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain. Cold Air Damming often implies that the trapped cold air mass is influencing the dynamics of the overlying air mass, e.g. in an overrunning scenario. Effects on the weather may include cold temperatures, freezing precipitation, and extensive cloud cover
Cold Air Funnel
A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.
Cold Weather Advisory
This product is issued for dangerously cold temperatures or wind chill values that are not expected to reach warning criteria.
Collar Cloud
A generally circular ring of cloud that may be observed on rare occasions surrounding the upper part of a wall cloud. This term sometimes is used (incorrectly) as a synonym for wall cloud.
Colorado Low
A low pressure storm system that forms in winter in southeastern Colorado or northeastern New Mexico and tracks northeastward across the central plains of the U.S. over a period of several days, producing blizzards and hazardous winter weather.
Columnar Ice
In hydrologic terms, ice consisting of columnar shaped grain. The ordinary black ice is usually columnar-grained.
Combined Seas
Generally referred to as SEAS. Used to describe the combination or interaction of wind waves and swells in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, Seas2 = S2+W2 where S is the height of all swell components and W is the height of the wind wave components. When used, SEAS should be considered as being the same as the significant wave height.
Comma Cloud
A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.
Comma Echo
A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo
Complex Gale/Storm
In the high seas and offshore forecasts, an area for which gale/storm force winds are forecast or are occurring but for which no single center is the principal generator of these winds.
Complex Terrain
Typically used to refer to mountainous terrain. In general usage, it may also refer to coastal regions and heterogeneous landscapes.
Composite Hydrograph
A stream discharge hydrograph which includes base flow, or one which corresponds to a net rain storm of duration longer than one unit period.
Comprehensive Flare Index (CFI)
In solar-terrestrial terms, the indicative of solar flare importance.
Condensation
In general, the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation, although on the molecular scale, both processes are always occurring.
Condensation Funnel
A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.).
Conditionally Unstable Air
An atmospheric condition that exists when the environmental lapse rate is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate.
Confined Ground Water
In hydrologic terms, ground water held under an aquiclude or an aquifuge, called artesian if the pressure is positive.
Congressional Organic Act of 1890
The act that assigned the responsibility of river and floor forecasting for the benefit of the general welfare of the Nation’s people and economy to the Weather Bureau, and subsequently the National Weather Service.
Conjugate Points
Two points on the earth's surface, at opposite ends of a geomagnetic field line
Conservation Storage
In hydrologic terms, storage of water for later release for usual purposes such as municipal water supply, power, or irrigation in contrast with storage capacity used for flood control.
Consolidated Ice Cover
In hydrologic terms, ice cover formed by the packing and freezing together of floes, brash ice and other forms of floating ice.
Constant Pressure Chart
Alternate term for Isobaric Chart; a weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York at a given time, and may also be at a different height over Dallas from one day to the next).
Continental Air Mass
A dry air mass originating over a large land area. Contrast with tropical air mass.
Continental Shelf
The zone bordering a continent and extending to a depth, usually around 100 FM, from which there is a steep descent toward greater depth.
Convective Boundary Layer
The unstable boundary layer that forms at the surface and grows upward through the day as the ground is heated by the sun and convective currents transfer heat upwards into the atmosphere.
Convective Condensation Level
(abbrev. CCL)- The level in the atmosphere to which an air parcel, if heated from below, will rise dry adiabatically, without becoming colder than its environment just before the parcel becomes saturated. See Lifted Condensation Level (LCL).
Convective Temperature
The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding.

Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.
Conveyance Loss
In hydrologic terms, the loss of water from a conduit due to leakage, seepage, evaporation, or evapo-transpiration.
Cooling Degree Days
(Abbrev. CDD) - A form of Degree Day used to estimate energy requirements for air conditioning or refrigeration. Typically, cooling degree days are calculated as how much warmer the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 75°F on a certain day, there were 10 CDD (Cooling Degree Days) that day because 75 - 65 = 10.
Cooperative Observer
An individual (or institution) who takes precipitation and temperature observations-and in some cases other observations such as river stage, soil temperature, and evaporation-at or near their home, or place of business. Many observers transmit their reports by touch-tone telephone to an NWS computer, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published.
Coordinated Universal Time (UTC)
By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".

More about UTC, and a table to convert UTC to your local time is posted at: http://www.srh.noaa.gov/jetstream/remote/radarfaq.htm#utc
Corona
1. The outermost layer of the solar atmosphere, characterized by low densities (< 1.0x109/cc) and high temperatures (> 1.0x106 K).

2. In solar-terrestrial terms, a white or colored circle or set of concentric circles of light of small radius seen around a luminous body, especially around the sun or moon. The color varies from blue inside to red outside and the phenomenon is attributed to diffraction of light by thin clouds or mist (distinguished from halo).
Coronal Hole
In solar-terrestrial terms, an extended region of the corona, exceptionally low in density and associated with unipolar photospheric regions.
Coronal Rain
(Abbrev. CRN) In solar-terrestrial terms, material condensing in the corona and appearing to rain down into the chromosphere as observed at the solar limb above strong sunspots.
Coronal Transients
In solar-terrestrial terms, a general term for short-time-scale changes in the corona, but principally used to describe outward-moving plasma clouds.
Correlated Shear
An output of the mesocyclone detection algorithm indicating a 3-dimensional shear region (i.e. vertically correlated) that is not symmetrical.
Correlation
A measure of similarity between variables of functions.
Cosmic Ray
An extremely energetic (relativistic) charged particle.
County Warning and Forecast Area
The group of counties for which a National Weather Service Forecast Office is responsible for issuing warnings and weather forecasts.
County Warning Area
The group of counties for which a National Weather Service Forecast Office is responsible for issuing warnings.
Coupled Atmosphere-Ocean Model
Same as Coupled Model; in the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.
Crepuscular Rays
The alternating bands of light and dark (rays and shadows) seen at the earth's surface when the sun shines through clouds.
Crest Gage
A gage used to obtain a record of flood crests at sites where recording gages are installed.
Critical Depth
In hydrologic terms, The depth of water flowing in an open channel or conduit, partially filled, corresponding to one of the recognized critical velocities.
Critical Flow
In hydrologic terms, a condition of flow where the mean velocity is at one of the critical values; ordinarily at Belanger's critical depth and velocity. Another important usage is in reference to the Reynolds' critical velocities which define the point at which the flow changes from streamline or nonturbulent to turbulent flow.
Critical Rainfall Probability
(Abbrev. CRP) - In hydrologic terms, the Probability that the actual precipitation during a rainfall event has exceeded or will exceed the flash flood guidance value.
Cross-Valley Wind System
A thermally driven wind that blows during daytime across the axis of a valley toward the heated sidewall.
CUFRA
Cumulus Fractus
Cumuliform Anvil
A thunderstorm anvil with visual characteristics resembling cumulus-type clouds (rather than the more typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid spreading of a thunderstorm updraft, and thus implies a very strong updraft. See anvil rollover, knuckles, mushroom.
Curtain Drain
In hydrologic terms, a drain constructed at the upper end of the area to be drained, to intercept surface or ground water flowing toward the protected area from higher ground, and carry it away from the area. Also called an Intercepting Drain.
CWA
County Warning Area
CWFA
County Warning and Forecast Area
Cyclonic Circulation
Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all mesocyclones and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, such as gustnadoes, occasionally rotate anticyclonically (clockwise). Compare with anticyclonic rotation.
Daily Climatological Report
As the name indicates, this climatological product is issued daily by each National Weather Service office. Most of the climatological data in this report are presented in a tabular form; however, some narrative statements may also be used in the product. The report is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, sunrise and sunset times, etc.).
Daily Flood Peak
In hydrologic terms, the maximum mean daily discharge occuring in a stream during a given flood event.
DALR
Dry Adiabatic Lapse Rate
Dam
In hydrologic terms, any artificial barrier which impounds or diverts water. The dam is generally hydrologically significant if it is:
1. 25 feet or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet.
2. Or has an impounding capacity of 50 acre-feet or more and is at least six feet above the natural bed of the stream.
Dam Failure
In hydrologic terms, catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.
DAMBRK
In hydrologic terms, the Dam Break Forecasting Model.
DAPM
In hydrologic terms, the Data Acquisition Program Manager.
Dark Surge on Disk (DSD)
In solar-terrestrial terms, dark gaseous ejections visible in H-alpha.
Dart Leader
A faint, negatively charged channel that travels more or less directly and continuously from cloud to ground.
Data Point
In the context of hydrologic observations, a location on a river/stream for which observed data is input to RFC or WFO hydrologic forecast procedures, or included in public hydro­logic products. Flood forecasts and warnings are not issued for data points (see /forecast point/).
DATACOL
In hydrologic terms, the Software System that supports RFC gateway functions.
DATANET
In hydrologic terms, it is the hydrologic Data Network Analysis Software.
Dawn
Same as Civil Dawn; the time of morning at which the sun is 6 degrees below the horizon. At this time, there is enough light for objects to be distiguishable and that outdoor activities can commence.
Day Length
Duration of the period from sunrise to sunset.
Dead Storage
In hydrologic terms, the volume in a reservoir below the lowest controllable level.
Decadal
Occurring over a 10-year period, such as an oscillation whose period is roughly 10 years ("Pacific Decadal Oscillation").
Declination
The latitude that the sun is directly over at a given time. The declination is ~23°N at the summer solstice, ~23°S at the winter solstice, and 0° (over the equator) at the spring and autumn equinoxes.
Deep Percolation Loss
In hydrologic terms, water that percolates downward through the soil beyond the reach of plant roots.
Deep Seepage
In hydrologic terms, infiltration which reaches the water table.
Deformation Zone
The change in shape of a fluid mass by variations in wind, specifically by stretching and/or shearing. Deformation is a primary factor in frontogenesis (evolution of fronts) and frontolysis (decay of fronts).
Degradation
In hydrologic terms, the geologic process by means of which various parts of the surface of the earth are worn down and carried away and their general level lowered, by the action of wind and water.
Degree Day
A measure that gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs. For more specific definitions and how to calculate degree days, see the definitions for Heating Degree Days and Cooling Degree Days.
Delta
In hydrologic terms, an alluvial deposit, often in the shape of the Greek letter "delta", which is formed where a stream drops its debris load on entering a body of quieter water.
Delta T
Change in temperature. 1) A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development. 2) The difference in temperature between the surface of a lake and 850mb, typically used to determine lake effect snow potential.
Dense Fog Advisory
This product is issued when fog reduces visibility to 1/4 mile or less over a widespread area. For marine products: an advisory for widespread or localized fog reducing visibilities to regionally or locally defined limitations not to exceed 1 nautical mile.
Dense Smoke Advisory
This product is issued for widespread or localized smoke reducing visibilities to ¼ mile or less.
Depression Storage
In hydrologic terms, the volume of water contained in natural depressions in the land surface, such as puddles.
Desertification
A tendency toward more prominent desert conditions in a region.
Design Criteria
In hydrologic terms, the hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially for the spillway and outlet works.
Detention Basins
Structures built upstream from populated areas to prevent runoff and/or debris flows from causing property damage and loss of life. They are normally dry, but are designed to attenuate storm flows or detain mud/debris during and immediately after a runoff event. They have no spillway gates or valves and do not store water on a long-term basis. Typical detention times for storm flows are on the order of 24 to 72 hours, but may be as long as 5 to 10 days. Basins designed for detention of mud and rock debris are periodically excavated to maintain their storage capacity.
Detention Storage
In hydrologic terms, the volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.
Developing Gale/Storm
In the high seas and offshore forecasts, a headline used in the warnings section to indicate that gale/storm force winds are not now occurring but are expected before the end of the forecast period.
Diabatic
A process which occurs with the addition or loss of heat. The opposite of adiabatic. Meteorological examples include air parcels warming due to the absorption of infrared radiation or release of latent heat.
Diablo Wind
Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.
Diagnostic Model
A computer model used to calculate air pollution concentrations. A diagnostic model produces a wind field over an area by interpolating from actual wind observations.
Diamond Dust
A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.
Differential Motion
Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.
Differential Rotation
In solar-terrestrial terms, the change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
Direct Flood Damage
In hydrologic terms, the damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.
Direct Solar Radiation
The component of solar radiation received by the earth's surface only from the direction of the sun's disk (i.e. it has not been reflected, refracted or scattered).
Directional Shear
The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.
Disappearing Solar Filament (DSF)
In solar-terrestrial terms, the sudden (timescale of minutes to hours) disappearance of a solar filament (prominence).
Discharge
In hydrologic terms, the rate at which water passes a given point. Discharge is expressed in a volume per time with units of L3/T. Discharge is often used interchangeably with streamflow.
Discharge Curve
In hydrologic terms, a curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location. Also called Rating Curve and Discharge Rating Curve.
Discharge Table
In hydrologic terms,

1. A table showing the relation between two mutually dependant quantities or variable over a given range of magnitude.

2. A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a Rating Table.
Distribution (Hydro)Graph
In hydrologic terms, a unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occur during successive equal units of time.
Diurnal
Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).
Diurnal Cycles
Variations in meteorological parameters such as temperature and relative humidity over the course of a day which result from the rotation of the Earth about its axis and the resultant change in incoming and outgoing radiation.
Diurnal Temperature Range
The temperature difference between the minimum at night (low) and the maximum during the day (high).
Dividing Streamline
In the blocked flow region upwind of a mountain barrier, the streamline that separates the blocked flow region near the ground from the streamlines above which go over the barrier.
Dividing Streamline Height
The height above ground of the dividing streamline, as measured far upwind of a mountain barrier. See dividing streamline.
Domain
In air pollution modeling, the geographical area over which a simulation is performed.
Domestic Use of water
In hydrologic terms, the use of water primarily for household purposes, the watering of livestock, the irrigation of gardens, lawns, shrubbery, etc., surrounding a house or domicile.
Doppler Radar
Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).
Down-Valley Wind
A thermally driven wind directed down a valley's axis, usually occurring during nighttime; part of the along-valley wind system.
Downdraft
(Abbrev. DWNDFT) - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.
Downstream
In the same direction as a stream or other flow, or toward the direction in which the flow is moving.
Downstream Slope
In hydrologic terms, the slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the erosive effects of rain and surface flow
Downwash
A deflection of air downward relative to an object that causes the deflection.
Downwelling Radiation
The component of radiation directed toward the earth's surface from the sun or the atmosphere, opposite of upwelling radiation.
DPVA
Differential Positive Vorticity Advection
Drainage Area
In hydrologic terms, an area having a common outlet for its surface runoff (also see Watershed and Catchment Area).
Drainage Basin
In hydrologic terms, a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.
Drainage Density
In hydrologic terms, the relative density of natural drainage channels in a given area. It is usually expressed in terms of miles of natural drainage or stream channel per square mile of area, and obtained by dividing the total length of stream channels in the area in miles by the area in square miles.
Drainage Divide
In hydrologic terms, the boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins.
Drainer
A valley or basin from which air drains continuously during nighttime rather than becoming trapped or pooled.
Drains (Relief Wells)
In hydrologic terms, a vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".
Drawdown
In hydrologic terms, the lowering of the surface elevation of a body of water, the water surface of a well, the water table, or the piezometric surface adjacent to the well, resulting from the withdrawl of water therefrom.
Drought Assessments
At the end of each month, CPC issues a long-term seasonal drought assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data Center, the United States Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought Monitor. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry.
Dry Adiabat
A line of constant potential temperature on a thermodynamic chart.
Dry Adiabatic Lapse Rate
The rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5°F per 1000 ft or 9.8°C per km.
Dry Crack
In hydrologic terms, a crack visible at the surface but not going right through the ice cover, and therefore it is dry.
Dry Weather Flow
In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as baseflow, or ground water flow.
Dry-adiabatic
1. An adiabatic process in a hypothetical atmosphere in which no moisture is present. 2. An adiabatic process in which no condensation of its water vapor occurs and no liquid water is present.
DSA
Special Tropical Disturbance Statement
Duration Curve
In hydrologic terms, a cumulative frequency curve that shows the percent of time during which specified units of items (e.g. discharge, head, power,etc.) were equaled or exceeded in a given period. It is the integral of the frequency diagram.
Duration of Ice Cover
In hydrologic terms, The time from freeze-up to break-up of an ice cover.
Duration of Sunshine
The amount of time sunlight was detected at a given point.
Dust Advisory
This product is issued when convectively-driven dust is reducing visibilities to 1 mile or less, but greater than ¼ mile. Winds of 25 mph or greater are usually required.
Dust Storm Warning
This product is issued when a convectively-driven dust storm is reducing visibilities to ¼ mile or less. Winds of 25 mph or greater are usually required.
Dynamic Ice
In hydrologic terms, pressure due to a moving ice cover or drifting ice. Pressure occuring at movement of first contact termed Ice Impact Pressure
Dynamic Lifting
The forced uplifting of air from various atmospheric processes, such as weather fronts, and cyclones.
Dynamic Wave Routing Model (DWOPER)
A computerized hydraulic routing program whose algorithms incorporate the complete one-dimensional equations of unsteady flow
Dynamics
Generally, any forces that produce motion or effect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.
E-19, Report on River Gage Station
In hydrologic terms, a report to be completed every 5 years providing a complete history of a river station and all gages that have been used for public forecasts since the establishment of the station.
E-19a, Abridged Report on River Gage Sta
In hydrologic terms, an abridged version of an E-19, an E-19a updates the E-19 as additional information, or changes occur at the station during the intervening five year period. An E-19a is to be completed anytime a significant change occurs at a forecast point. An E-19a is also used to take the place of an E-19 in documenting any gage history, or information of any non-forecast point (i.e; data point).
E-3, Flood Stage Report
In hydrologic terms, a form that a Service Hydrologist/ Hydrology Focal Point completes to document the dates in which forecast points are above flood stage, as well as the crest dates and stages. Discussion of the flood event must also be included in the E-5, Monthly Report of River and Flood conditions. An E-3 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
E-5, Monthly Report of River and Flood c
In hydrologic terms, a monthly narrative report covering flooding which occurred over the past month. Flood stage, flood crest and dates in which flooding occurred is covered within this report for each data point which was in flood. If the flooding involved a forecast point, an E-3 must be filled out as well. If no flooding has occurred within the past month, a climatic summary of the past month can be included as well as other interesting non-flood events, such as water supply, ice jams and the occurrence of drought. An E-5 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
E-7, Flood Damage Report
In hydrologic terms, a report to be completed anytime there is reported flood damage or loss of life as a direct result of flooding. An E-7 report is sent to Regional Headquarters, as well as the Office of Hydrology (OH).
Easterlies
Any winds with components from the east.
Easterly Wave
A low level disturbance of tropical origins. Easterly waves can develop into tropical cyclones. However, tropical cyclone development is not required in order for these systems to produce significant amounts of rainfall. The easterly waves are primarily a summer phenomenon.
Effective Precipitation
1) That part of the precipitation that produces runoff.

2) A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.

3) That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.
Effective Terrestrial Radiation
The difference between upwelling infrared or terrestrial radiation emitted from the earth and the downwelling infrared radiation from the atmosphere
Effective Topography
The topography as seen by an approaching flow, which may include not only the actual terrain but also cold air masses trapped within or adjacent to the actual topography.
Effluent Seepage
In hydrologic terms, diffuse discharge of ground water to the ground surface
Effluent Stream
In hydrologic terms, any watercourse in which all, or a portion of the water volume came from the Phreatic zone, or zone of saturation by way of groundwater flow, or baseflow
El Niño
A warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide.
Elevated Convection
Convection occurring within an elevated layer, i.e., a layer in which the lowest portion is based above the earth's surface. Elevated convection often occurs when air near the ground is relatively cool and stable, e.g., during periods of isentropic lift, when an unstable layer of air is present aloft.

In cases of elevated convection, stability indices based on near-surface measurements (such as the lifted index) typically will underestimate the amount of instability present. Severe weather is possible from elevated convection, but is less likely than it is with surface-based convection.
Embankment
In hydrologic terms, fill material, usually earth or rock, placed with sloping sides and usually with length greater than height. All dams are types of embankments
Emergency Action Plan
In hydrologic terms, a predetermined plan of action to be taken to reduce the potential for property damage and loss of life in an area affected by a dam break or excessive spillway.
Energy Dissipator
In hydrologic terms, a structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.
Enhanced V
A pattern seen on satellite infrared photographs of thunderstorms, in which a thunderstorm anvil exhibits a V-shaped region of colder cloud tops extending downwind from the thunderstorm core. The enhanced V indicates a very strong updraft, and therefore a higher potential for severe weather. Enhanced V should not be confused with V notch, which is a radar signature.
Enhanced Wording
1. An option used by the SPC in tornado and severe thunderstorm watches when the potential for strong/violent tornadoes, or unusually widespread damaging straight-line winds, is high. The text that accompanies a watch of this type will include the line "THIS IS A PARTICULARLY DANGEROUS SITUATION."

2. Strong wording or emphasis used in a zone forecast issued by a National Weather Service Forecast Office highlighting a potential condition (e.g., "some thunderstorms may be severe").
Ensemble Forecast
Multiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
Ensemble Hydrologic Forecasting
In hydrologic terms, a process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.
ENSO Diagnostic Discussion
The CPC issues the ENSO Diagnostic Discussion around the middle of the month. The discussion addresses the current oceanic and atmospheric conditions in the Pacific and the seasonal climate outlook for the following one to three seasons.
Entrainment Zone
A shallow region at the top of a convective boundary layer where fluid is entrained into the growing boundary layer from the overlying fluid by the collapse of rising convective plumes or bubbles.
Entrance Region
The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow).

This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. See also exit region, left exit region.
Environment Canada
The Canadian federal government department responsible for issuing weather forecasts and weather warnings in Canada.
Environmental Lapse Rate
The rate of decrease of air temperature with height, usually measured with a radiosonde.
Environmental Temperature Sounding
An instantaneous or near-instantaneous sounding of temperature as a function of height. This sounding or vertical profile is usually obtained by a balloon-borne instrument, but can also be measured using remote sensing equipment.
EPA
Environmental Protection Agency
Equi-Potential Line
In hydrologic terms, a line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is perpendicular to the line at all points.
Equilibrium Drawdown
In hydrologic terms, the ultimate, constant drawdown for a steady rate of pumped discharge.
Equilibrium Surface Discharge
In hydrologic terms, the steady rate of surface discharge which results from a long-continued, steady rate of net rainfall, with discharge rate equal to net rainfall rate
Equivalent Potential Temperature
The equivalent potential temperature is the temperature a parcel at a specific pressure level and temperature would have if it were raised to 0 mb, condensing all moisture from the parcel, and then lowered to 1000 mb.
Estuary
In hydrologic terms, the thin zone along a coastline where freshwater systems and rivers meet and mix with a salty ocean (such as a bay, mouth of a river, salt marsh, lagoon).
Esturine waters
In hydrologic terms, deepwater tidal habitats and tidal wetlands that are usually enclosed by land but have access to the ocean and are at least occasionally diluted by freshwater runoff from the land (such as bays, mouths of rivers, salt marshes, lagoons).
ETA
1. The Eta Model, now referred to as North Amercian Meso (NAM) an 84-hour numerical model of the atmosphere run four times daily by NCEP. This is one of the main forecast models used for short-term weather prediction in the United States.

2. Estimated Time of Arrival
Eta Model
Now referred to as North Amercian Meso (NAM) is one of the operational numerical forecast models run at NCEP. The Eta is run four times daily, with forecast output out to 84 hours.
Evaporation
The process of a liquid changing into a vapor or gas, usually water in meteorology.
Evaporation Pan
In hydrologic terms, a pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.
Evaporation Rate
In hydrologic terms, the quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in inches depth, per day, month, or year.
Evaporation-mixing Fog
Fog that forms when the evaporation of water raises the dew point of the adjacent air.
Evaporative Demand Drought Index (EDDI)
An experimental drought monitoring and early warning guidance tool. It examines how anomalous the atmospheric evaporative demand is for a given location and across a time period of interest.
Evaporimeter
In hydrologic terms, an instrument which measures the evaporation rate of water into the atmosphere.
Evapotranspiration
Combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere.
Excess Rain
In hydrologic terms, effective rainfall in excess of infiltration capacity.
Excessive Rainfall Outlook (ERO)
A graphical product in which the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG) within 40 kilometers (25 miles) of a point.
Exclusive Flood Control Storage Capacity
In hydrologic terms, the space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage
Experimental Product
An experimental product is in the final stages of testing and evaluation. If the product proves accurate and valuable to users then the next step is to make it an operational product.
Extended Forecast Discussion
This discussion is issued once a day around 2 PM EST (3 PM EDT) and is primarily intended to provide insight into guidance forecasts for the 3- to 5-day forecast period. The geographic focus of this discussion is on the United States (including Alaska and Hawaii). Although portions of this narrative will parallel the Hemispheric Map Discussion, a much greater effort is made to routinely relate the model forecasts and necessary modifications to weather forecasts, mainly in terms of temperature and precipitation.
Extraterrestrial Radiation
The theoretically-calculated radiation flux from the sun at the top of the atmosphere, before losses by atmospheric absorption.
Extratropical
A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.
Extratropical Cyclone
A cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers.
Extratropical Low
A low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity.
Extreme Cold Warning
This product is issued when dangerously cold air temperatures or wind chill values are expected or occurring. The criteria for this warning varies from place to place.
Extreme Cold Watch
This product is issued when an extreme cold event is possible, but its occurrence, location, and/or timing is still uncertain.
Extreme Heat
A period of abnormally hot and dangerous temperatures, with or without high humidities, that can result in negative impacts to people, animals, and infrastructure.
Extreme Heat Warning
This product is issued by the National Weather Service when extremely dangerous heat is expected or occurring. The criteria for this warning varies from place to place.
Extreme Heat Watch
This product is issued when conditions are favorable for an extreme heat event but its occurrence, location, and/or timing is still uncertain.
Extreme Ultraviolet (EUV)
A portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Extreme Wind Warning
Extreme Wind Warning (EWW) inform the public of the need to take immediate shelter in an interior portion of a well-built structure due to the onset of extreme tropical cyclone winds. An EWW for extreme tropical cyclone winds should be issued when both of the following criteria are met: a. Tropical cyclone is a category 3 or greater on the Saffir Simpson hurricane scale as designated by NHC, CPHC or JTWC. b. Sustained tropical cyclone surface winds of 100 knots (115 mph) or greater are occurring or are expected to occur in a WFO’s county warning area within one hour.
Eye Wall
It is an organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds and most intense rainfall typically occur near the eye wall. VIP levels 3 or greater are typical. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of thunderstorm.
F Corona
In solar-terrestrial terms, of the white-light corona (that is, the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in inter-planetary space.
F Scale
Abbreviation for Fujita Scale, a system of rating the intensity of tornadoes; for detailed information, see the definition for that term.
FA
Forecast Area
FAA
Federal Aviation Administration
Face
In hydrologic terms, the external surface of a structure, such as the surface of a dam.
Facula
In solar-terrestrial terms, a bright region of the photosphere seen in white light, seldom visible except near the solar limb.
Fahrenheit
(abbrev. F) The standard scale used to measure temperature in the United States. On this scale, the freezing point of water is 32°F and the boiling point is 212°F. To convert a Celsius temperature to Fahrenheit, multiply it by 9/5 and then add 32:

°F = (°C * 9/5) + 32
Fair
It is usually used at night to describe less than 3/8 opaque clouds, no precipitation, no extremes of visibility, temperature or winds. It describes generally pleasant weather conditions.
Fall
the season of the year which is the transition period from summer to winter occurring as the sun approaches the winter solstice. In the Northern Hemisphere, fall customarily includes the months of September, October and November.
Fall Line
A skiing term, indicating the line of steepest descent of a slope.
Fall Wind
A strong, cold, downslope wind.
Fallstreak
Same as Virga; streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.
FAN
AVN MOS Guidance (older version)
Fanning
A pattern of plume dispersion in a stable atmosphere, in which the plume fans out in the horizontal and meanders about at a fixed height.
FASTST
Fastest
Fathom
Unit of water depth equal to 6 feet.
FAWS
Flight Advisory Weather Service
Federal Snow Sampler
In hydrologic terms, a snow sampler consisting of five or more sections of sampling tubes, one which has a steel cutter on the end. The combined snowpack measuring depth is 150 inches. This instrument was formerly the Mount Rose Type Snow Sampling Set.
Feeder Bands
Lines or bands of low-level clouds that move (feed) into the updraft region of a thunderstorm, usually from the east through south (i.e., parallel to the inflow). Same as inflow bands. This term also is used in tropical meteorology to describe spiral-shaped bands of convection surrounding, and moving toward, the center of a tropical cyclone.
FEMA
Federal Emergency Management Agency. An agency of the federal government having responsibilities in hazard mitigation; FEMA also administers the National Flood Insurance Program.
Field (Moisture) Capacity
The amount of water held in soil against the pull of gravity
Filament
A mass of gas suspended over the photosphere by magnetic fields and seen as dark lines threaded over the solar disk. A filament on the limb of the sun seen in emission against the dark sky is called a prominence.
Filament Channel
A broad pattern of fibrils in the chromosphere, marking where a filament may soon form or where a filament recently disappeared.
Fill Dam
In hydrologic terms, any dam constructed of excavated natural materials or of industrial wastes
Firebrand
Any source of heat, natural or man made, capable of igniting wildland fuels; flaming or glowing fuel particles that can be carried naturally by wind, convection currents, or gravity into unburned fuels.
First Law of Thermodynamics
The law of physics that states that the heat absorbed by a system either raises the internal energy of the system or does work on the environment.
Flanking Line
A line of cumulus or towering cumulus clouds connected to and extending outward from the most active part of a supercell, normally on the southwest side. The line normally has a stair-step appearance, with the tallest clouds closest to the main storm
Flare
In solar-terrestrial terms, a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted
Flash
A sudden, brief illumination of a conductive channel associated with lightning, which may contain multiple strokes with their associated stepped leaders, dart leaders and return strokes.
Flash Flood
A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters.
Flash Flood Guidance
(FFG) Forecast guidance produced by the River Forecast Centers, often model output, specific to the potential for flash flooding (e.g., how much rainfall over a given area will be required to produce flash flooding).
Flash Flood Statement
(FFS) In hydrologic terms, a statement by the NWS which provides follow-up information on flash flood watches and warnings.
Flash Flood Table
In hydrologic terms, a table of pre-computed forecast crest stage values for small streams for a variety of antecedent moisture conditions and rain amounts. Soil moisture conditions are often represented by flash flood guidance values. In lieu of crest stages, categorical representations of flooding, e.g., minor, moderate, etc. may be used on the tables.
Flash Flood Warning
Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.
Flash Flood Watch
Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.
Flash Multiplicity
The number of return strokes in a lightning flash.
Flashboards
In hydrologic terms, a length of timber, concrete, or steel placed on the crest of a spillway to raise the retention water level but which may be quickly removed in the event of a flood by a tripping device, or by deliberately designed failure of the flashboard or its supports
Float Recording Precipitation gage
In hydrologic terms, a rain gage where the rise of a float within the instrument with increasing rainfall is recorded. Some of these gages must be emptied manually, while others employ a self-starting siphon to empty old rainfall amounts.
Flood Categories
Terms defined for each forecast point which describe or categorize the severity of flood impacts in the corres­ponding river/stream reach. Each flood category is bounded by an upper and lower stage (see Example 1). The severity of flooding at a given stage is not necessarily the same at all locations along a river reach due to varying channel/bank characteristics or presence of levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are usually associated with water levels corresponding to the most significant flood impacts some­where in the reach. The flood categories used in the NWS are: *Minor Flooding* - minimal or no property damage, but possibly some public threat. *Moderate Flooding* - some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations. *Major Flooding* - extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations. *Record Flooding* - flooding which equals or exceeds the highest stage or discharge at a given site during the period of record keeping. Note: all three of the lower flood categories (minor, moderate, major) do not necessarily exist for a given forecast point. For example, at the level where a river reaches flood stage, it may be considered moderate flooding. However, at least one of these three flood categories must start at flood stage.
Flood Control Storage
In hydrologic terms, storage of water in reservoirs to abate flood damage
Flood Hazard Outlook (FHO)
A high Level graphical depiction and key messages highlighting the potential threat of inland flood hazards (flash, urban, small stream and riverine) and their associated impacts (catastrophic, considerable, and limited) for the next seven days.
Flood Inundation Mapping (FIM)
In the context of the National Water Model (NWM) FIM, the expected maximum inundation extent over a particular period of time derived using the Height Above Nearest Drainage (HAND) method. Stage heights are interpolated from the NWM discharge values using synthetic rating curves, and are rounded to the nearest foot.
Flood Loss Reduction Measures
In hydrologic terms, the strategy for reducing flood losses. There are four basic strategies. They are prevention, property protection, emergency services, and structural projects. Each strategy incorporates different measures that are appropriate for different conditions. In many communities, a different person may be responsible for each strategy.
Flood Plain
In hydrologic terms, the portion of a river valley that has been inundated by the river during historic floods.
Flood Potential Outlook
(ESF on AFOS) (FPO for Acronym): In hydrologic terms, An NWS outlook that is issued to alert the public of potentially heavy rainfall that could send area rivers and streams into flood or aggravate an existing flood.
Flood Stage
An established gage height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull stage.
Flood Statement (FLS)
In hydrologic terms, a statement issued by the NWS to inform the public of flooding along major streams in which there is not a serious threat to life or property. It may also follow a flood warning to give later information.
Flood Warning
(FLW) In hydrologic terms, a release by the NWS to inform the public of flooding along larger streams in which there is a serious threat to life or property. A flood warning will usually contain river stage (level) forecasts.
Flood Watch
Issued to inform the public and cooperating agencies that current and developing hydrometeorological conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent.
Flood Wave
In hydrologic terms, a rise in streamflow to a crest and its subsequent recession caused by precipitation, snowmelt, dam failure, or reservoir releases
Floodwall
In hydrologic terms, a long, narrow concrete, or masonry embankment usually built to protect land from flooding. If built of earth the structure is usually referred to as a levee. Floodwalls and levees confine streamflow within a specified area to prevent flooding. The term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.
Floodway
In hydrologic terms,
(1) A part of the flood plain, otherwise leveed, reserved for emergency diversion of water during floods. A part of the flood plain which, to facilitate the passage of floodwater, is kept clear of encumbrances.
(2) The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and discharge the floodwater or floodflow of any river or stream.
Flow Duration Curve
In hydrologic terms, a cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.
Flow Separation
The process by which a separation eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides.
Foehn Pause
A temporary cessation of the foehn at the ground due to the formation or intrusion of a cold air layer which lifts the foehn off the ground.
Forced Channeling
Channeling of upper winds along a valley's axis when upper winds are diverted by the underlying topography. Compare pressure-driven channeling.
Forebay
In hydrologic terms, the water behind (upstream) of the dam.
Forecast
A statement of prediction.
Forecast Crest
In hydrologic terms, the highest elevation of river level, or stage, expected during a specified storm event.
Forecast Guidance
Computer-generated forecast materials used to assist the preparation of a forecast, such as numerical forecast models.
Forecast Issuance Stage
The stage which, when reached by a rising stream, represents the level where RFCs need to begin issuing forecasts for a non-routine (flood-only) forecast point. This stage is coordinated between WFO and RFC personnel and is not necessarily the same as action or alert stage. The needs of WFO/RFC partners and other users are considered in determining this stage.
Forecast Periods
Official definitions for NWS products:

Today...............................Sunrise to sunset
This afternoon..................noon till 6 p.m.
This evening.....................6 p.m. till sunset
Tonight.............................sunset till sunrise
Tomorrow.........................sunrise to sunset of the following day
Forecast Point
A location along a river or stream for which hydrologic forecast and warning services are provided by a WFO. The observed/forecast stage or discharge for a given forecast point can be assumed to represent conditions in a given reach (see /reach/).
Forecast valid for
The period of time the forecast is in effect beginning at a given day, date and time, and ending at a given day, date and time.
Forward Flank Downdraft
The main region of downdraft in the forward, or leading, part of a supercell, where most of the heavy precipitation is.
Fountainhead
In hydrologic terms, the upper end of a confined-aquifer conduit, where it intersects the land surface.
Fractocumulus
A cumulus cloud presenting a ragged, shredded appearance, as if torn.
Fractostratus
A stratus cloud presenting a ragged, shredded appearance, as if torn. It differs from a fractocumulus cloud in having a smaller vertical extent and darker color.
Fracture
In hydrologic terms, any break or rupture formed in an ice cover or floe due to deformation.
Fracture Zone
In hydrologic terms, an area which has a great number of fractures.
Fracturing
In hydrologic terms, deformation process whereby ice is permanently deformed, and fracture occurs.
Fractus
Ragged, detached cloud fragments; same as scud.
Frazil Ice
In hydrologic terms, fine spicules, plates, or discoids of ice suspended in water. In rivers and lakes, frazil is formed in supercooled, turbulent water.
Frazil Slush
In hydrologic terms, an agglomerate of loosely packed frazil which floats or accumulates under the ice cover.
Freak Wave
A wave of much greater height and steepness than other waves in the prevailing sea or swell system. See Rogue Wave.
Free Atmosphere
The part of the atmosphere that lies above the frictional influence of the earth's surface.
Free Ground Water
In hydrologic terms, unconfined ground water whose upper boundary is a free water table
Freeboard
In hydrologic terms, the vertical distance between the normal maximum level of the water surface in a channel, resrvoir, tank, canal, etc., and the top of the sides of a levee, dam, etc., which is provided so that waves and other movements of the liquid will not overtop the confining structure
Freeze Warning
Issued during the growing season when surface temperatures are expected to drop below freezing over a large area for an extended period of time, regardless whether or not frost develops.
Freeze Watch
This product is issued when there is potential for significant, widespread freezing temperatures, but the occurrence, location, and/or timing is still uncertain.
Freezeup date
In hydrologic terms, the date on which the water body was first observed to be completely frozen over
Freezing Drizzle Advisory
Issued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected. However, even small amounts of freezing rain or freezing drizzle may cause significant travel problems.
Freezing Fog Advisory
This product is issued when very light ice accumulations are expected from freezing fog.
Freezing Rain
Rain that falls as a liquid but freezes into glaze upon contact with the ground.
Freezing Rain Advisory
Issued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected. However, even small amounts of freezing rain or freezing drizzle may cause significant travel problems.
Freezing Spray
An accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Freezing Spray Advisory
An advisory for an accumulation of freezing water droplets on a vessel at a rate of less than 2 centimeters (cm) per hour caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Freezup jam
In hydrologic terms, ice jam formed as frazil ice accumulates and thickens
French Drain
In hydrologic terms, an underground passageway for water through the interstices among stones placed loosely in a trench
Friction Head
In hydrologic terms, the decrease in total head caused by friction
Friction Layer
Same as Planetary Boundary Layer; the layer within the atmosphere between the earth's surface and 1 km above the surface; this is the layer where friction affects wind speed and wind direction.
Frontal Inversion
A temperature inversion that develops aloft when warm air overruns the cold air behind a front.
FROPA
Frontal Passage
Frost Advisory
Issued during the growing season when widespread frost formation is expected over an extensive area. Surface temperatures are usually in the mid 30s Fahrenheit.
FTPMAIL
An Internet server application which provides access to Internet FTP server files via e-mail. The National Weather Service operates an FTPMAIL server which provides e -mail access to any product available on the tgftp.nws.noaa.gov FTP server including marine text and graphic forecasts. For further information see: http://weather.noaa.gov/pub/fax/ftpmail.txt, or send an e-mail to ftpmail@ftpmail.nws.noaa.gov with the word "help" in the body.
Fujita Scale
(or F Scale) - A scale of tornado intensity in which wind speeds are inferred from an analysis of wind damage:

RatingWind, Damage
F0 (weak)40-72 mph, light damage
F1 (weak)73-112 mph, moderate damage
F2 (strong)113-157 mph, considerable damage
F3 (strong)158-206 mph, severe damage
F4 (violent)207-260 mph, devestating damage
F5 (violent)260-318 mph (rare), incredible damage


All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.
Fujiwhara Effect
A binary interaction where tropical cyclones within a certain distance (300-750 nm depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint.
Full-Physics Numerical Model
A computer model used to calculate air pollution concentrations. A full-physics numerical model uses a full set of equations describing the thermodynamic and dynamic state of the atmosphere and can be used to simulate atmospheric phenomena.
Fumigation
A pattern of plume dispersion produced when a convective boundary layer grows upward into a plume trapped in a stable layer. The elevated plume is suddenly brought downward to the ground, producing high surface concentrations.
FZRA
freezing rain
Gage
In hydrologic terms,
1) A device for indicating the magnitude or position of a thing in specific units, when such magnitude or position undergoes change, for example: The elevation of a water surface, the velocity of flowing water, the pressure of water, the amount or intensity of precipitation, the depth of snowfall, etc.
(2) The act or operation of registering or measuring the magnitude or position of a thing when these characteristics are undergoing change.
(3) The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.
Gage Datum
A horizontal surface used as a zero point for measurement of stage or gage height. This surface usually is located slightly below the lowest point of the stream bottom such that the gage height is usually slightly greater than the maximum depth of water. Because the gage datum is not an actual physical object, the datum is usually defined by specifying the elevations of permanent reference marks such as bridge abutments and survey monuments, and the gage is set to agree with the reference marks. Gage datum is a local datum that is maintained indepen­dently of any national geodetic datum. However, if the elevation of the gage datum relative to the national datum (North American Vertical Datum of 1988 or National Geodetic Vertical Datum of 1929) has been determined, then the gage readings can be converted to elevations above the national datum by adding the elevation of the gage datum to the gage reading.
Gaging Station
In hydrologic terms, a particular site on a watercourse where systematic observations of stage/ and or flow are measured.
Gale
An extratropical low or an area of sustained surface winds of 34 (39 mph) to 47 knots (54 mph).
Gale Warning
A warning of sustained surface winds, or frequent gusts, in the range of 34 knots (39 mph) to 47 knots (54 mph) inclusive, either predicted or occurring, and not directly associated with a tropical cyclone.
Gale Watch
A watch for an increased risk of a gale force wind event for sustained surface winds, or frequent gusts, of 34 knots (39 mph) to 47 knots (54 mph), but its occurrence, location,and/or timing is still uncertain.
Gallery
In hydrologic terms, a passageway within the body of a dam or abutment.
Gamma
A unit of magnetic field intensity equal to 1 x 10.0-5 Gauss; also equal to 1 nanotelsa (nT).
Gamma Ray
A type of electromagnetic radiation with a very short wavelength and high energy level. Generally, emitted during radioactive decay of a substance.
Gap Winds
Strong winds channeled through gaps in the Pacific coastal ranges, blowing out into the Pacific Ocean or into the waterways of the Inside Passage. The winds blow through low passes where major river valleys issue onto the seaways when strong east-west pressure gradients exist between the coast and the inland areas, with low pressure over the ocean.
Gas Laws
The thermodynamic laws pertaining to perfect gases, including Boyle's law, Charles' law, Dalton's law and the equation of state.
Gate
In hydrologic terms, a device in which a leaf or member is moved across the waterway from an external position to control or stop flow. There are many different kinds of gates used on a dam
Gauss
The unit of magnetic induction in the cgs (centimeter-gram- second) system
Gaussian Plume Model
A computer model used to calculate air pollution concentrations. The model assumes that a pollutant plume is carried downwind from its emission source by a mean wind and that concentrations in the plume can be approximated by assuming that the highest concentrations occur on the horizontal and vertical midlines of the plume, with the distribution about these mid-lines characterized by Gaussian- or bell-shaped concentration profiles.
Gaussian Puff Model
A model used to calculate air pollution concentrations. The model assumes that a continuously emitted plume or instantaneous cloud of pollutants can be simulated by the release of a series of puffs that will be carried in a time- and space-varying wind field. The puffs are assumed to have Gaussian or bell-shaped concentration profiles in their vertical and horizontal planes.
GEMPAK
General Environmental Meteorological Package (programming language)
General Circulation
The totality of large-scale organized motion for the entire global atmosphere.
General Circulation Models
(GCMs) - These computer simulations reproduce the Earth's weather patterns and can be used to predict change in the weather and climate.
General Wind
Land management agency term for winds produced by synoptic-scale pressure systems on which smaller-scale or local convective winds are superimposed.
Geomagnetic Elements
In solar-terrestrial terms, the components of the geomagnetic field at the surface of the earth. In SESC use, the northward and eastward components are often called the H and D components, where the D component is expressed in gammas and is derived from D (the declination angle) using the small angle approximation.
Geomagnetic Field
The magnetic field observed in and around the earth. The intensity of the magnetic field at the earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole
Geomagnetic Storm
In solar-terrestraial terms, a worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations.
Minor Geomagnetic Storm: A storm for which the Ap index was greater than 29 and less than 50.
Major Geomagnetic Storm: A storm for which the Ap index was greater than 49 and less than 100.
Severe Geomagnetic Storm: A storm for which the Ap index was 100 or more.
Initial Phase: Of a geomagnetic storm, that period when there may be an increase of the middle-latitude horizontal intensity (H).
Main Phase: Of a geomagnetic storm, that period when the hori- zontal magnetic field at middle latitudes is generally decreasing.
Recovery Phase: Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels.
Geophysical Events
In solar-terrestrial terms, flares (Importance two or larger) with Centimetric Outbursts (maximum of the flux higher than the Quiet Sun flux, duration longer 10 minutes) and/or strong SID. Sometimes these flares are followed by Geomagnetic Storms or small PCA. (Class M Flares)
Geopotential Height
The height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.
Geostationary Satellite
A satellite that rotates at the same rate as the earth, remaining over the same spot above the equator.
Glaciation
The transformation of cloud particles from water drops to ice crystals. Thus, a cumulonimbus cloud is said to have a "glaciated" upper portion.
Glacier
In hydrologic terms, bodies of land ice that consist of recrystallized snow accumulated on the surface of the ground, and that move slowly downslope.
Glacier Dammed Lake
In hydrologic terms, the lake formed when a glacier flows across the mouth of an adjoining valley and forms an ice dam.
Glacier Wind
A shallow downslope wind above the surface of a glacier, caused by the temperature difference between the air in contact with the glacier and the free air at the same altitude. The glacier wind does not reverse diurnally like slope and along-valley wind systems.
Glaze
Ice formed by freezing precipitation covering the ground or exposed objects.
Global Ensemble Forecast System (GEFS)
A weather forecast model made up of 21 separate forecasts, or ensemble members. The National Centers for Environmental Prediction (NCEP) started the GEFS to address the nature of uncertainty in weather observations, which are used to initialize weather forecast models.
Global Forecast System
(GFS)- One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 384 hours.
Global Temperature Change
The net result of four primary factors including the greenhouse effect, changes in incoming solar radiation, altered patterns of ocean circulations, and changes in continental position, topography and/or vegetation. Three feedback mechanisms which affect global temperature change include cloud height and amount, snow and ice distribution, and atmospheric water vapor levels.
Global Warming
An overall increase in world temperatures which may be caused by additional heat being trapped by greenhouse gases.
GRAD
Gradient- A rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum change.
Gradient
(abbrev. GRAD) A rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum change.
Gradient High Winds
These high winds usually cover a large area and are due to synoptic-scale, extra-tropical low pressure systems.
Gradual Commencement
In solar-terrestrial terms, the commencement of a geomagnetic storm that has no well-defined onset
Granulation
In solar-terrestrial terms, the cellular structure of the photosphere visible at high spatial resolution.
Graupel
Same as snow pellets or small hail.
Gravity Dam
In hydrologic terms, a concrete structure proportioned so that its own weight provides the major resistance to the forces exerted on it.
Gravity Recovery and Climate Experiment (GRACE)
NASA satellites that detect small changes in the Earth’s gravitational field caused by the redistribution of water on and beneath the land surface.
Gravity Wave
A wave created by the action of gravity on density variations in the stratified atmosphere. A generic classification for lee waves, mountains waves, and many other waves that form in the atmosphere.
Graybody
A hypothetical "body" that absorbs some constant fraction of all electromagnetic radiation incident upon it.
Great Circle Track
A great-circle track is the shortest distance between two points on a sphere, and when viewed on a 2-dimensional map the track will appear curved. Swell waves travel along routes that mark out great circles.
Great Lakes Faxback
Dissemination systems housed at Weather Forecast Office (WFO) Cleveland by which Great Lakes customers request and receive hard copies of selected marine products.
Great Lakes Freeze-Up/Break-Up Outlook
(FBO) - A National Weather Service product to keep mariners informed of the projected freeze-up date or break-up date of ice on the Great Lakes.
Great Lakes Marine Forecast (MAFOR)
A National Weather Service coded summary appended to each of the Great Lakes Open Lakes forecasts.
Great Lakes Storm Summary
(GLS) - A National Weather Service forecast product providing updated information whenever a storm warning is in effect on any of the Great Lakes.
Great Lakes Weather Broadcast
(LAWEB) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.
Greenhouse Gases
The gases that absorb terrestrial radiation and contribute to the greenhouse effect; the main greenhouse gasses are water vapor, methane, carbon dioxide, and ozone.
Ground Blizzard Warning
When blizzard conditions are solely caused by blowing and drifting snow.
Ground Heat Flux
The flux of heat from the ground to the earth's surface; a component of the surface energy budget.
Ground Water
In hydrologic terms, water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table. Also termed Phreatic water.
Ground Water Divide
In hydrologic terms, A line on a water table where on either side of which the water table slopes downward. It is analogous to a drainage divide between two drainage basins on a land surface
Ground Water Flow
In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as baseflow, or dry-weather flow
Ground Water Hydrology
The branch of hydrology that specializes in ground water; its occurrence and movements; its replenishment and depletion; the properties of rocks that control ground water movement and storage; and the methods of investigation and utilization of ground water
Ground Water Outflow
In hydrologic terms, the part of the discharge from a drainage basin that occurs through the ground water. The term "underflow" is often used to describe the ground water outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measure at a gaging station.
Ground Water Overdraft
Pumpage of ground water in excess of safe yield.
Ground Water Runoff
That part of the runoff which has passed into the ground, has become ground water, and has been discharged into a stream channel as spring, or seepage water.
Grout Curtain
A barrier produced by injecting grout into a vertical zone, usually narrow (horizontally), and in the foundation to reduce seepage under a dam
Growing Degree Day
The number of degrees that the average temperature is above a baseline value. For example, 40 degrees for canning purposes; 45 degree for potatoes; and 50 degrees for sweet corn, snap beans, lima beans, tomatoes, grapes, and field corn. Every degree that the average temperature is above the baseline value becomes a growing degree day. Agricultural related interests use growing degree days to determine planting times.
Growing Season
the period of time between the last killing frost of spring and the first killing frost of autumn.
Gulf Stream
Warm water current extending from the Gulf of America and Florida up the U.S. east coast then east northeast to Iceland and Norway.
GUNA
Average of GHMI, EGRI, NGPI, and GFSI GHMI: Previous cycle GFDL, adjusted using a variable intensity offset correction that is a function of forecast time. GFDL: NWS/Geophysical Fluid Dynamics Laboratory GFDL) model EGRI: Previous cycle EGRR, adjusted EGRR: UKMET with subjective quality control applied to the tracker UKMET: United Kingdom Met Office NGPI: Previous cycle NGPS, adjusted NGPS: Navy Operational Global Prediction System NOGAPS: Navy Operational Global Prediction System GFSI: Previous cycle GFS, adjusted GFS: NWS/Global Forecast System
Gustnado
(or Gustinado) - A gustnado is a small, whirlwind which forms as an eddy in thunderstorm outflows. They do not connect with any cloud-base rotation and are not tornadoes. Since their origin is associated with cumuliform clouds, gustnadoes will be classified as Thunderstorm Wind events. Like dust devils, some stronger gustnadoes can cause damage.
H-Alpha
In solar-terrestrial terms, this absorption line of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The H-alpha line is universally used for patrol observations of solar flares.
H-component of the Geomagnetic Field
(Geomagnetic Elements) In solar-terrestrial terms, the components of the geomagnetic field at the surface of the earth. In SESC use, the northward and eastward components are often called the H and D components, where the D component is expressed in gammas and is derived from D (the declination angle) using the small angle approximation.
Hague Line
The North Atlantic boundary between the U.S. and Canada fishing waters as determined by the World Court in The Hague, Netherlands.
Hail
Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.
Hail Contamination
A limitation in NEXRAD rainfall estimates whereby abnormally high reflectivities associated with hail are converted to rainfall rates and rainfall accumulations. These high reflectivity values are mistaken by the radar for extremely heavy rain, thus "contaminating" (inflating) its estimation of how much rain has fallen over the affected area.
Hail Index
An indication of whether the thunderstorm structure of each storm identified is conducive to the production of hail.
Hail Size
Typically refers to the diameter of the hailstones. Warnings and reports may report hail size through comparisons with real-world objects that correspond to certain diameters:

DescriptionDiameter (inches)
Pea0.25
Marble or Mothball0.50
Penny or Dime0.75
Nickel0.88
Quarter1.00
Half Dollar1.25
Walnut or Ping Pong Ball1.50
Golfball1.75
Hen's Egg2.00
Tennis Ball2.50
Baseball2.75
Tea Cup3.00
Grapefruit4.00
Softball4.50
Hail Spike
An area of reflectivity extending away from the radar immediately behind a thunderstorm with extremely large hail. In an area of large hail, radiation from the radar can bounce from hailstone to hailstone before being reflected back to the radar. The time delay between the backscattered radiation from the storm and the bounced and scattered radiation from the large hail causes the reflectivity from the hail to appear to come from a farther range than the actual storm.
Haines Index
This is also called the Lower Atmosphere Stability Index. It is computed from the morning (12Z) soundings from RAOB stations across North America. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels. The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior. The Haines Indices range from 2 to 6 for indicating potential for large fire growth
Halo
Any of a variety of bright circles or arcs centered on the sun or moon, caused by the refraction or reflection of light by ice crystals suspended in the earth's atmosphere and exhibiting prismatic coloration ranging from red inside to blue outside.
Hanging (ice) dam
In hydrologic terms, a mass of ice composed mainly of frazil or broken ice deposited underneath an ice cover in a region of low flow velocity.
Hard Freeze
A hard freeze occurs when the surface air temperature reaches 28°F or lower for an extended period of time, killing most types of vegetation.
Hazardous Seas Warning
A warning for wave heights and/or wave steepness values meeting or exceeding locally defined warning criteria.
Hazardous Seas Watch
A watch for an increased risk of a hazardous seas warning event to meet Hazardous Seas Warning criteria but its occurrence, location, and/or timing is still uncertain.
Hazardous Weather Outlook
A narrative statement produced by the National Weather Service, frequently issued on a routine basis, to provide information regarding the potential of significant weather expected during the next 1 to 5 days.
Hazards Assessment
CPC's Hazards Assessment provides emergency managers, planners, forecasters and the public advance notice of potential hazards related to climate, weather and hydrological events.
Haze
(abbrev. HZ)- An aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors.
HDRAIN
An Hourly Digital Rainfall Product of the WSR-88D.
Head
In hydrologic terms, the difference between the pool height and tailwater height. Usually expressed in feet of head, or in lbs./sq. inch
Head Loss
In hydrologic terms, the decrease in total head caused by friction
Head Race
In hydrologic terms, a channel which directs water to a water wheel; a forebay.
Headward Erosion
In hydrologic terms, erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in such a direction.
Headwater Basin
In hydrologic terms, a basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.
Headwaters
In hydrologic terms, streams at the source of a river.
Heat Advisory
This product is issued for dangerous heat conditions that are not expected to reach warning criteria.
Heat Cramps
Painful, involuntary muscle spasms that can occur due to the overheating of the body during prolonged exertion and excessive sweating. Heat cramps may be the first sign of heat-related illness and can lead to heat exhaustion or stroke.
Heat Exhaustion
A heat-related illness characterized by the body's inability to effectively cool itself, typically occurring in high ambient temperatures or during intense physical exertion. It is characterized by headache, nausea, dizziness, weakness, irritability, thirst, and/or heavy sweating.
Heat Index
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature.
Heat Lightning
Lightning that occurs at a distance such that thunder is no longer audible.
Heat Stroke
A serious heat-related illness where the body can no longer control its temperature: the body's temperature rises rapidly, the sweating mechanism fails, and the body is unable to cool down. When heat stroke occurs, the body temperature can rise to 106°F or higher within 10 to 15 minutes. Heat stroke can cause permanent disability or death if the person does not receive emergency treatment.
Heat Wave
A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days.
Heating Degree Days
(abbrev. HDD) A form of degree day used to estimate energy requirements for heating. Typically, heating degree days are calculated as how much colder the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 55°F on a certain day, there were 10 HDD (Heating Degree Days) that day because 65 - 55 = 10.
HeatRisk
A color-numeric-based index that uses weather, climate, and Centers for Disease Control and Prevention (CDC) heat-health data to place the upcoming temperature forecast into climatological context and identify potentially dangerous heat that will lead to increased heat-related impacts.
Heavy Freezing Spray
An accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Heavy Freezing Spray Warning
A warning for an accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Heavy Freezing Spray Watch
A watch for an increased risk of a heavy freezing spray event to meet Heavy Freezing Spray Warning criteria but its occurrence, location, and/or timing is still uncertain.
Heavy Snow
This generally means...

In forecasts, snowfall amounts are expressed as a range of values, e.g., "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...".
Heavy Snow Warning
Issued by the National Weather Service when snowfall of 6 inches (15 cm) or more in 12 hours or 8 inches (20 cm) or more in 24 hours is imminent or occurring. These criteria are specific for the Midwest and may vary regionally.
Heavy Surf Advisory
An advisory issued by the National Weather Service for fast moving deep water waves which can result in big breaking waves in shallow water (the surf zone).
Hectopascal
A unit of pressure equal to a millibar (1 hPa = 1 mb). Abbreviated hPa.
Height Above the Nearest Drainage (HAND)
A relative elevation methodology used in inundation mapping, which determines the height of every point on a land surface above the nearest stream reach to which it drains.
High Flow Magnitude (HFM)
The magnitude of the maximum forecast streamflow conditions, measured in terms of annual exceedance probabilities.
High Latitudes
With specific reference to zones of geomagnetic activity, "high latitudes" refers to 50º to 80º geomagnetic.
High Resolution Ensemble Forecast (HREF)
An ensemble of products from several different models running at ~3 km horizontal grid spacing.
High Resolution Rapid Refresh (HRRR)
A real-time 3-km resolution, hourly updated, cloud-resolving, convection-allowing atmospheric model, initialized by 3km grids with 3km radar assimilation.
High Seas Forecast
(HSF) - Marine forecasts for the major oceans of the world. In this context, major gulfs or seas (e.g., the Gulf of Mexico or the Bering Sea) are included within these forecast areas. Areas of responsibility for the U.S. are determined by international agreements under the auspices of the World Meteorological Organization (WMO).
High Surf Advisory
A High Surf Advisory is issued when breaking wave action poses a threat to life and property within the surf zone. High surf criteria vary by region. High Surf Advisories are issued using the Coastal and Lakeshore Hazard Message (CFW) product.
High Surf Warning
A High Surf Warning is issued when breaking wave action results in an especially heightened threat to life and property within the surf zone. High surf criteria vary by region. High Surf Warnings are issued using the Coastal and Lakeshore Hazard Message (CFW) product.
High Water Arrival Time
The time at which a stream reach is forecast to achieve its high water flow within the forecast period.
High Water Flow
The Office of Water Prediction (OWP) uses the term “high water” as an indicator of elevated flows for a given stream reach. This regionally varied threshold is used in many National Water Model (NWM) data services, such as maximum high flows, peak and high flow arrival times, rapid onset flooding, and flood inundation mapping. Filtering on flows that are greater than or equal to the high water threshold allows these services to focus on areas where flows are significant enough to monitor - where there is potential flooding, for example. High water thresholds are established for eleven clusters that categorize runoff efficiencies across the Continental United States (ConUS). Hawaii, Puerto Rico, and U.S. Virgin Islands domains utilize the 2.0 year recurrence interval for the high water threshold.
High Water Probability
The probability (%) that a stream will reach its high water flow within the forecast period.
High Wind Warning
This product is issued when sustained, strong winds with even stronger gusts are expected or occurring. Generally for sustained wind speeds of 40 mph or greater lasting for 1 hour or longer, or winds of 58 mph or greater for any duration.
High Wind Watch
This product is issued when a high wind event is possible, but its occurrence, location, and/or timing is still uncertain.
High-Speed Stream
In solar-terrestrial terms, a feature of the solar wind having velocities that are about double average solar wind values.
Hinge Crack
In hydrologic terms, a crack caused by significant changes in water level.
Hoar Frost
A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling.
Hodograph
A polar coordinate graph which shows the vertical wind profile of the lowest 7000 meters of the atmosphere. These plots are used to determine the advection patterns aloft, whether a thunderstorm will rotate, and the type of thunderstorms that you will likely see that day.
Homologous Flares
In solar-terrestrial terms, solar flares that occur repetitively in the same active region, with essentially the same position and with a common pattern of development
Hourly Precipitation Data (HPD)
It contains data on nearly 3,000 hourly precipitation stations (National Weather Service, Federal Aviation Administration, and cooperative observer stations) in inches to tenths or inches to hundredths at local standard time. HPD includes maximum precipitation for nine (9) time periods from 15 minutes to 24 hours, for selected stations.
hPa
Hectopascal- A unit of pressure equal to a millibar (1 hPa = 1 mb).
HSA (Hydrologic Service Area)
A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.
Hurricane
(abbrev. HURCN) A tropical cyclone in the Atlantic, Caribbean Sea, Gulf of Mexico, or eastern Pacific, which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater.
Hurricane Force Wind Warning
A warning for sustained winds, or frequent gusts, of 64 knots (74 mph) or greater, either predicted or occurring, and not directly associated with a tropical cyclone.
Hurricane Force Wind Watch
A watch for an increased risk of a hurricane force wind event for sustained surface winds, or frequent gusts, of 34 knots 64 knots (74 mph) or greater, but its occurrence, location, and/or timing is still uncertain.
Hurricane Local Statement
A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on
(1) weather conditions
(2) evacuation decisions made by local officials
(3) other precautions necessary to protect life and property.
Hurricane Season
The part of the year having a relatively high incidence of tropical cyclones. In the Atlantic, Caribbean, and Gulf of Mexico, and central North Pacific, the hurricane season is the period from June through November; in the eastern Pacific, May 15 through November 30. Tropical cyclones can occur year-round in any basin.
Hurricane Warning
An announcement that hurricane conditions (sustained winds of 74 mph or higher) are expected somewhere within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds. The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force.
Hurricane Watch
An announcement that hurricane conditions (sustained winds of 74 mph or higher) are possible within the specified coastal area. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.
Hyder Flare
In solar-terrestrial terms, a filiment-associated two-ribbon flare, often occurring in spotless regions. The flare presumably results from the impact on the chromosphere of infalling filament material.
Hydraulic Fill Dam
In hydrologic terms, a dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water
Hydraulic Flow
Atmospheric flow that is similar in character to the flow of water over an obstacle.
Hydraulic Grade Line
In hydrologic terms, a line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a given fluid flow path, such as along a pipeline or a ground water streamline.
Hydraulic Head
In hydrologic terms,
(1) The height of the free surface of a body of water above a given point beneath the surface.
(2) The height of the water level at the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream.
(3) The height of a hydraulic grade line above the center line of a pressure pipe, at a given point.
Hydraulic Jump
A steady disturbance in the lee of a mountain, where the airflow passing over the mountain suddenly changes from a region of low depth and high velocity to a region of high depth and low velocity.
Hydraulic Permeability
In hydrologic terms, the flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
Hydro­meteorology
An inter­disci­plinary science involving the study and analysis of the inter­relation­ships between the atmo­spheric and land phases of water as it moves through the hydro­logic cycle.
Hydrograph
In hydrologic terms, a graph showing the water level (stage), discharge, or other property of a river volume with respect to time.
Hydrograph Separation
In hydrologic terms, the process where the storm hydrograph is separated into baseflow components and surface runoff components.
Hydrographic Survey
In hydrologic terms, an instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and elevation of high water marks; location and depth of wells, etc.
Hydrologic Ensemble Forecast System (HEFS)
A probabilistic forecast tool with the goals to provide hydrologic forecasts including an analysis of “probable outcomes” and to minimize biases in the atmospheric models and in the hydrologic models.
Hydrologic Equation
In hydrologic terms, the water inventory equation (Inflow = Outflow + Change in Storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.
Hydrologic Service Area
HSA. A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.
Hydrostatic Head
In hydrologic terms, a measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure
Hyetograph
A graphical representation of rainfall intensity with respect to time.
Hypothermia
A rapid, progressive mental and physical collapse that accompanies the lowering of body temperature.
Ice Age
A time of widespread glaciation.
Ice Crystals
A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.
Ice Jam
In hydrologic terms, a stationary accumulation that restricts or blocks streamflow.
Ice Storm Warning
This product is issued by the National Weather Service when freezing rain produces a significant and possibly damaging accumulation of ice. The criteria for this warning varies from state to state, but typically will be issued any time more than 1/4" of ice is expected to accumulate in an area.
Icelandic Low
A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.
Ideal Gas Laws
The thermodynamic laws applying to perfect gases.
Impermeable
Material that does not permit fluids to pass through it.
Inactive Storage Capacity
In hydrologic terms, the portion of capacity below which the reservoir is not normally drawn, and which is provided for sedimentation, recreation, fish and wildlife, aesthetic reasons, or for the creation of a minimum controlled operational or power head in compliance with operating agreements or restrictions.
Industrial Consumption
The quantity of water consumed in a municipality or district for mechanical, trade, and manufacturing purposes, in a given period, generally one day. The per capita use is generally based on the total population of the locality, municipality, or district.
Infiltration
In hydrologic terms, movement of water through the soil surface into the soil
Infiltration Capacity
In hydrologic terms, the maximum rate at which water can enter the soil at a particular point under a given set of conditions.
Infiltration Index
In hydrologic terms, an average rate of infiltration, in inches per hour, equal to the average rate of rainfall such as that the volume of rainfall at greater rates equals the total direct runoff.
Infiltration Rate
In hydrologic terms,
(1) The rate at which infiltration takes place expressed in depth of water per unit time, usually in inches per hour.
(2) The rate, usually expressed in cubic feet per second, or million gallons per day per mile of waterway, at which ground water enters an infiltration ditch or gallery, drain, sewer, or other underground conduit.
Inflow Bands
Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone
Influent Seepage
In hydrologic terms, movement of gravity water in the zone of aeration from the ground surface toward the water table.
Influent Stream
In hydrologic terms, any watercourse in which all, or a portion of the surface water flows back into the ground namely the, vadose zone, or zone of aeration
Infrared Satellite Imagery
This satellite imagery senses surface and cloud top temperatures by measuring the wavelength of electromagnetic radiation emitted from these objects. This energy is called "infrared". High clouds are very cold, so they appear white. Mid-level clouds are somewhat warmer, so they will be a light gray shade. Low cloud are warmer still, so they appear as a dark shade of gray or black. Often, low clouds are the same temperature as the surrounding terrain and cannot be distinguished at all. The satellite picks up this infrared energy between 10.5 and 12.6 micrometer (um) channels.
Initial Detention
In hydrologic terms, the volume of water on the ground, either in depressions or in transit, at the time active runoff begins.
Inland freshwater wetlands
In hydrologic terms, swamps, marshes, and bogs found inland beyond the coastal saltwater wetlands.
Insolation
Incoming solar radiation. Solar heating; sunshine.
Instability
(abbrev. INSTBY)- The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.
Intangible Flood Damage
In hydrologic terms, estimates of the damage done by disruption of business, danger to health, shock, and loss of life and in general all costs not directly measurable which require a large element of judgment for estimating.
Interbasin Transfer
In hydrologic terms, the physical transfer of water from one watershed to another.
Interception Storage Requirements
In hydrologic terms, water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.
Intermediate Synoptic Times
The times of 0300, 0900, 1500, and 2100 UTC.
Intermittent Stream
In hydrological terms, a stream that flows periodically
International Date Line
The line of longitude located at 180 degrees East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.
Interplanetary Magnetic Field
(abbrev. IMF) In solar-terrestrial terms, the magnetic field carried with the solar wind.
Interpolate
To estimate a value within an interval between two known values. This technique is sometimes used with computer models for locations in between the model's "gridpoints."
Intertropical Convergence Zone
(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.
Intraseasonal Oscillation
Oscillation with variability on a timescale less than a season. One example is the Madden-Julian Oscillation.
Inundation
The process of covering normally dry areas with flood waters.
Irrigation
In hydrologic terms, the controlled application of water to arable lands to supply water requirements not satisfied by rainfall
Irrigation Requirement
In hydrologic terms, the quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable wastes.
Isallobar
A line of equal change in atmospheric pressure during a specified time period.
Isentropic Analysis
A way in the forecaster can look at the atmosphere in 3-dimensions instead of looking at constant pressure surfaces (such as the 850 mb, 700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method, the forecaster looks at constant potential temperature (the temperature that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available.
Isentropic Surface
A two-dimensional surface containing points of equal potential temperature.
Isobar
A line connecting points of equal pressure.
Isobaric Chart
A weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York).
Isobaric Process
Any thermodynamic change of state of a system that takes a place at constant pressure.
Isobath
In hydrologic terms, an imaginary line on the earth's surface or a line on a map connecting all points which are the same vertical distance above the upper or lower surface of a water-bearing formation or aquifer
Isolated
A National Weather Service convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch). Isolated is used interchangeably with few.
Isotach
A line connecting points of equal wind speed.
Issuance Time
The time the product is transmitted.
ITWAS
Integrated Terminal Weather System
January Thaw
A period of mild weather popularly supposed to recur each year in late January.
Jet Max
( or Jet Streak) A point or area of relative maximum wind speeds within a jet stream.
Jet Streak
Same as Jet Max; a point or area ("streak") of relative maximum wind speeds within a jet stream.
Jet Stream
(abbrev. JSTR) Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.
Jet Stream Cirrus
A loose term for filamentous cirrus that appears to radiate from a point in the sky, and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.
JMA
Japanese Meteorological Agency
Jokulhlaup
In hydrologic terms, an Icelandic term meaning glacier dammed lake outburst flood
Juvenile Water
In hydrologic terms, water formed chemically within the earth and brought to the surface in intrusive rock
K AMS
Cold Air Mass
K Corona
In solar-terrestrial terms, of the white-light corona (that is, the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered by electrons in the hot outer atmosphere of the sun.
Katabatic Wind
A wind that is created by air flowing downhill.
Katafront
A front where the warm air descends the frontal surface (except in the low layers of the atmosphere).
Kelvin Temperature Scale
An absolute temperature scale in which a change of 1 Kelvin equals a change of 1 degree Celsius; 0ºK is the lowest temperature on the Kelvin scale. The freezing point of water is +273ºK (Kelvin) and the boiling point of +373ºK. It is used primarily for scientific purposes. It is also known as the Absolute Temperature Scale.
Kelvin Waves
Fluctuations in wind speed at the ocean surface at the Equator result in eastward propagating waves, known as Kelvin Waves. Kelvin Waves cause variations in the depth of the oceanic thermocline, the boundary between warm waters in the upper ocean and cold waters in the deep ocean. They play an important role in monitoring and predicting El Niño episodes.
Kelvin-Helmholtz Waves
Vertical waves in the air associated with wind shear across statically-stable regions. Can appear as breaking waves and as braided patterns in radar images and cloud photos.
Kilopascal
The internationally recognized unit used by the Atmospheric Environment Service for measuring atmospheric pressure. Abbreviated kPa.
La Niña
La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further west than the climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal.
Lag
1) The measure of the time between the center of mass of precipitation to the center of mass of runoff (on the hydrograph); basin lag is a function of not only basin characteristics, but also of storm intensity and movement. Some hydrologic texts define lag from the center of mass of rainfall to the hydrograph peak.
2) The time it takes a flood wave to move downstream.
Lake Breeze
A thermally produced wind blowing during the day from the surface of a large lake to the shore, caused by the difference in the rates of heating of the surfaces of the lake and of the land.
Lake Effect Snow
Snow showers that are created when cold, dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.
Lake Effect Snow Advisory
This product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not because of a low pressure system) may pose a hazard or it is life threatening. The criteria for this advisory varies from area to area.
Lake Effect Snow Squall
A local, intense, narrow band of moderate to heavy snow squall that can extend long distances inland. It may persist for many hours. It may also be accompanied by strong, gusty, surface winds and possibly lightning. Accumulations can be 6 inches or more in 12 hours.
Lake Effect Snow Warning
This product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not because of a synoptic storm or low pressure system) may pose a hazard or it is life threatening.
Lake Effect Storm
A fall or winter storm that produces heavy but localized precipitation as a result of temperature differences between the air over snow-covered ground and the air over the open waters of a lake.
Lake Wind Advisory
This product is issued for sustained wind speeds, generally 20 to 29 mph, lasting for 1 hour or longer for regions which have a significant user community. The need for this product is locally determined.
Lakeshore Flood Advisory
See Coastal/Lakeshore Flood Advisory.
Lakeshore Flood Watch
See: COASTAL/LAKESHORE FLOOD WATCH
Lakeshore Flooding
See COASTAL/LAKESHORE FLOODING
LALs
(L)ightning (A)ctivity (L)evels.
LAL 1 - No thunderstorms.
LAL 2 - Few building cumulus with isolated thunderstorms.
LAL 3 - Much building cumulus with scattered thunderstorms. Light to moderate rain.
LAL 4 - Thunderstorms common. Moderate to heavy rain reaching the ground.
LAL 5 - Numerous thunderstorms. Moderate to heavy rain reaching the ground.
LAL 6 - Dry lightning (same as LAL 3 but without the rain).
Laminar
Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.
Laminar Flow
Streamline flow in which successive flow particles follow similar path lines and head loss varies with velocity to the first power.
Land Breeze
A coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.
Landfall
The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water. Compare direct hit, indirect hit, and strike.
Landspout
[Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.
Lapse Rate
The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.
Large Scale
(Synoptic Scale) Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems.
Last Update
The time and date in which the forecast was issued or updated. The forecast may be updated at any time as weather conditions warrant.
LAT
Latitude- The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.
Latent Heat
Heat absorbed or released during a change of phase at constant temperature and pressure.
Latent Heat Flux
The flux of heat from the earth's surface to the atmosphere that is associated with evaporation or condensation of water vapor at the surface; a component of the surface energy budget.
Latitude
(abbrev. LAT) The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.
LAWEB
(Great Lakes Weather Broadcast) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.
Layer Composite Reflectivity Average
This WSR-88D radar product displays the average reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.
Layer Composite Reflectivity Maximum
This WSR-88D radar product displays the maximum reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). Currently, the low layer extends from the surface to 24,000 feet, the mid layer extends from 24,000 feet to 33,000 feet, and high layer extends above 33,000 feet. It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.
Layered Haze
Haze produced when air pollution from multiple line, area or point sources is transported long distances to form distinguishable layers of discoloration in a stable atmosphere.
LCD (Local Climatological Data)
This National Climatic Data Center (NCDC) publication is produced monthly and annually for some 270 United States cities and it's territories. The LCD summarizes temperature, relative humidity, precipitation, cloudiness, wind speed and direction observation.
Leader
The streamer which initiates the first phase of each stroke of a lightning discharge. The first stroke is led by a steeped leader, which may be preceded by a pilot streamer. All subsequent strokes begin with a dart leader.
Leader Spot
In solar-terrestrial terms, in a magnetically bipolar or multipolar sunspot group, the western part precedes and the main spot in that part is called the leader.
Lee Wave
The wavelike effect, characterized by severe updrafts and downdrafts, that occurs in the lee of a mountain range when rapidly flowing air is lifted up the steep front of a mountain range. Compare mountain wave.
Leeward
The side away from the wind. Compare windward.
Left Front Quadrant
Used interchangably with Left Exit Region; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.
Lenticular Cloud
A very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.
Lifting Condensation Level
(LCL) - The level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.
Lightning Channel
The irregular path through the air along which a lightning discharge occurs. A typical discharge of flash between the ground and the cloud is actually a composite flash which is composed of several sequential lightning strokes, each of which is initiated by a leader and terminated by a return streamer.
Lightning Discharge
The series of electrical processes by which charge is transferred along a channel of high ion density between electrical charge centers of opposite sign. This can be between a cloud and the Earth's surface of a cloud-to-ground discharge.
Limb Flare
In solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.
Line Echo Wave Pattern
(abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.
Liquid Water Equivalent
Same as Water Equivalent; the liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.
Littoral Zone
In hydrologic terms, the area on, or near the shore of a body water
Live Capacity
In hydrologic terms, the total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation storage and exclusive flood control storage.
Loaded Gun (Sounding)
[Slang], a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.
Local Convective Wind
In fire weather terminology, local thermally driven winds arising over a comparatively small area and influenced by local terrain. Examples include sea and land breezes, lake breezes, diurnal mountain wind systems and columnar convective currents.
Lock & Dam (L&D)
Used in navigable rivers, lock and dams allow for large ships to move from one level of water to another.
Longwave Radiation
A term used to describe the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers. Compare shortwave radiation.
Longwave Trough
A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration.
LORAN
Long Range Navigation, a system of long range navigation whereby latitude and longitude are determined from the time displacement of radio signals from two or more fixed transmitters.
Low Water Advisory
An advisory to describe water levels which are significantly below average levels over the Great Lakes, coastal marine zones, and any tidal marine area, waterway, or river inlet within or adjacent to a marine zone that would potentially be impacted by low water conditions creating a hazard to navigation.
Lowland Flooding
In hydrologic terms, inundation of low areas near the river, often rural, but may also occur in urban areas.
Mackeral Sky
The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales.
Macroburst
A convective downdraft with an affected outflow area of at least 2½ miles wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage of up to F3 intensity.
Macroscale
Large scale, characteristic of weather systems several hundred to several thousand kilometers in diameter.
Madden-Julian Oscillation
(abbrev. MJO)- Tropical rainfall exhibits strong variability on time scales shorter than the seasonal El Niño-Southern Oscillation (ENSO). These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as the Madden-Julian Oscillation or intraseasonal oscillations. The intraseasonal oscillations are a naturally occurring component of our coupled ocean-atmosphere system. They significantly affect the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affect the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, they have an important impact on storminess and temperatures over the United States. During the summer these oscillations have a modulating effect on hurricane activity in both the Pacific and Atlantic basins.
MAFOR
(Great Lakes Marine Forecast) - A National Weather Service coded summary appended to each of the Great Lakes Open Lakes forecasts.
Magflare
In solar-terrestrial terms, a geomagnetic and/or cosmic storm has been associated with this flare
Magnetic Bay
In solar-terrestrial terms, a relatively smooth excursion of the H (horizontal) component of the geomagnetic field away from and returning to quiet levels.
Magnetogram
In solar-terrestrial terms, solar magnetograms are a graphic representation of solar magnetic field strengths and polarity
Magnetopause
In solar-terrestrial terms, the boundary layer between the solar wind and the magnetosphere.
Magnetosphere
In solar-terrestrial terms, the magnetic cavity surrounding the earth, carved out of the passing solar wind by virtue of the geomagnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity
Main Stem
In hydrologic terms, the reach of a river/stream formed by the tributaries that flow into it.
Main Synoptic Times
The times of 0000, 0600, 1200, and 1800 UTC. Also known as the standard synoptic times.
MAINT
Maintain
Major Flare
In solar-terrestrial terms, this flare is the basis for the forecast of geomagstorm, cosmic storm and/or protons in the earth's vicinity
Major Flooding
A general term including extensive inundation and property damage. (Usually characterized by the evacuation of people and livestock and the closure of both primary and secondary roads.)
Major Hurricane
A hurricane which reaches Category 3 (sustained winds greater than 110 mph) on the Saffir/Simpson Hurricane Scale.
MALR
Moist Adiabatic Lapse Rate- The rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.
Mammatus Clouds
Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well.
MAP
Mean Areal Precipitation- The average rainfall over a given area, generally expressed as an average depth over the area.
MARC Velocity Signature
A Doppler radar-velocity based precursor towards forecasting the initial onset of damaging straight-line winds in a linear Quasi_Linear Convective System (QLCS) or bowing convective system.
Mare's Trail
The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.
MAREP
(MArine REPort) A voluntary marine observation program of the National Weather Service whose goal is to solicit meteorological and oceanographic observations in plain language from recreational and small commercial mariners who are not part of Voluntary Observing Ship program.
Marginal Visual Flight Rules
(Abbrev. MVFR) - In an aviation product, refers to the general weather conditions pilots can expect at the surface. VFR stands for Visual Flight Rules and MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility.
Marine Inversion
Temperature inversion produced when cold marine air underlies warmer air.
Marine Push
A replacement of the current air mass with air from off the ocean. Temperatures are much cooler and relative humidities much higher. The air mass is generally much more stable in this situation.
Marine Small Craft Thunderstorm Advisory
A marine warning issued by Environment Canada Atmospheric Environment Branch when the possibility of thunderstorms is greater than 40 percent.
Marine Small Craft Wind Warning
A marine warning issued by Environment Canada Atmospheric Environment Branch for winds which are forecasted to be in the 20-33 knot range inclusive.
Marine Weather Statement
A National Weather Service product to provide mariners with details on significant or potentially hazardous conditions not otherwise covered in existing marine warnings and forecasts. Marine weather statements are also used to supplement special marine warnings.
Marine Zone
Specific, defined over-water areas contained in the various NWS marine forecasts. These are the equivalent of "zones" in the public forecast program.
Maritime Air Mass
An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.
Maritime Polar Air Mass
An air mass characterized by cold, moist air. Abbreviated mP.
Maritime Tropical Air Mass
An air mass characterized by warm, moist air. Abbreviated mT.
MAROB
A voluntary marine observation program of the National Weather Service in the early stages of development whose goal is to solicit meteorological and oceanographic observations in coded format from recreational and small commercial mariners who are not part of the more in-depth Voluntary Observing Ship program.
MARS
A voluntary marine observation program of the National Weather Service whereby U.S. Coast Guard Sector Stations report marine weather conditions from several shore locations within their operating area. The reports are in an abbreviated plain language format with fixed fields.
Massif
A compact portion of a mountain range, containing one or more summits.
MAV
AVN MOS Guidance
MAX
Maximum
Max Parcel Level (MPL)
This signifies the highest attainable level that a convective updraft can reach; therefore, it is a good indication of how tall a thunderstorm may reach.
Maximum Spillway Discharge
In hydrologic terms, spillway discharge (cfs) when reservoir is at maximum designed water surface elevation.
Maximum Sustained Surface Wind
When applied to a particular weather system, refers to the highest one-minute average wind (at an elevation of 10 meters with an unobstructed exposure) associated with that weather system at a particular point in time.
Maximum Temperature
This is the highest temperature recorded during a specified period of time. Common time periods include 6, 12 and 24 hours. The most common reference is to the daily maximum temperature, or "high."
Maximum Unambiguous Range
The range from the radar at which an echo can be known unquestionably as being at that range. As the radar sends out a pulse of energy, the pulse hits a target and part of the energy bounces back to the radar, but part of the energy may continue to travel away from the radar. The distance to the target is computed by knowing the time that has elapsed since the pulse was emitted. Then a second pulse of energy is transmitted. If some of the energy from the first pulse strikes a target at a far range and returns to the radar when radiation from the second pulse arrives, the RDA misinterprets the returned first pulse as arriving from a target near the returned second pulse. The maximum unambiguous range is related to the amount of time that elapses between successive pulses of emitted energy.
Maximum Unambiguous Velocity
The highest radial velocity that can be measured unambiguously by a pulsed Doppler radar. The maximum unambiguous velocity is related to the radar's successive pulses of emitted energy. When a target's velocity exceeds the maximum unambiguous velocity, the velocity will be "folded" to appear as a different velocity.
MAXT
High temperature for the day. Usually forecast out 7 days in advance.
Mean
The arithmetic average of a set of data (numbers), or the middle point between its two extremes.
Mean Annual Temperature
The average temperature for the entire year at any given location.
Mean Areal Precipitation
(abbrev. MAP)- The average rainfall over a given area, generally expressed as an average depth over the area.
Mean Daily Temperature
The average of the highest and lowest temperatures during a 24-hour period.
Mean Depth
In hydrologic terms, the average depth of water in a stream channel or conduit. It is equal to the cross-sectional area divided by the surface width.
Mean Doppler Velocity
Reflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive radar pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity refers to spectral density first moment, radial velocity to base data.
Mean Low Water
(MLW) - A tidal datum. The average of all the low water heights observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch.
Mean Lower Low Water
(MLLW) - A tidal datum. The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, simultaneous observational comparisons are made with a control tide station in order to derive the equivalent datum of the National Tidal Datum Epoch.
Mean Sea Level
(MSL) - The arithmetic mean of hourly water elevations observed over a specific 19-year tidal epoch.
Meander
In hydrologic terms, the winding of a stream channel
Meander Belt
In hydrologic terms, the area between lines drawn tangential to the extreme limits of fully developed meanders
Measured Ceiling
A ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. See variable ceiling.
Medium Range
In forecasting, (generally) three to seven days in advance.
Medium Range Forecast (MRF)
A configuration of the National Water Model (NWM) that runs every 6 hours and produces 3-hourly deterministic forecasts of streamflow and hydrologic states for the contiguous United States (ConUS). This configuration is an ensemble forecast with 7 members; member 1 extends out to 10 days, while members 2-7 extend out to 8.5 days. Meteorological forcing data are drawn from the GFS.
Mercury Barometer
An instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the pressure of the atmosphere increases or decreases.
Meridian
An imaginary line on the earth's surface passing through both geographic poles and through any given point on the planet, also called a line of longitude.
Meridional Flow
Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.
Mesoclimate
The climate of a small area of the earth's surface which may differ from the general climate of the district.
Mesopause
The top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.
Mesoscale
Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems
Mesoscale Convective Complex
(abbrev. MCC)- MCC - Mesoscale Convective Complex. A large Mesoscale Convective System (MCS), generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs: * Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. * Duration: Size criteria must be met for at least 6 hours. * Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.
Mesoscale Convective System
(MCS): A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.
Mesoscale Discussion
When conditions actually begin to shape up for severe weather, SPC (Storm Prediction Center) often issues a Mesoscale Discussion (MCD) statement anywhere from roughly half an hour to several hours before issuing a weather watch. SPC also puts out MCDs for hazardous winter weather events on the mesoscale, such as locally heavy snow, blizzards and freezing rain (see below). MCDs are also issued on occasion for heavy rainfall, convective trends, and other phenomena, when the forecaster feels he/she can provide useful information that is not readily available or apparent to field forecasters. MCDs are based on mesoscale analysis and interpretation of observations and of short term, high resolution numerical model output.

The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs can help you get a little extra lead time on the weather and allow you to begin gearing up operations before a watch is issued. The MCD begins with a numerical string that gives the LAT/LON coordinates of a polygon that loosely describes the area being discussed.
Mesoscale High Winds
These high winds usually follow the passage of organized convective systems and are associated with wake depressions or strong mesohighs.
METAR
An international code (Aviation Routine Weather Report) used for reporting, recording and transmitting weather observations.
Meteogram
A graphical depiction of trends in meteorological variables such as temperature, dew point, wind speed and direction, pressure, etc. The time series meteogram can be constructed using observed data or forecast data.
Meteoric Water
Water derived from precipitation.
Meteorological Model Ensemble River Forecast (MMEFS)
An automated short-term hydrologic ensemble forecast system which utilizes temperature and precipitation output from the National Centers for Environmental Prediction (NCEP) GEFS and NAEFS meteorological models as inputs to River Forecast Center hydrologic models.
Microbarograph
A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.
Microclimate
The climate of a small area such as a cave, house, city or valley that may be different from that in the general region.
Microscale
Pertaining to meteorological phenomena, such as wind circulations or cloud patterns, that are less than 2 km in horizontal extent.
Microwave
A type of electromagnetic radiation with wavelengths between those of infrared radiation and radio waves.
Microwave Burst
In solar-terrestrial terms, a radiowave signal associated with optical and/or X-ray flares
Mid-Flame Wind
Wind measured at the midpoint of the flames, considered to be most representative of the speed of the wind that is affecting fire behavior.
Mid-Latitude Areas
Areas between 30o and 60o north and south of the Equator.
Middle Latitudes
1) The latitude belt roughly between 35 and 65 degrees North and South. Also referred to as the temperate region.
or
2) With specific reference to zones of geomagnetic activity, "middle latitudes" refers to 20º to 50º geomagnetic
Mie Scattering
Any scattering produced by spherical particles whose diameters are greater than 1/10 the wavelength of the scattered radiation. This type of scattering causes the clouds to appear white in the sky. Often, hail exhibits in this type of scattering.
Millibar
A unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.
Minimum Discernible Signal
In a receiver, it is the smallest input signal that will a produce a detectable signal at the output. In radar terms, it is the minimal amount of back scattered energy that is required to produce a target on the radar screen. In other words, MDS is a measure of the radar's sensitivity.
Minimum Temperature
This is the lowest temperature recorded during a specified period of time. The time period can be 6, 12 or 24 hours. The most common reference is to the daily minimum temperature, or "low."
Minor Tidal Overflow
Minor flooding caused by high tides that results in little if any damage.
Misoscale
The scale of meteorological phenomena that ranges in size from 40 meters to about 4 kilometers. It includes rotation within a thunderstorm.
Mixed Layer
An atmospheric layer, usually the layer immediately above the ground, in which pollutants are well mixed by convective or shear-produced turbulence.
Mixed Precipitation
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
Mixing Ratio
The ratio of the weight of water vapor in a specified volume (such as an air parcel) to the weight of dry air in that same volume.
MLCAPE
Mean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level. See Convective Available Potential Energy (CAPE).
Model Output Statistics
(abbrev. MOS) - the Hydrometeorological Center (HPC) produces a short range (6 to 60 hours) MOS (Model Output Statistics) guidance package generated from the NGM, GFS, and ETA models for over 300 individual stations in the continental United States. These alphanumeric messages are made available at approximately 0400 and 1600 UTC for the 0000 and 1200 UTC forecast cycles, respectively. Model Output Statistics are a set of statistical equations that use model output to forecast the probability of precipitation, high and low temperature, cloud cover, and precipitation amount for many cities across the USA. The statistical equations were specifically tailored for each location, taking into account factors such as each location's climate.
Moderate Flooding
The inundation of secondary roads; transfer to higher elevation necessary to save property -- some evacuation may be required.
Moderate Risk (of severe thunderstorms)
Severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See high risk, slight risk, convective outlook.
Moist Adiabat
The line on a Skew T-Log P chart that depicts the change in temperature of saturated air as it rises and undergoes cooling due to adiabatic expansion. As saturated air rises, the temperature changes at a rate of 0.55 degrees Celsius per 100 meters (2-3 degrees Fahrenheit per 1,000 feet). Contrast with a dry adiabat.
Moist Adiabatic Lapse Rate
(abbrev. MALR)- The rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.
Moist-adiabatic
(Also known as saturation-adiabatic process.) An adiabatic process for which the air is saturated and may contain liquid water. A distinction is made between the reversible process, in which total water is conserved, and the pseudoadiabatic or irreversible moist adiabatic process, in which liquid water is assumed to be removed as soon as it is condensed.
Moisture Advection
Transport of moisture by horizontal winds.
Moisture Equivalent
In hydrologic terms, the ratio of the weight of water which the soil, after saturation, will retain against a centrifugal force 1,000 times the force of gravity, to the weight of the soil when dry. The ratio is stated as a percentage.
Monitor Stage
The stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. (Used if different from action stage, and at the discretion of the WFO or river forecast center [RFC].) The term "alert stage" is to be used instead of warning stage. Monitor stage or caution stage may be used instead of alert stage in some parts of the country. see/ alert stage/.
Monostatic Radar
A radar that uses a common antenna for both transmitting and receiving.
Monthly Climatological Report
This climatological product is issued once a month by each National Weather Service office. It is a mix of tabular and narrative information. It is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, heating/cooling degree information, etc.).
Mostly Clear
When the 1/8th to 2/8ths of the sky is covered by with opaque (not transparent) clouds. Sometimes referred to as Mostly Sunny if this condition is present during daylight hours.
Mount Wilson Magnetic Classifications
In solar-terrestrial terms, a classification system for sunspots:
Mountain Wave
The wavelike effect, characterized by updrafts and downdrafts, that occurs above and behind a mountain range when rapidly flowing air encounters the mountain range's steep front.
Mountain Wind System
The system of diurnal winds that forms in a complex terrain area, consisting of mountain-plain, along-valley, cross-valley and slope wind systems.
Mountain-Plain Wind System
A closed, large-scale, thermally driven circulation between the mountains and the surrounding plain. The mountain-to-plain flow making up the lower branch of the closed circulation usually occurs during nighttime, while the plain-to-mountain flow occurs during daytime.
Mountainado
A vertical-axis eddy produced in a downslope windstorm by the vertical stretching of horizontal roll vortices produced near the ground by vertical wind shear. Mountainadoes, when carried by the mean wind, can produce strong horizontal shears and wind gusts that are much more damaging than the general prevailing wind speeds.
Movable Bed
In hydrologic terms, a stream bed made up of materials readily transportable by the stream flow
Multi-Radar Multi-Sensor (MRMS)
An automated system that rapidly and intelligently integrates data from multiple radars and radar networks, surface observations, numerical weather prediction (NWP) models, and climatology to generate seamless, high spatio-temporal resolution mosaics.
Multiple Doppler Analysis
The use of more than one radar (and hence more than one look angle) to reconstruct spatial distributions of the 2D or 3D wind field, which cannot be measured from a single radar alone. Includes dual Doppler, triple Doppler, and overdetermined multiple Doppler analysis.
Multiple Vortex Tornado
A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.
Municipal Use of Water
In hydrologic terms, the various uses to which water is put to use developed urban areas, including domestic use, industrial use, street sprinkling, fire protection, etc.
Nacreous Clouds
Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the Earth's surface. They are also called "mother-of-the-pearl clouds."
Nadir
The point on any given observer's celestial sphere diametrically opposite of one's zenith.
NAM
The operational North American Meso (NAM, formerly Eta) is run four times per day (00,06,12,18Z), all cycles run to 84-h.
Nanotesla (nT)
A unit of magnetism equal to 10-9 tesla, equivalent to a gamma (10-5 gauss).
NAO
North Atlantic Oscillation - the NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa.
NAO Index
This index measures the anomalies in sea level pressure between the Icelandic low pressure system and the Azores high pressure system in the North Atlantic Ocean.

When the NAO is in its is positive phase (+NAO), the northeastern United States sees an increase in temperature and a decrease in snow days; the central US has increased precipitation, the North Sea has an increase in storms; and Norway along with Northern Europe has warmer temperatures and increased precipitation.

When the NAO is in its negative phase (-NAO), the Tropical Atlantic and Gulf coast have increased number of strong hurricanes; northern Europe is drier, and Turkey along with other Mediterranean countries has increased precipitation.
National Ambient Air Quality Standards
In the United States, national standards for the ambient concentrations in air of different air pollutants designed to protect human health and welfare.
National Blend of Models (NBM)
The National Blend of Models (NBM) is a nationally consistent and skillful suite of calibrated forecast guidance based on a blend of both NWS and non-NWS numerical weather prediction model data and post-processed model guidance.
National Climatic Data Center
The agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations.
National Digital Forecast Database
(NDFD)- The National Weather Service's NDFD provides access to gridded forecasts of sensible weather elements (e.g., wind, wave height) through the National Digital Forecast Database (NDFD). NDFD contains a seamless mosaic of digital forecasts from NWS field offices working in collaboration with the National Centers for Environmental Prediction (NCEP). The database is made available to all customers and partners from the public, private and academic sectors. Those customers and partners may use this data to create a wide range of text, graphic, gridded and image products of their own.
National Fire Danger Rating System
A uniform fire danger rating system used in the United States that focuses on the environmental factors that impact the moisture content of fuels. Fire danger is rated daily over large administrative areas, such as national forests.
National Flood Summary
This NWS daily product (abbreviated FLN) contains nationwide information on current flood conditions. It is issued by the Hydrometeorological Information Center of the Office of Hydrology.
National Hurricane Center
One of three branches of the Tropical Prediction Center (TPC). This center maintains a continuous watch on tropical cyclones over the Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific from 15 May through November 30. The Center prepares and distributes hurricane watches and warnings for the general public, and also prepares and distributes marine and military advisories for other users. During the "off-season" NHC provides training for U.S. emergency managers and representatives from many other countries that are affected by tropical cyclones. NHC also conducts applied research to evaluate and improve hurricane forecasting techniques, and is involved in public awareness programs.
National Hurricane Operations Plan
(NHOP) - The NHOP is issued annually by the Federal Coordinator for Meteorological Services and Supporting Research. It documents interdepartmental agreements relating to tropical cyclone observing, warning, and forecasting services. National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and the JTWC serve as the principal offices in coordinating the day-to-day activities of the NWS in support of the Plan in their region of responsibility.
National Hydrologic Discussion (NHD)
A discussion of the current and forecast hydrologic conditions across the nation, including a variety of short and medium range (Days 1-10) observed and modeled hydrologic guidance.
National Severe Storms Laboratory
This is one of NOAA's internationally known Environmental Research Laboratories, leading the way in investigations of all aspects of severe weather. Headquartered in Norman OK with staff in Colorado, Nevada, Washington, Utah, and Wisconsin, the people of NSSL, in partnership with the National Weather Service, are dedicated to improving severe weather warnings and forecasts in order to save lives and reduce property damage.
National Water Model (NWM)
A hydrologic modelling framework that simulates observed and forecast streamflow over the entire continental United States and Hawaii.
National Water Model Medium-Range Forecast (NWM MRF)
A 10-day streamflow forecast for the over 3.6 million waterway miles across the Nation, forced by the GFS and updated every 6 hours.
National Water Model Short-Range Forecast (NWM SRF)
An 18-hour streamflow forecast for the over 3.4 million waterway miles across the Nation, forced by the HRRR and updated hourly.
National Weather and Crop Summary
A product of the National Agricultural Statistics Service, Agricultural Statistics Board, and U.S. Department of Agriculture. It contains weekly national agricultural weather summaries, including the weather's effect on crops; summaries and farm progress for 44 states and New England area.
Natural Control
In hydrologic terms, a stream gaging control which is natural to the stream channel, in contrast to an artificial control structure by man.
Nautical Dawn
The time at which the sun is 12 degrees below the horizon in the morning. Nautical dawn is defined as that time at which there is just enough sunlight for objects to be distiguishable.
Nautical Dusk
The time at which the sun is 12 degrees below the horizon in the evening. At this time, objects are no longer distinguishable.
Nautical Mile
A unit of distance used in marine navigation and marine forecasts. It is equal to 1.15 statue miles or 1852 meters. It is also the length of 1 minute of latitude.
Nautical Twilight
The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars.
Navigation Methods
In hydrologic terms, there are three basic methods of providing and managing inland waterways -
1) Run-of-the-River: no provision of upstream storage;
2) Slack-Water: locks and dams provide slack water or pools with adequate depth for the draft of heavy barges and area to prevent excessive velocities;
3) Canalization: in lieu of a series of dams on the river a canal with locks adjoins the river.
NAVTEX
An international automated medium frequency (518 kHz) direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 200 nautical miles of shore and is an element of the Global Maritime Distress and Safety System (GMDSS). NAVTEX stations in the U.S. are operated by the U.S. Coast Guard. There are no user fees associated with receiving NAVTEX broadcasts. The International System uses the frequency of 518 kHz worldwide and optional National Systems use frequencies of 490 and 4205.5 kHz. Safety messages on 518 kHz have to be in the English language worldwide but broadcasts on the 490 and 4209.5 kHz frequency can be in the local language.
NAVTEX Forecast
(NAV) - A National Weather Service marine forecast combining various Coastal Waters and Offshore forecasts, optimized to accommodate transmission via NAVTEX.
NCAR
National Center for Atmospheric Research
Neap Tide
A minimum tide occurring at the first and third quarters of the moon.
Nearshore Forecast
(NSH) - National Weather Service seasonal marine forecasts for an areas of the Great Lakes extending from a line approximating mean low water datum along the coast or an island, including bays, harbors, and sounds, out to 5 nm. These forecasts are normally issued from Daylight Savings Time ~April 7 through December 31, though the dates may be shortened or extended based on local/regional requirements.
Negative Vorticity Advection
(Abbrev. NVA) - the advection of lower values of vorticity into an area.
Negative-tilt Trough
An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.
Net All-Wave Radiation
The net or resultant value of the upward and downward longwave and shortwave radiative fluxes through a plane at the earth-atmosphere interface; a component of the surface energy budget.
Net Rainfall
In hydrologic terms, the portion of rainfall which reaches a stream channel or the concentration point as direct surface flow.
Neutral Line
The line that separates longitudinal magnetic fields of opposite polarity.
Neutral Stability
An atmospheric condition that exists in unsaturated air when the environmental lapse rate equals the dry adiabatic rate, or in saturated air when the environmental lapse rate equals the moist adiabatic rate.
NEXRAD
NEXt Generation RADar. A NWS network of about 140 Doppler radars operating nationwide.
Nimbostratus
(abbrev. NS)- A cloud of the class characterized by a formless layer that is almost uniformly dark gray; a rain cloud of the layer type, of low altitude, usually below 8000 ft (2400 m).
NOAA
National Oceanic and Atmospheric Administration
NOAA Weather Radio
"The voice of the National Weather Service" - NOAA Weather Radio broadcasts National Weather Service warnings, watches, forecasts and other hazard information 24 hours a day. It is provided as a public service by NOAA. The NOAA Weather Radio network has more than 480 stations in the 50 states and near adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands and U.S. Pacific Territories.
NOAA Weather Wire
Mass dissemination via satellite of National Weather Service products to the media and public.
Nocturnal
Related to nighttime; occurring at night.
Nocturnal Inversion
Used interchangably with Radiational Inversion; a temperature inversion that develops during the night as a result of radiational cooling of the surface. Because the immediate surface (lower Boundary Layer) cools much more rapidly during radiational cooling conditions than the air just above (upper Boundary Layer), a temperature inversion can be created overnight, but typically erodes quickly after sunrise.
Nocturnal Jet
This wind speed maximum occurs just above the nocturnal inversion at night. It is typically found in the south central United States during the late spring and summer months. It is important in the development of Mesoscale Convective Complexes (MCC) or Mesoscale Convective Systems (MCS).
Nocturnal Thunderstorms
Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.
NOGAPS
Navy Operational Global Atmospheric Prediction System; a 144-hour numerical model of the atmosphere run by the U.S. Navy twice daily.
Nonattainment Area
An area that does not meet ambient air quality standards.
Normal
The long-term average value of a meteorological paramater (i.e., temperature, humidity, etc.) for a certain area. For example, "temperatures are normal for this time of year" means that temperatures are at or near the average climatological value for the given date. Normals are usually taken from data averaged over a 30-year period (e.g., 1971-2000 average), and are concerned with the distribution of data within limits of common occurrence.
Normal Water Surface Elevation
In hydrologic terms, the lowest crest level of overflow on a reservoir with a fixed overflow level (spillway crest elevation). For a reservoir whose outflow is controlled wholly or partly by movable gates, siphons, or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.
Normal Year
A year during which the precipitation or stream flow approximates the average for a long period of record.
North American Ensemble Forecast System (NAEFS)
An atmospheric ensemble of 20 members each from the NCEP GEFS and CMC EPS ensemble systems.
North American Mesoscale Forecast System (NAM)
One of the major weather models run by the National Centers for Environmental Prediction (NCEP) for producing weather forecasts.
North Atlantic Oscillation
(Abbrev. NAO) - the NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland and is quantified in the NAO Index. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa.
North Pacific High
A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Comparable systems are the Azores High and the Bermuda High.
North Wall
The north side boundary of the Gulf Stream generally extending northeast from Cape Hatteras where the Gulf Stream turns northeast.
Nor\'easter
A strong low pressure system that affects the Mid Atlantic and New England States. It can form over land or over the coastal waters. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.
Nowcast
A short-term weather forecast, generally out to six hours or less. This is also called a Short Term Forecast.
Numerical Forecasting
A computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time.
Numerical Weather Prediction
Same as Numerical Forecasting; a computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time.
NVA
Negative Vorticity Advection - the advection of lower values of vorticity into an area.
Obscuration
Any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Obscuring Phenomena
Any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Observation Well
In hydrologic terms, a non-pumping well used for observing the elevation of the water table or piezometric surface
Oceanography
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.
Office of Global Programs
The Office of Global Programs (OGP) sponsors focused scientific research, within approximately eleven research elements, aimed at understanding climate variability and its predictability. Through studies in these areas, researchers coordinate activities that jointly contribute to improved predictions and assessments of climate variability over a continuum of timescales from season to season, year to year, and over the course of a decade and beyond.
Offshore Waters
That portion of the oceans, gulfs, and seas beyond the coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by specific latitude and longitude points.
Offshore Waters Forecast
(OFF) - A National Weather Service marine forecast product for that portion of the oceans, gulfs, and seas beyond the coastal waters extending to a specified distance from the coastline, to a specified depth contour, or covering an area defined by specific latitude and longitude points.
Okta
Used for the measurement of total cloud cover. One okta of cloud cover is the equivalent of 1/8 of the sky covered with cloud.
Omega
A term used to describe vertical motion in the atmosphere. The "omega equation" used in numerical weather models is composed of two terms, the "differential vorticity advection" term and the "thickness advection" term. Put more simply, omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere. On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion (UVV) in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.
Omega High
A warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts. It is an example of a blocking high.
Opaque
A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.
Open Lakes Forecast
(GLF) - A National Weather Service marine forecast product for the U.S. waters within a Great Lake not including the waters covered by an existing Nearshore Waters Forecast (NSH). When the seasonal Nearshore forecast is not issued, the Open Lake forecast includes a forecast of nearshore waters.
Operational Products
A product that has been fully tested and evaluated and is produced on a regular and ongoing basis.
Orographic
Related to, or caused by, physical geography (such as mountains or sloping terrain).
Orographic Lifting
Same as Upslope Flow; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.
Orographic Precipitation
Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.
Orographic Uplift
Same as Orographic Lifting; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.
Orographic Waves
A wavelike airflow produced over and in the lee of a mountain barrier.
Orphan Anvil
Slang for an anvil from a dissipated thunderstorm, below which no other clouds remain.
Oscillation
A shift in position of various high and low pressure systems that in climate terms is usually defined as an index (i.e., a single numerically-derived number, that represents the distribution of temperature and pressure over a wide ocean area, such as the El Niño-Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation).
Outer Convective Band
Bands in a hurricane that occur in advance of main rain shield and up to 300 miles from the eye of the hurricane. The typical hurricane has two or three bands (and sometimes more) which are comprised of cells resembling ordinary thunderstorms. Wind gusts are usually higher in these bands than in the Pre-Hurricane Squall Line.
Outflow Boundary
A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.

New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).
Outflow Channel
In hydrologic terms, a natural stream channel which transports reservoir releases.
Outgoing Longwave Radiation
Outgoing Longwave Radiation is a polar satellite derived measurement of the radiative character of energy radiated from the warmer earth surface to cooler space. This measurement provides information on cloud-top temperature which can be used to estimate tropical precipitation amounts which is important in forecasting weather and climate.
Outlet Discharge Structure
In hydrologic terms, protects the downstream end of the outlet pipe from erosion and is often designed to slow down the velocity of released water to prevent erosion of the stream channel
Overcast
(Abbrev. OVC)- An official sky cover classification for aviation weather observations, when the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.
Overhang
Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).
Overland Flow
In hydrologic terms, the flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse it becomes runoff.
Ozone Action Day
A "heads-up" message issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels may reach dangerous levels the next day. This message encourages residents to prevent air pollution by postponing the use of lawn mowing, motor vehicles, boats, as well as filling their vehicle gas tanks.
Ozone Layer
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.
PAC
Pacific
Pacific Decadal Oscillation
(Abbrev. PDO) - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands.
Palmer Drought Severity Index
(Abbrev. PDSI) - an index used to gage the severity of drought conditions by using a water balance equation to track water supply and demand. This index is calculated weekly by the National Weather Service.
Pan Pan
A headline within National Weather Service high seas forecasts transmitted via the GMDSS to indicate that a hurricane or hurricane force winds are forecast.
Pancake Ice
In hydrologic terms, circular flat pieces of ice with a raised rim; the shape and rim are due to repeated collisions
Panhandle Hook
Low pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track.
Parameter
A subset of the group of evaluations that constitute each element of an observation.
Parapet Wall
In hydrologic terms, a solid wall built along the top of the dam for ornament, safety, or to prevent overtopping
Parcel
A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.
Parhelion
The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns.
Partial Beam Filling
A limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.
Partial-Duration Flood Series
In hydrologic terms, a list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year.
Particle Trajectory Model
A computer sub-model that tracks the trajectories of multiple particles that are released into an atmospheric flow model.
Partly Cloudy
Between 3/8 and 5/8 of the sky is covered by clouds.
Partly Sunny
Between 3/8 and 5/8 of the sky is covered by clouds. The term "Partly Sunny" is used only during daylight hours.
Pascal
The unit of pressure produced when one newton acts on one square meter (1 N/m2). It is abbreviated Pa.
PAT
Pattern
PDS Watch
The Particularly Dangerous Situation (PDS) wording is used in rare situations when long-lived, strong and violent tornadoes are possible. This enhanced wording may also accompany severe thunderstorm watches for intense convective wind storms.
Peak Discharge
In hydrologic terms, the rate of discharge of a volume of water passing a given location
Peak Flow Arrival Time (PFAT)
The time at which a stream reach is forecast to achieve its maximum flow within the forecast period.
Peak Gust
The highest instantaneous wind speed observed or recorded.
Peak Pulse
The amount of power transmitted by a radar during a given pulse. Note that because these pulses are widely spaced, the average power will be much smaller.
Peak Wind Speed
The maximum instantaneous wind speed since the last observation that exceeded 25 knots.
Pendant Echo
Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.
Penetrating Top
Same as Overshooting Top; a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm.
Penumbra
In solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae. It consists of linear bright and dark elements radial from the sunspot umbra.
Perched Groundwater
In hydrologic terms, local saturated zones above the water table which exist above an impervious layer of limited extent.
Percolation
In hydrologic terms, the movement of water, under hydrostatic pressure, through the interstices of a rock or soil, except the movement through large openings such as caves
Percolation Path
In hydrologic terms, the course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a permeable foundation.
Perennial Stream
In hydrologic terms, a stream that flows all year round.
Permafrost
A layer of soil at varying depths below the surface in which the temperature has remained below freezing continuously from a few to several thousands of years.
Permeability
In hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.
Permeability Coefficient
In hydrologic terms, the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
Permeameter
In hydrologic terms, a laboratory instrument for determining permeability by measuring the discharge through a sample of the material when a known hydraulic head is applied.
Persistence Forecast
A forecast that the current weather condition will persist and that future weather will be the same as the present (e.g., if it is raining today, a forecast predicting rain tonight).
Perturbation Model
A computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of equations that describe atmospheric motions.
Phenomenological Model
A computer model used to calculate air pollution concentrations. A phenomenological model focuses on an individual phenomenon, such as plume impingement or fumigation.
Photochemical Smog
Air pollution containing ozone and other reactive chemical compounds formed by the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight.
Phreatic water
In hydrologic terms, water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table. Also termed Groundwater.
PIBAL
Pilot balloon. A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Pilot Balloon
(Abbrev. PIBAL)- A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Plage
In solar-terrestrial terms, an extended emission feature of an active region that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background.
Plage Corridor
In solar-terrestrial terms, a space in chromospheric plage lacking plage intensity, coinciding with polarity inversion line.
Plagenil
In solar-terrestrial terms, spotless disc free of calcium plage.
Plan Position Indicator
An acronym for Plan Position Indicator. A PPI displays radar data horizontally using a map projection. In PPI mode, the radar makes a 360-degree sweep with the antenna at a specific elevation angle. A PPI display is the familiar radar display shown on the television weather programs.
Planetary Boundary Layer
The layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.
Plasma
Any ionized gas; that is, any gas containing ions and electrons.
Platform
A generic radar term, often used to encompass the pedestal and antenna assembly; sometimes including the radar control, display and analysis hardware and software as well.
Plume-dominated Fire
A fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the general wind.
Pluvial
In hydrology, anything that is brought about directly by precipitation.
PNA
Pacific North American teleconnection
Point Precipitation
Precipitation at a particular site, in contrast to the mean precipitation over an area.
Polar Cap Absorption (PCA)
In solar-terrestrial terms, an anomalous condition of the polar ionosphere whereby HF and VHF (3 - 300 MHz) radiowaves are absorbed, and LF and VLF (3 - 300 kHz) radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.
Polar Front
A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.
Polar Jet
Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.
Polar Jet Stream
Used interchangably with Polar Jet; a jet stream marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.
Polar Orbiting Satellite
A weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 km) above the Earth's surface. A satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit.
Polarization Radar
A radar which takes advantage of ways in which the transmitted waves' polarization affect the backscattering. Such radars may alternately transmit horizontal and vertically polarized beams, and measure differential reflectivity.
Pollutant
Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.
Pondage
In hydrologic terms,
(1) The holding back of water for later release for power development above the dam of a hydroelectric plant to
(a) equalize daily or weekly fluctuations of streamflow or
(b) to permit irregular hourly use of water by the wheels to care for fluctuations in the load demand.
(2) In general the holding back of water for later releases.
(3) The storage capacity available for the use of such water.
Positive Area
The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. Positive area is a measure of the energy available for convection; see CAPE.
Positive Vorticity Advection
(Abbrev. PVA) - Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.
Post-Flare Loops
In solar-terrestrial terms, a loop prominence system often seen after a major two-ribbon flare, which bridges the ribbons.
Potential Temperature
The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.
Potential Vorticity
This plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.
Pre-Frontal Squall Line
A line of thunderstorms that precedes an advancing cold front.
Pre-Frontal Trough
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.
Pre-Hurricane Squall Line
It is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front. It is as much as 50 miles or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles ahead of the eye, but it has been observed to be as much as 500 miles ahead of the eye in the largest hurricanes.
Precipitable Water
Measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
Precipitation
The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.
Precipitation Attenuation
The loss of energy that radar beam experiences as it passes through an area of precipitation.
Precipitation Mode
The standard, or default, operational mode of the WSR-88D. The radar automatically switches into precipitation mode from clear-air mode if the measured reflectivity exceeds a specific threshold value. The precipitation mode of NEXRAD is more sensitive than previous weather radars. The minimum detectable reflectivity in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.
Precipitation Processing System
The WSR-88D system that generates 1-hour running, 3-hourly, and running storm total precipitation accumulations. Five functional steps are performed to calculate the best estimate of precipitation: 1) development of a sectorized hybrid scan, 2) conversion to precipitation rate, 3) precipitation accumulation, 4) adjustment using rain gages, 5) product update.
Predominant Wind
The wind that prevails and generates the local component of the significant sea conditions across the forecast area. This is the wind included in all marine forecast products and is defined as a 10-meter wind, except over the nearshore marine zones where it is defined to be the wind at a 3-meter height.
Preliminary Report
Now known as the "Tropical Cyclone Report". A report summarizing the life history and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on forecasts and warnings associated with the cyclone. NHC writes a report on every tropical cyclone in its area of responsibility.
Present Weather
The type of weather observed at the reporting time. These conditions may include types and intensity of precipitation such as light rain or heavy snow, as well as the condition of the air environment such as foggy, hazy or blowing dust.
Pressure Altimeter
An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.
Pressure Altitude
The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.
Pressure Change
The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.
Pressure Characteristic
The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.
Pressure Falling Rapidly
A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.
Pressure Gage
A device for registering the pressure of solids, liquids, or gases. It may be graduated to register pressure in any units desired.
Pressure Gradient
The amount of pressure change occurring over a given distance.
Pressure Gradient Force
A three-dimensional force vector operating in the atmosphere that accelerates air parcels away from regions of high pressure and toward regions of low pressure in response to an air pressure gradient. Usually resolved into vertical and horizontal components.
Pressure Head
Energy contained by fluid because of its pressure, usually expressed in feet of fluid (foot pounds per pound).
Pressure Induced Wave
A rare type of wave that does not develop from wind or seismic activity. Instead, these waves develop as a pressure perturbation moves over the water surface. The water surface adjusts to account for the atmospheric pressure change. As atmospheric pressure decreases, the force exerted upward by the water increases, creating a pressure induced wave.
Pressure Rising Rapidly
An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.
Pressure Unsteady
A pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement.
Pressure-driven Channeling
Channeling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley's axis. Compare forced channeling.
Prevailing Visibility
The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Prevailing Westerlies
The westerly winds that dominant in middle latitudes.
Prevailing Winds
A wind that consistently blows from one direction more than from any other.
Prevention of Significant Deterioration
A program, specified in the Clean Air Act, whose goal is to prevent air quality from deteriorating significantly in areas of the country that are presently meeting the ambient air quality standards.
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
Primary Control Tide Station
A tide station where continuous observations have been made for a minimum of 19 years. Its purpose is to provide data for computing accepted values essential to tide predictions and for determining tidal datums for coastal and marine boundaries. The data series from primary control tide stations serves as a primary control for the reduction of tidal datum for subordinate tide stations with a shorter period of record. The 19 year period is the official tidal epoch for calculating tidal datums.
Primary Pollutant
Substances that are pollutants immediately on entering the atmosphere. Compare secondary pollutant.
Primary Swell Direction
Prevailing direction of swell propagation.
Probability
A chance, or likelihood, that a certain event might happen.
Probability Forecast
A forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur.
Probability of Hail
(Abbrev. POH) - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Probability of Tropical Cyclone Conditio
The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.
PROPA
Propagation (movement)
Propagation
1. The movement of an atmospheric phenomenon. This term is frequently applied to the motion of thunderstorms into regions favorable for their continued development (into a maritime tropical airmass).

2. The transmission of electromagnetic energy as waves through or along a medium.
Proton Flare
In solar-terrestrial terms, any flare producing significant fluxes of greater-than-10 MeV protons in the vicinity of the earth.
Pseudo-Warm Front
A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone.
Public Information Statement
A narrative statement issued by a National Weather Service Forecast Office that can be used for:

1) A current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).

2) Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc.

3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.
Public Severe Weather Outlook
These are issued when the Storm Prediction Center (SPC) in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.
Pulse Duration
The time over which a radar pulse lasts. The pulse duration can be multiplied by the speed of light to determine the pulse length or pulse width.
Pulse Radar
A type of radar, designed to facilitate range (distance) measurements, in which are transmitted energy emitted in periodic, brief transmission.
Pulse-Pair Processing
Nickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".
PVA
Positive Vorticity Advection - the advection of higher values of vorticity into an area.
PWAT
Precipitable Water - measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
QUAD
Quadrant
Quadrature
The component of the complex signal that is 90 degrees out of phase with the inphase component. This component lies along the imaginary axis the complex plane.
Quality of Snow
The amount of ice in a snow sample expressed as a percent of the weight of the sample.
Quantitative Precipitation Estimate (QPE)
A spatial and temporal analysis estimating the amount of precipitation that has occurred using a variety of techniques including observational and remote sensing data.
Quantitative Precipitation Forecast
A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.
Quasi-stationary
(abbrev. QSTNRY)- Describes a low or high pressure area or a front that is nearly stationary.
Quasi-stationary Front
A front which is nearly stationary or moves very little since the last synoptic position. Also known as a stationary front.
Quiescent Prominence (Filament)
Long, sheet-like prominences nearly vertical to the solar surface
RA
Rain
RADAP II
RAdar DAta Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, accumulated rainfall, etc.
RADAR
Acronym for RAdio Detection And Ranging; a radio device or system for locating an object by means of ultrahigh-frequency radio waves reflected from the object and received, observed, and analyzed by the receiving part of the device in such a way that characteristics (as distance and direction) of the object may be determined.
Radar Beam
The straight line that a radar pulse travels along. As the radar beam gets further away from the radar, it gets wider and wider. In order for a precipitation target to be detected by the radar, it must fill the entire radar beam; therefore, the radar will have a difficult time detecting small showers and thunderstorms at a great distance from the radar.
Radar Coded Message
This is an alphanumeric coded message which will be used in preparation of a national radar summary chart. It is automatically produced by the WSR-88D's Radar Product Generator (RPG) in 3 parts (reflectivities, storm motion, and echo tops).
Radar Cross Section
The area of a fictitious, perfect reflector of electromagnetic waves (e.g., metal sphere) that would reflect the same amount of energy back to the radar as the actual target (e.g., lumpy snowflake).
Radar Data Acquisition
An acronym for Radar Data Acquisition. The RDA is the hardware component of the NEXRAD system that consists of the radar antenna, transmitter, receiver, tower, and controlling computer. The RDA collects the unprocessed, analog voltages from the radar antenna and converts the signal to base reflectivity , base velocity, and spectrum width (in polar coordinate form). These "wide-band" products are transmitted to the RPG, which creates and disseminates end-user products. Also: The RDA is the origination point of the WSR-88D radar data that will be eventually used by the radar operator. This WSR-88D component group is made up of several subcomponents which generate and radiate radio frequency (RF) pulses, receive reflected energy from those pulses, and process this received energy into digital base data. The RDA is also the site of the first two of four data recording levels used by the WSR-88D to record and store radar data.
Radar Meteorology
Branch of meteorology that uses radars for weather observations and forecasts.
Radar Mosaic
A radar product that combines information from multiple radars to give a regional or national view of reflectivity or precipitation. An individual NEXRAD radar is limited to a range of about 200 miles. Typically, a mosaic product is produced for regions spanning several hundreds to several thousands of miles. Mosaic products are produced by vendors external to the NEXRAD system.
Radar Product Generator (RPG)
The RPG is the computer in the NEXRAD system that receives polar-coordinate base radar data from the RDA and processes these data into end-user products. Algorithms are utilized for pattern-recognition, rainfall estimation, computation of VIL and other products. The RPG communicates these products to end-users. A specific subset of available products is always generated for the NIDS vendors for distribution outside of the NWS, DoD, and FAA. Other products are generated by the RPG upon request from a PUP.
Radar Range
Distance from the radar antenna. The WSR-88D radar has a range for velocity products out to 124 nautical miles and reflectivity products out to 248 nautical miles.
Radar Reflectivity
The sum of all backscattering cross-sections (e.g., precipitation particles) in a pulse resolution volume divided by that volume. The radar reflectivity can be related to the radar reflectivity factor through the dielectric constant term |K|^2, and the radar wavelength.
Radar Reflectivity Factor (z)
z = the sum (over i) of (N_i * D_i^6), where N_i is the number of drops of diameter D_i in a pulse resolution volume. Note that z may be expressed in linear or logarithmic units. The radar reflectivity factor is simply a more meteorologically meaningful way of expressing the radar reflectivity.
Radial Velocity
Component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, can not be detected.)
Radiance
A measure of the intensity of the radiant energy flux emitted by a body in a given direction.
Radiation
Energy transport through electromagnetic waves. See shortwave radiation and longwave radiation.
Radiation Fog
A fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.
Radiation Laws
The four physical laws which fundamentally describe the behavior of blackbody radiation: Kirchhoff's law, Planck's law, Stefan-Boltzmann law and Wien's displacement law.
Radiational Cooling
The cooling of the Earth's surface. At night, the Earth suffers a net heat loss to space due to terrestrial cooling. This is more pronounced when you have a clear sky.
Radiational Inversion
Used interchangably with Nocturnal Inversion; a temperature inversion that develops during the night as a result of radiational cooling of the surface. Because the immediate surface (lower Boundary Layer) cools much more rapidly during these conditions than the air just above (upper Boundary Layer), a temperature inversion can be created overnight, but typically erodes quickly after sunrise.
Radio Emission
Emissions of the sun in radio wavelengths from centimeters to dekameters, under both quiet and disturbed conditions. Type I. A noise storm composed of many short, narrow-band bursts in the metric range (300 - 50 MHz). Type II. Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward deka- meter wavelengths (10 MHz). Type II emissions occur in loose association with major FLAREs and are indicative of a shock wave moving through the solar atmosphere. Type III. Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500 - 0.5 MHz). They often occur in groups and are an occasional feature of complex solar ACTIVE REGIONs. Type IV. A smooth continuum of broad-band bursts primarily in the meter range (300 - 30 MHz). These bursts are associated with some major flare events beginning 10 to 20 minutes after the flare maximum, and can last for hours
Radio Event
Flares with Centimetric Bursts and/or definite Ionospheric Event (SID)
Radiofacsimile
Also known as HF FAX, radiofax or weatherfax, is a means of broadcasting graphic weather maps and other graphic images via HF radio. HF radiofax is also known as WEFAX, although this term is generally used to refer to the reception of weather charts and imagery via satellite. Maps are received using a dedicated radiofax receiver or a single sideband shortwave receiver connected to an external facsimile recorder or PC equipped with a radiofax interface and application software.
Radiofax
Abbreviation for radiofacsimile
Radioisotope Snow Gage
A snow water equivalent gage based on the absorption of gamma radiation by snow; this gage can measure up to 55 inches water equivalent with a 2 to 5 percent error.
Radiosonde
An instrument that is carried aloft by a balloon to send back information on atmospheric temperature, pressure and humidity by means of a small, expendable radio transmitter. Radiosondes can be tracked by radar, radio direction finding, or navigation systems (such as the satellite Global Positioning System) to obtain wind data. See also rawinsonde.
Radius of Maximum Winds
The distance from the center of a tropical cyclone to the location of the cyclone's maximum winds. In well-developed hurricanes, the radius of maximum winds is generally found at the inner edge of the eyewall.
RAFC
Regional Area Forecast Center
RAFS
Regional Analysis and Forecasting System
Rain
Precipitation that falls to earth in drops more than 0.5 mm in diameter.
Rain Foot
Slang for a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.
Rain Forest
A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.
Rain Gauge
An instrument for measuring the quantity of rain that has fallen.
Rain Induced Fog
When warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues. Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.
Rain Shadow
An area of reduced precipitation on the lee side of a mountain barrier caused by warming of air and dissipation of cloudiness as air descends the barrier.
Rain Shield
In a hurricane, a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye. The outer edge is well defined and its distance from the eye varies greatly from storm to storm. The wind, both sustained and peak gusts, keeps increasing as much as one moves through the rain shield toward the storm's eye.
Rain-free Base
A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.
Rainbow
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.
Rainfall
The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge. Refer to rain for rates of intensity and the quantitative precipitation for forecasting.
Rainfall Estimates
A series of NEXRAD products that employ a Z-R relationship to produce accumulations of surface rainfall from observed reflectivity.
Range
Distance from the radar antenna. The WSR-88D radar has a range for velocity products out to 124 nm and reflectivity products out to 248 nm.
Range Folding
This occurs when the radar receives a signal return from a pulse other than the most recent pulse. In this case, the radar sends out a pulse (a short burst of energy). This pulse will continue to go in a straight line until it strikes a target. When it strikes the target, a portion of the pulse will be back scattered towards the radar. If the target it strikes is well beyond the normal range of the radar, it will take longer for the back scattered energy to arrive back at the radar. As a result, the radar will most likely have sent out another pulse in the same direction before the back scattered energy arrives back at the radar. Therefore, when the radar receives the back scattered energy, it will assume that it came from an object much closer to the radar and it will improperly locate the echo. A multiple-trip return appears at the difference of the true range and a multiple of the unambiguous range, i.e., R_displayed = R_true - n * R_max, where n = 0,1,2,...
Range Gate
The discrete point in range along a single radial of radar data at which the received signal is sampled. Range gates are typically spaced at 100-1000 meter intervals. A "radial" of radar data is composed of successive range gates, out to the maximum unambiguous range.
Range Height Indicator
The RHI is a radar display in which the radar scans vertically, with the antenna pointing at a specific azimuth or radial. NEXRAD does not support RHI, but the PUP software allows the NEXRAD operator to construct a vertical cross-section using data from multiple scans of the radar.
Range Normalization
A receiver gain function in the radar which compensates for the effect of range (distance) on the received power for an equivalent reflectivity.
Range Resolution
The ability of the radar to distinguish two targets along the same radial but at different ranges. it is approximately ½ the pulse length.
Range Unfolding
Process of removing range ambiguity in apparent range of a multitrip target on the radar.
Rankine Vortex
Velocity profile for a symmetric circulation in which the inner core is in solid rotation and tangential winds outside the core vary inversely with radial distance from the center.
RAOB
Radiosonde Observation (Upper-Air Observation)
Rapid Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours
Rapid Onset Flooding (ROF)
In the context of the National Water Model (NWM), rapid onset flooding refers to stream reaches that are forecast to at least double their flow within an hour, and meet or exceed their high water flow within six hours of this flow increase.
Rapidly Intensifying
Any maritime cyclone whose central pressure is dropping, or is expected to drop, at a rate of 1 MB per hour for 24 hours.
Rawinsonde
A radiosonde that is tracked to measure winds.
Rawinsonde Observation
A radiosonde observation which includes wind data.
RAWS
Remote Automated Weather Stations
Rayleigh Scattering
Changes in directions of electromagnetic energy by particles whose diameters are 1/16 wavelength or less. This type of scattering is responsible for the sky being blue.
Reach
A section of river or stream between an upstream and downstream location, for which the stage or flow measured at a point somewhere along the section (e.g., gaging station or forecast point) is representative of conditions in that section of river or stream.
Reach
In hydrologic terms, the distance between two specific points outlining that portion of the stream, or river for which the forecast applies. This generally applies to the distance above and below the forecast point for which the forecast is valid.
Real-Time
Refers to the rapid retrieval, processing and transmission of data.
Rear Flank Downdraft
A region of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant; thus the presence of a hook or pendant may indicate the presence of an RFD.
Recharge
The replenishment of groundwater through infiltration of precipitation or snowmelt into the soil and gravity flow of streams into valley alluvium, sinkholes, or other large openings.
Reconnaissance Code
An aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight.
Recreation Report
This National Weather Service product is used to relay reports on conditions for resorts and recreational areas and/or events. This report may also contain forecast information. Reports for recreational areas and resorts are often routine products, typically for a season, but possibly year-round.
Red Flag
This a fire weather program which highlights the onset of critical weather conditions conducive to extensive wildfire occurrences.
Red Flag Warning
A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met:
1) a sustained wind average 15 mph or greater
2) relative humidity less than or equal to 25 percent and
3) a temperature of greater than 75 degrees F.
In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.
Red Watch or Red Box
Slang for Tornado Watch.
Reference Mark
A relatively permanent point of known elevation which is tied to a benchmark.
Reflectivity Factor
The result of a mathematical equation (called the Weather Radar Equation) that converts the analog power (in Watts) received by the radar antenna into a more usable quantity. The reflectivity factor (denoted by Z) takes into account several factors, including the distance of a target from the radar, the wavelength of the transmitted radiation, and certain assumptions about the kind and size of targets detected by the radar. The reflectivity factor ranges over several orders of magnitudes, so it is usually expressed on a logarithmic scale called dBZ (decibels of reflectivity).
Refraction
Changes in the direction of energy propagation as a result of density changes within the propagating medium. In weather terms, this is important on determining how a radar beam reacts in the atmosphere.
Refractive Index
A measure of the amount of refraction. Numerically equal to the ratio of wave velocity in a vacuum to a wave speed in the medium, i.e., n = c / v where: v is actual speed, and c is speed of light in a vacuum.
Refractivity
Expressed as N; N = (n-1)*106, where n is refractive index and N is a function of temperature, pressure and vapor pressure (in the atmosphere).
Regional Haze
Haze that is mixed uniformly between the surface and the top of a convective boundary layer.
Relative Humidity
A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.
Relative Vorticity
The sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis.
Relative Wind
The wind with reference to a moving point. Sometimes called APPARENT WIND. See also APPARENT WIND, TRUE WIND.
Relocated
A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest known position is not necessarily a reasonable representation of the cyclone's movement.
Remote Observing System Automation
A type of automated data transmitter used by NWS Cooperative Program observers.
Replace and Route (R&R)
A methodology that ingests the official streamflow forecasts issued by the NWS RFCs at AHPS gauge locations, and utilizes the National Water Model (NWM) to route these forecasts downstream. This method is used to delineate the River Forecast Center Flood Inundation Map (RFC FIM).
Residual Layer
the elevated portion of a convective boundary layer that remains after a stable boundary layer develops at the ground (usually in late afternoon or early evening) and cuts off convection.
Residual Moisture
Atmospheric moisture which lingers over an area after the main weather system has departed.
Resonance
The state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same, or nearly the same, as the natural vibration frequency of the system.
Right Ascension
The celestial longitude of the sun. This value is 0 at the vernal equinox, 90 at the summer solstice, 180 at the autumnal equinox and 270 at the winter solstice.
Right Entrance Region
Used interchangably with Right Rear Quadrant; the area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.
Right Rear Quadrant
(Abbrev. RRQ) - Used interchangably with Right Entrance Region; the area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also exit region, left front quadrant.
Riparian Zone
In hydrologic terms, a stream and all the vegetation on its banks.
River Basin
In hydrologic terms, drainage area of a river and its tributaries.
River Flood Statement
This product is used by the local National Weather Service Forecast Office (NWFO) to update and expand the information in the River Flood Warning. This statement may be used in lieu of a warning if flooding is forecasted, imminent, or existing and it presents no threat to life or property. The statement will also be used to terminate a River Flood Warning.
River Flood Warning
This is product is issued by the local National Weather Service Forecast Office (NWFO) when forecast points (those that have formal gaging sites and established flood stages) at specific communities or areas along rivers where flooding has been forecasted, is imminent, or is in progress. Flooding is defined as the inundation of normally dry areas as a result of increased water levels in an established water course. The flood warning is based on the RVF product from the River Forecast Center (RFC) in Minneapolis, Minnesota. The flood warning normally specifies crest information. It usually occurs 6 hours or later after the causative event and it is usually associated with widespread heavy rain and/or snow melt or ice jams.

It will contain the forecast point covered by the warning, the current stage (if it is available), and the established flood stage. It will also contain the forecasted crest from the River Forecast Center (RFC) in Minneapolis, Minnesota. From this forecasted crest, the NWFO will be able to determine which areas will be affected by the river flooding. This information will be included in the warning. Finally, the statement will include a site/event specific call to action.
River Forecast
An internal product issued by RFCs to other NWS offices. An RVF contains stage and/ or flow forecasts for specific locations based on existing, and forecasted hydrometeorologic conditions. The contents of these products are used by the HSA office to prepare Flood Warnings (FLW), Flood Statements (FLS), River Statements (RVS), as well as other products available to the public.
River Forecast Center
Centers that serve groups of Weather Service Forecast offices and Weather Forecast offices, in providing hydrologic guidance and is the first echelon office for the preparation of river and flood forecasts and warnings.
River Gage
A device for measuring the river stage.
River Gage Datum
The arbitrary zero datum elevation which all stage measurements are made from.
River Ice Statement
A public product issued by the RFC containing narrative and numeric information on river ice conditions.
River Observing Station
An established location along a river designated for observing and measuring properties of the river.
River Recreation Statement
A statement released by the NWS to inform river users of current and forecast river and lake conditions. These statements are especially useful for planning purposes.
River Statement
A NWS product issued to communicate notable hydrologic conditions which do not involve flooding, i.e., within river bank rises, minor ice jams, etc.
Rockfill Dam
In hydrologic terms, an embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment.
Rogue Wave
Commonly used term by mariners of a wave of an unexpected wave of much greater height or steepness than other waves in the prevailing sea or swell system. Rogue waves have been part of marine folklore for centuries. They are generally considered to be unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local area. A single rogue wave has certainly been known to spell disaster for the mariner. They have, over the past twenty or thirty years, come to be recognized as unique phenomena albeit with several possible causes.
(1) Constructive interference. Several different wave trains of differing speeds and directions meet at the same time. The heights of the crests are additive so that an extreme wave may result when very high waves are included in the wave trains. The effect is normally short lived since the wave trains continue to separate and move on.
(2) Focusing of wave energy. When storm forced waves are developed in a water current counter to the wave direction an interaction can take place which results in a shortening of the wave frequency. The result is the superimposing of the wave trains and the generation of extreme waves. Examples of currents where these are sometimes seen are the Gulf Stream and Agulhas current. Extreme wave developed in this regime tend to be longer lived.
(3) Normal part of the wave spectrum. The generation of waves on water results not in a single wave height but in a spectrum of waves distributed from the smallest capillary waves to large waves indeed. Within this spectrum there is a finite possibility of each of the wave heights to occur with the largest waves being the least likely. The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The probability of encountering such a wave is about 1 in 10 while 1 in 1000 waves will be nearly double the significant wave height or higher. This is thought to be the source of at least some reports of rogue waves.
Rolled Filled Dam
In hydrologic terms, an embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment
Rope Stage
The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.
ROSA
Remote Observing System Automation. A type of automated data transmitter used by NWS Cooperative Program observers.
Rossby Waves
A series of troughs and ridges on quasi-horizontal surfaces in the major belt of upper tropospheric westerlies. The waves are thousands of kilometers long and have significant latitudinal amplitude.
Rotation
The spinning of a body, such as the earth, about its axis.
Rough Seas
Sea conditions associated with regionally defined wind thresholds over bays, inlets, harbors, inland waters, and estuaries where larger waves are forming with whitecaps and spray everywhere.
Runway Visual Range
The maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. This value is normally determined by visibility sensors located alongside and higher than the center line of the runway. RVR is calculated from visibility, ambient light level, and runway light intensity.
RVA
River Summary, a NWS summary of river and/or crest stages for selected forecast points along the river.
S-Band Radar
These were in use as network radars in the National Weather Service prior to the installation of the WSR 88-D radars. They were 10-centimeter wavelength radars.
SAB
Satellite Analysis Branch, part of NESDIS NESDIS: National Environmental Satellite, Data and Information Service
Sacramento Soil Moisture Accounting Model (SAC-SMA)
A continuous soil moisture accounting model with spatially lumped parameters that simulates runoff within a basin.
SafetyNET
Inmarsat-C SafetyNET is an internationally adopted, automated satellite system for promulgating weather forecasts and warnings, marine navigational warnings and other safety related information to all types vessels and is part of the Global Maritime Distress and Safety System (GMDSS).
Saffir-Simpson Hurricane Wind Scale
The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. In general, damage rises by about a factor of four for every category increase. The maximum sustained surface wind speed (peak 1-minute wind at the standard meteorological observation height of 10 m [33 ft] over unobstructed exposure) associated with the cyclone is the determining factor in the scale. The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes.
Salinity (SAL)
In oceanography, conductivity is measured and converted to salinity by a known functional relationship between the measured electrical conductivity of seawater temperature and pressure.
SAME
(Specific Area Message Encoding) - A tone alert system which allows NOAA Weather Radio receivers equipped with the SAME feature to sound an alert for only certain weather conditions or within a limited geographic area such as a county.
SAMEX
Storm and Mesoscale Ensemble Experiment
Sampling Frequency
The rate at which sensor data is read or sampled.
Sandstorm
Particles of sand carried aloft by strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.
Santa Ana Wind
In southern California, a weather condition in which strong, hot, dust-bearing winds descend to the Pacific Coast around Los Angeles from inland desert regions.
Sastrugi
Ridges of snow formed on a snow field by the action of the wind.
SAT
1. Satellite (imagery)

2. Saturday
Satellite Hydrology Program
A NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.
SATL
Satellite
Saturation Vapor Pressure
The vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase.
SAWRS
Supplementary Aviation Reporting Station - the SAWRS program addresses the concerns of users who depend on weather observations for air operations. If the cooperator is collocated with a commissioned automated system, they ensure continuity during outage periods of the automated system. The requirement for a SAWRS arises from the FAA validated need for observations to satisfy FAR 121 or 135 operations or for the safe conduct of other aircraft.
SBCAPE
Surface Based CAPE; CAPE calculated using a Surface based parcel.
SCA
Small Craft Advisory
Scattered
When used to describe precipitation (for example: "scattered showers") - Area coverage of convective weather affecting 30 percent to 50 percent of a forecast zone (s). When used to describe sky cover: 3/8th to 4/8th (sky cover is measured in eighths or oktas) of the sky covered by clouds. In U.S. weather observing procedures, this is reported with the contraction “SCT.”
Scattering
The process in which a beam of light is diffused or deflected by collisions with particles suspended in the atmosphere.
Sea Breeze
A thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.
Sea Breeze Convergence Zone
The zone at the leading edge of a sea breeze where winds converge. The incoming air rises in this zone, often producing convective clouds.
Sea Breeze Front
The leading edge of a sea breeze, whose passage is often accompanied by showers, a wind shift, or a sudden drop in temperature.
Sea Fog
Common advection fog caused by transport of moist air over a cold body of water.
Sea Ice
Any form of ice found at sea which has originated from the freezing of sea water (sea ice does NOT include superstructure icing). Ice formed from the freezing of the waters of the Great Lakes will be considered the same as sea ice.
Sea Level Pressure
The sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
Sea Surface Temperatures
The term refers to the mean temperature of the ocean in the upper few meters.
Seas
The combination of both wind waves and swell. Used to describe the combination or interaction of wind waves and swell in which the separate components are not distinguished. This includes the case when swell is negligible or is not considered in describing sea state. Specifically, Seas2 = S2+W2 where S is the height of the swell and W is the height of the wind wave. When used, Seas should be considered as being the same as the Significant Wave Height
Second-Day Feet
In hydrologic terms, the volume of water represented by a flow of one cubic foot per second for 24 hours; equal to 86,400 cubic feet. This is used extensively as a unit of runoff volume. Often abbreviated as SDF.
Secondary Ambient Air Quality Standards
Air quality standards designed to protect human welfare, including the effects on vegetation and fauna, visibility and structures.
Secondary Pollutant
Pollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.
Sector Boundary
In solar-terrestrial terms, in the solar wind, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward or away from the sun.
Sectorized Hybrid Scan
A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.
Sediment Storage Capacity
In hydrologic terms, the volume of a reservoir planned for the deposition of sediment.
Seepage
In hydrologic terms, the interstitial movement of water that may take place through a dam, its foundation, or abutments.
Sensible Heat Flux
The flux of heat from the earth's surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.
Separation Eddy
An eddy that forms near the ground on the windward or leeward side of a bluff object or steeply rising hillside; streamlines above this eddy go over the object.
Serial Derecho
Type of derecho that consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows. A Serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the "progressive derecho."

It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low pressure system and strong short wave trough at 500 mb (strong dynamic forcing). Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (3-7 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential. The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots. Squall line movement is often less than 30 knots.
Severe Local Storm
A convective storm that usually covers a relatively small geographic area, or moves in a narrow path, and is sufficiently intense to threaten life and/or property. Examples include severe thunderstorms with large hail, damaging wind, or tornadoes. Although cloud-to-ground lightning is not a criteria for severe local storms, it is acknowledged to be highly dangerous and a leading cause of deaths, injuries, and damage from thunderstorms. A thunderstorm need not be severe to generate frequent cloud-to-ground lightning. Additionally, excessive localized convective rains are not classified as severe storms but often are the product of severe local storms. Such rainfall may result in related phenomena (flash floods) that threaten life and property.
Severe Local Storm Watch
An alert issued by the National Weather Service for the contiguous U.S. and its adjacent waters of the potential for severe thunderstorms or tornadoes.
Severe Thunderstorm Warning
This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail one inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criteria for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.

Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning. If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning. If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.

After it has been issued, the affected NWFO will follow it up periodically with Severe Weather Statements. These statements will contain updated information on the severe thunderstorm and they will also let the public know when the warning is no longer in effect.
Severe Thunderstorm Watch
This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces one inch hail or larger in diameter and/or winds equal or exceed 58 miles an hour. The size of the watch can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.

A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement. During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.
Severe Weather Analysis
This WSR-88D radar product provides 3 base products (reflectivity (SWR), radial velocity (SWV), and spectrum width (SWW)) at the highest resolution available along with radial shear (SWS). These products are mapped into a 27 nm by 27 nm region centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. It is most effective when employed as an alert paired product with the product centered on alert at height that caused the alert. It is used to examine 3 base products simultaneously in a 4 quadrant display; and analyze reflectivity and velocity products at various heights to gain a comprehensive vertical analysis of the thunderstorm.
Severe Weather Potential Statement
This statement is designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance. It is issued by the local National Weather Service office.
Severe Weather Probability
This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.
Severe Weather Statement
A National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.
Shallow Fog
Fog in which the visibility at 6 feet above ground level is 5/8ths statute mile or more and the apparent visibility in the fog layer is less than 5/8ths statute mile.
SHARS
Subtle Heavy Rainfall Signature
Shear
Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.
Short Range Forecast (SRF)
A configuration of the National Water Model (NWM) that runs hourly and produces hourly deterministic forecasts of streamflow and hydrologic states out to 18 hours for the contiguous United States (ConUS), and out 48 hours for Hawaii and Puerto Rico/U.S. Virgin Islands. For ConUS, meteorological forcing data are drawn from the HRRR and RAP. For Hawaii and Puerto Rico/U.S. Virgin Islands, meteorological forcing data are drawn from the NAM-Nest with HIRESW WRF-ARW.
Short Term Forecast
A product used to convey information regarding weather or hydrologic events in the next few hours.
Short Wave Fade (SWF)
In solar-terrestrial terms, a particular ionospheric solar flare effect under the broad category of sudden ionospheric disturbances (SIDs) whereby short-wavelength radio transmissions, VLF, through HF, are absorbed for a period of minutes to hours.
Short-Fuse Warning
A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.
Short-Term Prediction Research and Transition (SPoRT)
A NASA- and NOAA-funded activity to transition experimental/quasi-operational satellite observations and research capabilities to the operational weather community to improve short-term weather forecasts on a regional and local scale.
Short-Term Prediction Research and Transition Center - Land Information System (SPoRT-LIS)
Provides high-resolution (~3 km) gridded soil moisture products in real-time to support regional and local modeling and improve situational awareness.
Shortwave
Also known as Shortwave Trough; a disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
Shortwave Radiation
In solar-terrestrial terms, shortwave radiation is a term used to describe the radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths (between about 0.1 and 2 micrometers).
Shortwave Trough
Also called Shortwave; A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave trough.
Showalter Index
(Abbrev. SWI) - a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.
SHRA
Rain showers
SHRAS
showers
Signal-to-Noise Ratio
A ratio that measures the comprehensibility of data, usually expressed as the signal power divided by the noise power, usually expressed in decibels (dB).
Significant Wave Height
The mean or average height of the highest one third of all waves in a swell train or in a wave generating region. It approximates the value an experienced observer would report if visually estimating sea height. When expressed as a range (e.g. Seas 2-4 ft) , indicates a degree of uncertainty in the forecast and/or expected changing conditions (not that all waves are between 2-4 ft). Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.

Example: Significant Wave Height = 10 ft 1 in 10 waves will be larger than 11 ft 1 in 100 waves will be larger than 16 ft 1 in 1000 waves will larger than 19 ft

Therefore, assuming a wave period of 8 seconds, for a significant wave height of 10 feet, a wave 19 feet or higher will occur every 8,000 seconds (2.2 hours).
Significant Weather Outlook
A narrative statement produced by the National Weather Service, frequently issued on a routine basis, to provide information regarding the potential of significant weather expected during the next 1 to 5 days.
SKYWARN
A nationwide network of volunteer weather spotters who report to and are trained by the National Weather Service. These spotters report many forms of significant or severe weather such as Severe Thunderstorms, Tornadoes, Hail, Heavy Snow, or Flooding. Contact your local National Weather Service Forecast Office to learn about SKYWARN activities in your area.
Sleet Warning
Issued when accumulation of sleet in excess of 1/2" is expected; this is a relatively rare scenario. Usually issued as a winter storm warning for heavy sleet.
Slight Chance
In probability of precipitation statements, usually equivalent to a 20 percent chance.
SMA
The Soil Moisture Accounting Model.
Small Craft
There is no precise definition for small craft. Any vessel that may be adversely affected by Small Craft Advisory criteria should be considered a small craft. Other considerations include the experience of the vessel operator, and the type, overall size, and sea worthiness of the vessel. See Small Craft Advisory.
Small Craft Advisory
(SCA) - An advisory issued by coastal and Great Lakes Weather Forecast Offices (WFO) for areas included in the Coastal Waters Forecast or Nearshore Marine Forecast (NSH) products. Thresholds governing the issuance of small craft advisories are specific to geographic areas. A Small Craft Advisory may also be issued when sea or lake ice exists that could be hazardous to small boats. There is no precise definition of a small craft. Any vessel that may be adversely affected by Small Craft Advisory criteria should be considered a small craft. Other considerations include the experience of the vessel operator, and the type, overall size, and sea worthiness of the vessel. Exact thresholds may be found in NWSI 10-303:https://www.nws.noaa.gov/directives/sym/pd01003003curr.pdf. For a list of NWS Weather Offices by Region, refer to the following website: http://www.nws.noaa.gov/organization.php
Small Craft Should Exercise Caution
Precautionary statement issued to alert mariners with small, weather sensitive boats.
Small Hail
Technically used to refer to snow pellets or graupel.
Small Stream Flooding
In hydrologic terms, flooding of small creeks, streams, or runs.
Smoke Dispersal
Describes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere, i.e., a combination of mixing heights and transport winds.
Smoke Management
The use of meteorology, fuel moisture, fuel loading, fire suppression and burn techniques to keep smoke impacts from prescribed fires within acceptable limits.
Snow Accumulation and Ablation Model
In hydrologic terms, a model which simulates snow pack accumulation, heat exchange at the air-snow interface, areal extent of snow cover, heat storage within the snow pack, liquid water retention, and transmission and heat exchange at the ground-snow interface.
Snow Advisory
This product is issued by the National Weather Service when a low pressure system produces snow that may cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations. The advisory criteria varies from area to area. If the forecaster feels that it is warranted, he or she can issued it for amounts less than the minimum criteria. For example, it may be issued for the first snow of the season or when snow has not fallen in long while.
Snow Data Assimilation System (SNODAS)
A physically based, mass conserving snow water equivalent (SWE) model.
Snow Grains
Precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.
Snow Pack
Same as Snowcover; the combined layers of snow and ice on the ground at any one time.
Snow Squall
A snow squall is an intense, but limited duration, period of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning (generally moderate to heavy snow showers). Snow accumulation may be significant.
Snow Stake
A 1-3/4 inch square, semi-permanent stake, marked in inch increments to measure snow depth.
Snow Water Equivalent
The water content obtained from melting accumulated snow.
Snowboard
A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.
Snowflake
An agglomeration of snow crystals falling as a unit.
Snowpack
The total snow and ice on the ground, including both the new snow and the previous snow and ice which has not melted.
Solar Coordinates
In solar-terrestrial terms, Central Meridian Distance (CMD). The angular distance in solar longitude measured from the central meridian
Solar Cycle
In solar-terrestrial terms, the approximately 11-year quasi-periodic variation in frequency or number of solar active events.
Solar Maximum
In solar-terrestrial terms, the month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a maximum.
Solar Minimum
In solar-terrestrial terms, the month(s) during the solar cycle when the 12-month mean of monthly average sunspot numbers reaches a minimum.
Solar Noon
The time of day at which the sun is the highest in the sky. This time varies through the year due to the change in speed of the earth's orbit around the sun.
Solar Sector Boundary (SSB)
In solar-terrestrial terms, the apparent solar origin, or base, of the interplanetary sector boundary marked by the larger-scale polarity inversion lines.
Solar Wind
The outward flux of solar particles and magnetic fields from the sun. Typically, solar wind velocities are near 350 km/s.
Southern Oscillation
(SO) - a "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia. The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation". The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Nino occurs. The cold phase of the SO, called "La Nina" by some, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Nino Southern Oscillation or ENSO.
Southern Oscillation Index
A numerical index measuring the state of the Southern Oscillation. The SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin, Australia. It is highly correlated with tropical sea surface temperature anomaly indices recorded in Niño3.
Space Environment Center
(SEC) - This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.
Spearhead Echo
A radar echo associated with a downburst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During it's mature stage, the appendage turns into a major echo and the parent echo loses its identity.
Special Avalanche Warning
Issued by the National Weather Service when avalanches are imminent or occurring in the mountains. It is usually issued for a 24 hour period.
Special Fire Weather
Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.
Special Marine Warning
(SMW) A warning product issued for potentially hazardous weather conditions usually of short duration (up to 2 hours) producing sustained marine thunderstorm winds or associated gusts of 34 knots or greater; and/or hail 3/4 inch or more in diameter; and/or waterspouts affecting areas included in a Coastal Waters Forecast, a Nearshore Marine Forecast, or an Great Lakes Open Lakes Forecast that is not adequately covered by existing marine warnings. Also used for short duration mesoscale events such as a strong cold front, gravity wave, squall line, etc., lasting less than 2 hours and producing winds or gusts of 34 knots or greater.
Special Tropical Disturbance Statement
This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).
Specific Gravity
The ratio of the density of any substance to the density of water.
Spectral Density
A radar term for the distribution of power by frequency.
Spectral Wave Density
On a buoy report, energy in (meter*meter)/Hz, for each frequency bin (typically from 0.03 Hz to 0.40 Hz).
Spectral Wave Direction
On a buoy report, mean wave direction, in degrees from true North, for each frequency bin.
Speed Shear
The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.
Sphere Calibration
Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.
Spillway
In hydrologic terms, a structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.
Spillway Crest
In hydrologic terms, the elevation of the highest point of a spillway.
Sporadic E
In solar-terrestrial terms, a phenomenon occurring in the E region of the ionosphere, which significantly affects HF radiowave propagation. Sporadic E can occur during daytime or nighttime and it varies markedly with latitude.
Spray
An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.
Squall
A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. 2. In nautical use, a severe local storm considered as a whole, that is, winds and cloud mass and (if any) precipitation, thunder and lightning.
Squall Line
A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.
St Lawrence Freeze-Up Outlook
A National Weather Service forecast product to keep mariners informed of the projected freeze-up date of ice the St. Lawrence River.
Stability
The degree of resistance of a layer of air to vertical motion.
Stability Index
The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.
Stable
An atmospheric state with warm air above cold air which inhibits the vertical movement of air.
Stable Boundary Layer
The stably-stratified layer that forms at the surface and grows upward, usually at night or in winter, as heat is extracted from the atmosphere's base in response to longwave radiative heat loss from the ground. Stable boundary layers can also form when warm air is advected over a cold surface or over melting ice.
Stable Core
Post-sunrise, elevated remnant of the temperature inversion that has built up overnight within a valley.
Staccato Lightning
A Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.
Stage
The level of the water surface of a river or stream above an established datum at a given location.
Stair Stepping
In hydrologic terms, the process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared.
Standard Atmosphere
A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.
Standard Synoptic Times
The times of 0000, 0600, 1200, and 1800 UTC. Also known as the main synoptic times.
State Forecast Product
This National Weather Service product is intended to give a good general picture of what weather may be expected in the state during the next 5 days. The first 2 days of the forecast is much more specific than the last 3 days. In comparison with the Zone Forecast Product, this product will be much more general.
State Weather Roundup
This is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service (NWWS). It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point in Fahrenheit, the relative humidity, wind speed and direction, and finally additional information (wind chill, heat index, a secondary weather condition). These reports are broken up regionally. When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet with no significant weather and/or obstructions to visibility.
Station ID
Five-digit WMO Station Identifier used by the Buoy Data Center since 1976. ID's can be reassigned to future deployments within the same 1 degree square.
Station Model
A specified pattern for plotting, on a weather map, the meteorological symbols that represent the state of the weather at a particular observing station.
Station Pressure
The absolute air pressure at a given reporting station. The air pressure is directly proportional to the combined weight of all air in the atmosphere located in a column directly above the reporting site. Consequently, the station pressure may vary tremendously from one location to another in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.
Stationary Front
A front between warm and cold air masses that is moving very slowly or not at all.
Steam Fog
Fog formed when water vapor is added to air which is much colder than the source of the vapor. It may be formed when very cold air drifts across relatively warm water. At temperatures below about-20°F, ice particles or droxtals may be formed in the air producing a type of ice fog known as frost smoke.
Stepped Leader
A faint, negatively charged channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.
Stilling basin
In hydrologic terms, a basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.
Storm Data
This National Climatic Data Center (NCDC) monthly publication documents a chronological listing, by states, of occurrences of storms and unusual weather phenomena. Reports contain information on storm paths, deaths, injuries, and property damage. An "Outstanding storms of the month" section highlights severe weather events with photographs, illustrations, and narratives. The December issue includes annual tornado, lightning, flash flood, and tropical cyclone summaries.
Storm Relative
Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.
Storm Relative Mean Radial Velocity Map
(SRM): This WSR-88D radar product depicts a full 360º sweep of radial velocity data with the average motion of all identified storms subtracted out. It is available for every elevation angle sampled. It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms.
Storm Relative Mean Radial Velocity Regi
(SRR): This WSR-88D radar product depicts a 27 nm by 27 nm region of storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm radius of the radar. The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator. It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product
Storm Scale
Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale and mesoscale.
Storm Total Precipitation
This radar image is an estimate of accumulated rainfall since the last time there was a one-hour, or more, break in precipitation. It is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event and is available only for the short range (out to 124 nm). To determine accumulated precipitation at greater distances you should link to an adjacent radar.
Storm Tracking Information
This WSR-88D radar product displays the previous, current, and projected locations of storm centroids (forecast and past positions are limited to one hour or less). Forecast tracks are based upon linear extrapolation of past storm centroid positions, and they are intended for application to individual thunderstorms not lines or clusters. It is used to provide storm movement: low track variance and/or 2 or more plotted past positions signify reliable thunderstorm movement.
Storm Warning
A warning of sustained surface winds, or frequent gusts, in the range of 48 knots (55 mph) to 63 knots (73 mph) inclusive, either predicted or occurring, and not directly associated with a tropical cyclone.
Storm Watch
A watch for an increased risk of a storm force wind event for sustained surface winds, or frequent gusts, of 48 knots (55 mph) to 63 knots (73 mph), but its occurrence, location, and/or timing is still uncertain.
Stormwater Discharge
In hydrologic terms, precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces but instead flows onto adjacent land or water areas and is routed into drain/sewer systems.
Straight-Line Hodograph
The name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.
Straight-line Winds
Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.
Stratiform
Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).
Stratiform Rings and Bands
These occur between the active convective bands of a hurricane outside of the eye wall. Inner stratiform bands often exhibit the bright band aloft, a VIP Level 2, and in the lower layers typically show a VIP Level 1.
Stratocumulus
Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.
Stratopause
The boundary between the stratosphere and mesosphere.
Stratosphere
The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.
Stratospheric Ozone
In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.
Stratus
A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.
Stream line
Arrows on a weather chart showing wind speed and direction. The head of the arrow points toward where the wind is blowing and the length of the arrow is proportional to the wind speed. Sometimes shows wind direction and trajectory only.
Stream Reach
A significant segment of surface water that has similar hydrologic characteristics, such as a stretch of stream/river between two confluences.
Streamflow
In hydrologic terms, water flowing in the stream channel. It is often used interchangeably with discharge.
Striations
Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of a Low Precipitation (LP) storm.
Sublimation
The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid.
Sublimation of ice
The transition of water from solid to gas without passing through the liquid phase.
Subrefraction
The bending of the radar beam in the vertical which is less than under standard refractive conditions. This causes the beam to be higher than indicated, and lead to the underestimation of cloud heights.
Substation
A location where observations are taken or other services are furnished by people not located at NWS offices who do not need to be certified to take observations.
Subsurface Storm Flow
In hydrologic terms, the lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called interflow.
Subtle Heavy Rainfall Signature
This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68ºF with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.
Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.
Subtropical Depression
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Subtropical Jet
(Abbrev. STJ) - this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.
Subtropical Storm
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 34 knots (39 mph) or more.
Sudden Ionospheric Disturbance (SID)
In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.
Summation Principle
This principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover provided at all successively higher layers up to and including the layer in question.
Sun Pillar
A bright column above or below the sun produced by the reflection of sunlight from ice crystals.
Sunspot Group Classification
Supercooled Liquid Water
In the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.
Superrefraction
Bending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.
Surcharge Capacity
In hydrologic terms, the volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.
Surf Zone Forecast
(SRF) - A National Weather Service routine or event driven forecast product geared toward non-boating marine users issued for an area extending from the area of water between the high tide level on the beach and the seaward side of the breaking waves.
Surface Energy Budget
The energy or heat budget at the earth's surface, considered in terms of the fluxes through a plane at the earth-atmosphere interface. The energy budget includes radiative, sensible, latent and ground heat fluxes.
Surface impoundment
In hydrologic terms, an indented area in the land's surface, such as a pit, pond, or lagoon.
Surface Runoff
In hydrologic terms, the runoff that travels overland to the stream channel. Rain that falls on the stream channel is often lumped with this quantity.
Surface Water
Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.
Surface Weather Chart
An analyzed synoptic chart of surface weather observations. A surface chart shows the distribution of sea-level pressure (therefore, the position of highs, lows, ridges and troughs) and the location and nature of fronts and air masses. Often added to this are symbols for occurring weather phenomena. Although the pressure is referred to mean sea level, all other elements on this chart are presented as they occur at the surface point of observation.
Surface-based Convection
Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.
Sustained Overdraft
In hydrologic terms, long-term withdrawal from the aquifer of more water than is being recharged.
Sustained Wind
Wind speed determined by averaging observed values over a two-minute period.
SWEAT
Severe Weather ThrEAT index; a stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds as follows:

SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f 850) + f 500 + (125 [s+0.2]) where

SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers.

The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed upper air sounding analysis programs.
Synoptic Scale
The spatial scale of the migratory high and low pressure systems of the lower troposphere, with wavelengths of 1000 to 2500 km.
Synoptic Track
Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.
Synoptic Weather
Weather occurring over a wide region on time scales exceeding 12 hours.
Synthetic Aperture Radar (SAR)
A radar mounted on a moving platform (aircraft or satellite) used for imaging. Since the radar antenna moves a significant distance between transmission and receiving the signal back, the radar antenna acts as if it is larger than its physical dimensions, providing better range and azimuth resolution.
Synthetic Aperture Radar River Ice Surveillance (SARRIS)
An experimental river ice mapping experiment using Synthetic Aperture Radar (SAR).
TAF
Terminal Aerodrome Forecast
TAFB
Tropical Analysis and Forecast Branch (of the TPC)
Tail Cloud
A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud).
Tail-End Charlie
Slang for the thunderstorm at the southernmost end of a squall line or other line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.
Tailwater Height
In hydrologic terms, height of water immediately downstream of the dam.
Target
Precipitation or other phenomena which produces echoes on a radar display.
TDA
Today
Temperature
(Abbrev. TEMP)- The temperature is a measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.
Temperature Inversion
(surface-based or elevated) : a layer of the atmosphere in which air temperature increases with height. When the layer's base is at the surface, the layer is called a surface-based temperature inversion; when the base of the layer is above the surface, the layer is called an elevated temperature inversion.
Temperature Recovery
The change in temperature over a given period of time. Generally, the period between late evening and sunrise. Windy or cloudy conditions will tend to produce slow temperature recovery, while clear, calm weather can cause rapid recovery.
Terminal Aerodrome Forecast
This NWS aviation product is a concise statement of the expected meteorological conditions at an airport during a specified period (usually 24 hours). Each country is allowed to make modifications or exceptions to the code for use in each particular country. TAFs use the same weather code found in METAR weather reports.
Terrain Forced Flow
An airflow that is modified or channeled as it passes over or around mountains or through gaps in a mountain barrier.
Texas Hooker
Same as Panhandle Hook - low pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track.
Thalweg
In hydrologic terms, the line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration.
Thermal
A relatively small-scale, rising air current produced when the Earth's surface is heated. Thermals are a common source of low level turbulence for aircraft.
Thermal Belt
A zone of high nighttime temperatures (and relatively low humidities) that is often experienced within a narrow altitude range on valley sidewalls, especially evident during clear weather with light winds.
Thermal High
Area of high pressure that is shallow in vertical extent and produced primarily by cold surface temperatures.
Thermal Low
Area of low pressure that is shallow in vertical extent and produced primarily by warm surface temperatures.
Thermal Wind
It is a theoretical wind that blows parallel to the thickness lines, for the layer considered, analogous to how the geostrophic wind blows parallel to the height contours. The closer the thickness isopleths, the stronger the thermal wind. Cold air is always located to the left of the thermal wind (as you face downstream) and the warm air is located on the right. Since thickness contours are tighter on the cold side of thermal wind, your lower thickness values will be found on the left side of the thermal wind. The speed and direction of the thermal wind are determined by vector geometry where the geostrophic wind at the upper level is subtracted from the geostrophic wind at the lower level.
Thermally Driven Circulation
A diurnally reversing closed cellular wind current resulting from horizontal temperature contrasts caused by different rates of heating or cooling over adjacent surfaces; includes along-slope, cross-valley, along-valley, mountain-plain and sea breeze circulations.
Thermodynamic Chart
A chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.
Thermodynamic Diagram
Used interchangably with Thermodynamic Chart; a chart containing contours of pressure, temperature, moisture, and potential temperature, all drawn relative to each other such that basic thermodynamic laws are satisfied. Such a chart typically is used to plot atmospheric soundings, and to estimate potential changes in temperature, moisture, etc. if air were displaced vertically from a given level. A thermodynamic chart thus is a useful tool in diagnosing atmospheric instability.
Thermodynamics
In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.
Thermograph
An instrument that measures and records air temperature.
Theta-e
(or Equivalent Potential Temperature) - The temperature a parcel of air would have if
a) it was lifted until it became saturated,
b) all water vapor was condensed out, and
c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.
Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.
Theta-e Ridge
An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge.
THETAE
Abbreviation for Theta-e; the temperature a parcel of air would have if
a) it was lifted until it became saturated,
b) all water vapor was condensed out, and
c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars.
Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability.
Three-Hour Rainfall Rate
This WSR-88D Radar product displays precipitation total (in inches) of the current and past two clock hours as a graphical image. It displays hourly precipitation total (in inches) as a graphical image (polar format with resolution 1.1 nm by 1 degree). It is updated once an hour. It is used to:
1) Assess rainfall intensities and amounts over a longer viewing interval; and
2) Possibly adjust flash flood guidance values since the product corresponds to the timing of Flash Flood Guidance values.
Tidal Cycle
The periodic changes in the intensity of tides caused primarily by the varying relations between the earth, moon, and sun.
Tidal Piling
Occurs when unusually high water levels occur as the result of an accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.
Tidal Wave
See TSUNAMI
Tide Anomaly
Actual water level minus the prediction.
Tilted Updraft
A thunderstorm updraft which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.
Tipping-Bucket Rain Gage
A precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.
Toe of Dam
(Upstream and Downstream) : The junction of the face of a dam with the ground surface
Topography
The shape of the land.
Tornado
A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena.
Tornado Emergency
An exceedingly rare tornado warning issued when there is a severe threat to human life and catastrophic damage from an imminent or ongoing tornado. This tornado warning is reserved for situations when a reliable source confirms a tornado, or there is clear radar evidence of the existence of a damaging tornado, such as the observation of debris.
Tornado Family
A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line.
Tornado Vortex Signature
An image of a tornado on the Doppler radar screen that shows up as a small region of rapidly changing wind speeds inside a mesocyclone. The following velocity criteria is normally required for recognition: velocity difference between maximum inbound and outbound (shear) is greater than or equal to 90 knots at less than 30 nmi and is greater than or equal to 70 knots between 30 and 55 nmi. It shows up as a red upside down triangle on the Storm Relative Velocity Display. Existence of a TVS strongly increases the probability of tornado occurrence, but does not guarantee it; therefore, the feature triggering it must be examined closely by the radar operator. A TVS is not a visually observable feature.
Tornado Warning
This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.

A Tornado Warning is issued by your local National Weather Service office (NWFO). It will include where the tornado was located and what towns will be in its path. If the tornado will affect the nearshore or coastal waters, it will be issued as the combined product--Tornado Warning and Special Marine Warning. If the thunderstorm which is causing the tornado is also producing torrential rains, this warning may also be combined with a Flash Flood Warning. If there is an ampersand (&) symbol at the bottom of the warning, it indicates that the warning was issued as a result of a severe weather report.

After it has been issued, the affected NWFO will followed it up periodically with Severe Weather Statements. These statements will contain updated information on the tornado and they will also let the public know when warning is no longer in effect.
Tornado Watch
This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. Their size can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

A Tornado Watch is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. Prior to the issuance of a Tornado Watch, SPC will usually contact the affected local National Weather Forecast Office (NWFO) and they will discuss what their current thinking is on the weather situation. Afterwards, SPC will issue a preliminary Tornado Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public. After adjusting the watch, the NWFO will let the public know which counties are included by way of a Watch Redefining Statement. During the watch, the NWFO will keep the public informed on what is happening in the watch area and also let the public know when the watch has expired or been cancelled.
Total-Totals Index
A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. The total-totals index is the arithmetic sum of two other indices: the Vertical Totals Index (temperature at 850 mb minus temperature at 500 mb) and the Cross Totals Index (dew point at 850 mb minus temperature at 500 mb). As with all stability indices there are no magic threshold values, but in general, values of less than 50 or greater than 55 are considered weak and strong indicators, respectively, of potential severe storm development.
Trace
In hydrologic terms, a hydrograph or similar plot for an extended-range time horizon showing one of many scenarios generated through an ensemble forecast process.
Track
The path that a storm or weather system follows.
Trade Winds
Persistent tropical winds that blow from the subtropical high pressure centers towards the equatorial low.
Training
Repeated areas of rain, typically associated with thunderstorms, that move over the same region in a relatively short period of time and are capable of producing excessive rainfall totals. Train(ing) echoes can frequently be a source of flash flooding.
Transmitter
The radar equipment used for generating and amplifying a radio frequency (RF) carrier signal, modulating the carrier signal with intelligence, and feeding the modulated carrier to an antenna for radiation into space as electromagnetic waves. Weather radar transmitters are usually magnetrons or klystrons.
Transpiration
Water discharged into the atmosphere from plant surfaces.
Transport Wind
The average wind over a specified period of time within a mixed layer near the surface of the earth.
Transverse Bands
Bands of clouds oriented perpendicular to the flow in which they are embedded. They often are seen best on satellite photographs. When observed at high levels (i.e., in cirrus formations), they may indicate severe or extreme turbulence. Transverse bands observed at low levels (called transverse rolls or T rolls) often indicate the presence of a temperature inversion (or cap) as well as directional shear in the low- to mid-level winds. These conditions often favor the development of strong to severe thunderstorms.
Transverse Rolls
Elongated low-level clouds, arranged in parallel bands and aligned parallel to the low-level winds but perpendicular to the mid-level flow. Transverse rolls are one type of transverse band, and often indicate an environment favorable for the subsequent development of supercells. Since they are aligned parallel to the low-level inflow, they may point toward the region most likely for later storm development.
Trapper
A valley or basin in which cold air becomes trapped or pooled.
Travel Time
In hydrologic terms, the time required for a flood wave to travel from one location to a subsequent location downstream.
Tropical Advisory
Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropical cyclones.
Tropical Analysis and Forecast Branch
One of three branches of the Tropical Prediction Center (TPC). It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.
Tropical Cyclone
A warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center.
Tropical Cyclone Plan of the Day
A coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.
Tropical Cyclone Position Estimate
The National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center. As far as is possible, the position estimate is issued hourly near the beginning of the hour. The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point.
Tropical Cyclone Update
This brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings.
Tropical Depression
A tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Tropical Disturbance
A discrete tropical weather system of apparently organized convection--generally 100 to 300 mi in diameter--originating in the tropics or subtropics, having a nonfrontal migratory character and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
Tropical Storm
A tropical cyclone in which the maximum 1-minute sustained surface wind ranges from 34 to 63 knots (39 to 73 mph) inclusive.
Tropical Storm Summary
Written by the Hydrometeorological Prediction Center* (HPC) after subtropical and names tropical cyclones have moved inland and advisories have been discontinued. These advisories will be terminated when the threat of flash flooding has ended or when the remnants of these storms can no longer be distinguished from other synoptic features capable of producing flash floods. Storm summaries will not be issued for storms that enter the coast of Mexico and do not pose an immediate flash flood threat to the coterminous United States. They will be initiated when and if flash flood watches are posted in the United States because of an approaching system. Storm summaries will continue to be numbered in sequence with tropical cyclone advisories and will reference the former storm's name in the text. Summaries will be issued at 0100, 0700, 1300, and 1900 Eastern Daylight Time (EDT). The only exception will be the first one in the series may be issued at a nonscheduled time.
Tropical Storm Warning
An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are expected somewhere within the specified coastal area within 36 hours.
Tropical Storm Watch
An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are possible within the specified coastal area within 48 hours.
Tropical Wave
(formerly known as inverted trough) - A trough or cyclonic curvature maximum in the trade wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere or may be the reflection of an upper tropospheric cold low or an equatorward extension of a mid-latitude trough.
Tropical Weather Discussion
These messages are issued 4 times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics. One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West. Plain language is used in these discussions.
Tropical Weather Outlook
This outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time. In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time. For the Central Pacific, transmission times are 1000 and 2200 UTC. Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land. Given for each system are its location, size, intensity, and movement. For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it.
Tropical Weather Summary
The National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why.
Tropopause
The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).
Tropopause Jet
Type of jet stream found near the tropopause. Examples of this type of jet are the subtropical and polar fronts.
TROWAL
TROugh of Warm Air ALoft. Typically used during winter weather, it is a "tongue" of relatively warm/moist air aloft that wraps around to the north and west of a mature cyclone. It is best analyzed between 750-550 milibars using equivalent potential temperature (theta-e). Areas of intense lift and frontogenesis are commonly associated with TROWALs, hence they are favored regions for heavy and/or prolonged precipitation. During a winter storm, the heaviest snowfall amounts frequently occur along and north of the TROWAL axis.
TSRA
Thunderstorms with rain
Tsunami
A series of long-period waves (on the order of tens of minutes) that are usually generated by an impulsive disturbance that displaces massive amounts of water, such as an earthquake occurring on or near the sea floor. Underwater volcanic eruptions and landslides can also cause tsunami. The resultant waves much the same as waves propagating in a calm pond after a rock is tossed. While traveling in the deep oceans, tsunami have extremely long wavelengths, often exceeding 50 nm, with small amplitudes (a few tens of centimeters) and negligible wave steepness, which in the open ocean would cause nothing more than a gentle rise and fall for most vessels, and possibly go unnoticed. Tsunami travel at very high speeds, sometimes in excess of 400 knots. Across the open oceans, these high-speed waves lose very little energy. As tsunami reach the shallow waters near the coast, they begin to slow down while gradually growing steeper, due to the decreasing water depth. The building walls of destruction can become extremely large in height, reaching tens of meters 30 feet or more as they reach the shoreline. The effects can be further amplified where a bay, harbor, or lagoon funnels the waves as they move inland. Large tsunami have been known to rise to over 100 feet! The amount of water and energy contained in tsunami can have devastating effects on coastal areas.
Tsunami Advisory
For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Center will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami advisory is issued due to the threat of a potential tsunami which may produce strong currents or waves dangerous to those in or near the water. Coastal regions historically prone to damage due to strong currents induced by tsunamis are at the greatest risk. The threat may continue for several hours after the arrival of the initial wave, but significant widespread inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.
Tsunami Information Statement
A tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.
Tsunami Warning
For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert. Warnings advise that appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami warning is issued when a potential tsunami with significant widespread inundation is imminent or expected. Warnings alert the public that widespread, dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after arrival of the initial wave. Warnings also alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
Tsunami Watch
For products of the Pacific Tsunami Warning Center (PTWC - Pacific (except Alaska, British Columbia and Western States) Hawaii, Caribbean (except Puerto Rico, Virgin Is.), Indian Ocean): The second highest level of tsunami alert. Watches are issued based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves. Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area. For products of the West Coast/Alaska Tsunami Warning Center (WC/ATWC - Alaska, British Columbia and Western States, Canada, Eastern and Gulf States, Puerto Rico, U.S Virgin Islands): A tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
Two-Ribbon Flare
In solar-terrestrial terms, a flare that has developed as a pair of bright strands (ribbons) on both sides of the main inversion ("neutral") line of the magnetic field of the active region.
Typhoon Season
The part of the year having a relatively high incidence of tropical cyclones. In the western North Pacific, the typhoon season is from July 1 to December 15. Tropical cyclones can occur year-round in any basin.
U.S. Geological Survey
(Abbrev. USGS)- The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation’s rivers and streams that are useful for mitigation of hazards associated with floods and droughts.
Ultra High Frequency (UHF)
Those radio frequencies exceeding 300 MHz
Ultraviolet Index
This index provides important information to help you plan your outdoor activities in ways that prevent overexposure to the sun's rays. It was designed by the National Weather Service (NWS) and the Environmental Protection Agency (EPA). Unlike some countries' indices, the United States UV Index is not based upon surface observations. Rather, it is computed using forecasted ozone levels, a computer model that relates ozone levels to UV incidence on the ground, forecasted cloud amounts, and the elevation of the forecast cities. The calculation starts with measurements of current total ozone amounts for the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). These data are then used to produce a forecast of ozone levels for the next day at various points around the country.

CategoryUV IndexTime to BurnActions to Take
Minimal0 - 260 min. +Apply SPF sunscreen.
Low3 - 445 min.Apply SPF sunscreen, wear a hat.
Moderate5 - 630 min.Apply SPF 15, wear a hat.
High7 - 915 - 24 min.Apply SPF 15 to 30, wear a hat and sunglasses. Limit midday exposure.
Very High10+10 min.Apply SPF 30; wear a hat, sunglasses, and protective clothing; limit midday exposure.
Ultraviolet Radiation
Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays.
Umbra
In solar-terrestrial terms, the dark core or cores (umbrae) in a sunspot with penumbra, or a sunspot lacking penumbra.
Unimodal
A distribution having only one localized maximum, i.e., only one peak.
Unit Hydrograph
The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period
United States Drought Monitor (USDM)
A multi-agency product that defines and highlights the severity of drought across the ConUS and OConUS based on expert interpretation of a multitude of indices.
Universal Geographic Code
(UGC) - UGC's, (e.g. ANZ300 for Western Long Island Sound) are used in many National Weather Service text products to provide geographical information. This allows users easy automated processing and redistribution of the information. More specifically, the purpose of the UGC are to specify the affected geographic area of the event, typically by state, county (or parish), or unique NWS zone (land and marine). The only exception to the above is to define the weather synopsis part of certain marine products.
Universal Time (UT)
By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".

More about UTC, and a table to convert UTC to your local time is posted at: http://www.srh.noaa.gov/srh/jetstream/doppler/radarfaq.htm#utc
Unstable Air
Air that is able to rise easily, and has the potential to produce clouds, rain, and thunderstorms.
Up-valley Wind
A diurnal thermally driven flow directed up a valley's axis, usually occurring during daytime; part of the along-valley wind system.
Updraft
A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
Upper Level Disturbance
A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.
Upper-air Weather Chart
Weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.
Upstream
Towards the source of flow, or located in the area from which the flow is coming.
Upstream Slope
The part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock riprap or concrete.
Urban and Small Stream Flood Advisory
This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.
Urban and Small Stream Flooding
Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.
Urban Flash Flood Guidance
A specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area.
Urban Flooding
Flooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening.
Urban Heat Island
The increased air temperatures in urban areas in contrast to cooler surrounding rural areas.
VAAC
Volcanic Ash Advisory Centers
VAD
Velocity Azimuth Display
VAD Wind Profile
A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets). This product often is referred to erroneously as a VAD.
Vadose Zone
The locus of points just above the water table where soil pores may either contain air or water. This is also called the zone of aeration
VALDRIFT
An air pollution transport and diffusion model developed to determine pesticide drift from aerial spraying operations in valleys.
Valid Time
The period of time during which a forecast or warning, until it is updated or superseded by a new forecast issuance, is in effect.
Valid Time Event Code
(VTEC) - The Valid Time Event Code (VTEC) always is used in conjunction with, and provides supplementary information to, the Universal Geographic Code (UGC), to further aid in the automated delivery of National Weather Service text products to users. The VTEC is included in many event driven or non-routine products and in some routine Marine forecasts. The VTEC provides information on the event, while the UGC describes the affected geographic area.
Valley Exit Jet
A strong elevated down-valley air current issuing from a valley above its intersection with the adjacent plain.
Valley Volume Effect
The reduction in volume of a valley (or basin) as compared to an equal depth volume with a horizontal floor. Because the valley volume is smaller, equivalent heat fluxes will cause larger changes in temperature in the valley volume than in the flat-floor volume.
Valve
In hydrologic terms, a device fitted to a pipeline or orifice in which the closure member is either rotated or moved in some way as to control or stop flow.
Vapor Pressure
The partial pressure of water vapor in an air-water system.
Variable Infiltration Capacity (VIC)
A large-scale, semi-distributed hydrologic model that solves full water and energy balances. As such, it shares several basic features with other land surface models that are commonly coupled to global circulation models.
Variable Wind
Same as Variable Wind Direction; a condition when
(1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Variable Wind Direction
A condition when
(1) the wind direction fluctuates by 60° or more during the 2-minute evaluation period and the wind speed is greater than 6 knots; or
(2) the direction is variable and the wind speed is less than 6 knots.
Variance
A measure of variability.
Velocity Azimuth Display
A WSR 88-D product which shows the radar derived wind speeds at various heights. This radar product shows the wind speeds from 2,000 to 55,000 feet above the ground. VAD and EVAD (Extended VAD) are methods of guessing the large scale two-dimensional winds from one-dimensional radial velocity data. They are essentially multivariate regressions which fit a simple, large scale wind model to the observed winds. EVAD also estimates the large scale horizontal divergence and particle fall speed. See VWP.
Ventilation Index
Product of the mixing depth and transport wind speed, a measure of the potential of the atmosphere to disperse airborne pollutants from a stationary source. Sometimes referred to as a Clearing Index.
Vertical Velocity
The component of velocity (motion) in the vertical. The evaluation of areas of upward vertical velocity is key to forecasting areas of active weather.
Vertical Wind Shear
the change in the wind's direction and speed with height. This is a critical factor in determining whether severe thunderstorms will develop.
Vertically Stacked System
A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. Such systems typically are weakening and are slow-moving, and are less likely to produce severe weather than tilted systems. However, cold pools aloft associated with vertically-stacked systems may enhance instability enough to produce severe weather.
Virga
Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.
Virtual Potential Temperature
The virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.
Virtual Temperature
The virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.
Visible Infra-Red Imaging Radiometer (VIIRS)
A medium-resolution sensor flown aboard the NOAA-20 and Suomi-NP satellites.
Visible Satellite Imagery
This type of satellite imagery uses reflected sunlight (this is actually reflected solar radiation) to see things in the atmosphere and on the Earth's surface. Clouds and fresh snow are excellent reflectors, so they appear white on the imagery. Clouds can be distinguished from snow, because clouds move and snow does not move. Meanwhile, the ground reflects less sunlight, so it appears black on the imagery. The satellite uses its 0.55 to 0.75 micrometer (um) channel to detect this reflected sunlight. Since this imagery relies on reflected imagery, it cannot be used during night.
Visual Spectrum
The portion of the electromagnetic spectrum to which the eye is sensitive, i.e., light with wavelengths between 0.4 and 0.7 micrometers. Compare shortwave radiation and longwave radiation.
Volcanic Ash
Fine particles of mineral matter from a volcanic eruption which can be dispersed long distances by winds aloft. The chemical composition and abrasiveness of the particles can seriously affect aircraft and also machinery on the ground. If it is blown into the stratosphere and it is thick enough, it can decrease the global temperature.
Volume Scan
A radar scanning strategy in which sweeps are made at successive antenna elevations (i.e., a tilt sequence), and then combined to obtain the three-dimensional structure of the echoes.
Voluntary Observing Ship Program
(VOS) - An international voluntary marine observation program under the auspices of the World Meteorological Organization (WMO). Observations are coded in a special format known as the ships synoptic code, or "BBXX" format. They are then distributed for use by meteorologists in weather forecasting, by oceanographers, ship routing services, fishermen, and many others.
Vort Max
Common slang reference to Vorticity Maximum; a center, or maximum, in the vorticity field of a fluid.
WAA
Warm Air Advection - the advection (movement) of warm air into a region.
Wake
The region of turbulence immediately to the rear of a solid body caused by the flow of air over or around the body.
Wall Cloud
A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation.

However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion.

"Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.
Warm Advection
Transport of warm air into an area by horizontal winds. Low-level warm advection sometimes is referred to (erroneously) as overrunning. Although the two terms are not properly interchangeable, both imply the presence of lifting in low levels.
Warm Core Low
A low pressure area which is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern. Unlike cold core lows, these lows produce much of their cloud cover and precipitation during the nighttime.
Warm Front
A transition zone between a mass of warm air and the colder air it is replacing.
Warm Occlusion
A frontal zone formed when a cold front overtakes a warm front and, finding colder air ahead of the warm front, leaves the ground and rises up and over this denser air. Compare with cold occlusion.
Warm Sector
A region of warm surface air between a cold front and a warm front.
Warning
A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property.
Wasatch Wind
A strong easterly wind blowing out of the mouths of the canyons of the Wasatch Mountains onto the plains of Utah. Also called canyon wind.
Watch
A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so that those who need to set their plans in motion can do so.
Watch Box
(or simply "Box") - slang for a Severe Thunderstorm Watch or Tornado Watch issued by the SPC.
Watch Cancellation
This product will be issued to let the public know when either a Tornado Watch or Severe Thunderstorm Watch has been canceled early. It is issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. In the text of the statement it will specify the severe weather watch number and the area which the watch covered.
Watch Redefining Statement
This product tells the public which counties/parishes are included in the watch. This is done not only by writing them all out, but by using the county FIPS codes in the Header of the product. It is issued by the local National Weather Service Forecast Office (WFO).
Watch Status Reports
This product lets the NWFO know of the status of the current severe weather watch (Tornado or Severe Thunderstorm). During the severe weather watch, the Storm Prediction Center (SPC) will issue these reports periodically. These reports will describe, in plain language, the current evaluation of the severe weather situation and whether the watch will expire or be reissued. A status report is not issued if a cancellation or replacement has been issued at least 1 hour prior to the expiration time of the original watch.
Water Equivalent
The liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.
Water Pollution
The alteration of the constituents of a body of water by man to such a degree that the water loses its value as a natural resource.
Water Supply Outlook
A seasonal volume forecast, generally for a period centered around the time of spring snowmelt (e.g., April-July). The outlooks are in units of acre-feet and represent the expected volume of water to pass by a given point during a snowmelt season. The outlook categories include Most Probable, Reasonable Maximum, and Reasonable Minimum.
Water Table
The level below the earth's surface at which the ground becomes saturated with water. The water table is set where hydrostatic pressure equals atmospheric pressure.
Water Vapor Plume
This appears in the water vapor satellite imagery. It is a plume-like object that extends from the Intertropical Convergence Zone (ITCZ) northward or southward into the higher latitudes. It is usually located over a 850 to 700 mb theta-e ridge axis. As a result, it is a favored location for the development of a Mesoscale Convective Complex (MCC). Researchers have found it to be a favored region for very heavy rain. It is thought that the ice crystals located in this plume help thunderstorms to become highly efficient rainfall producers. In North America, this is sometimes called the "Mexican Connection".
Water Year
The time period from October 1 through September 30.
Watercourse
Any surface flow such as a river, stream, tributary.
Watershed
Land area from which water drains toward a common watercourse in a natural basin.
Waterspout
In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.

The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.
Watt
Unit of power in the MKS system of units; energy per unit of time, one Joule per second (1 J/s). Abbreviated W.
Wave Crest
The highest part of a wave
Wave Height
Distance from wave trough to wave crest.
Wave Period
Time, in seconds, between the passage of consecutive wave crests past a fixed point.
Wave Spectrum
The distribution of wave energy with respect to wave frequency or period. Wave spectra assist in differentiating between wind waves and swell.
Wave Steepness
The ratio of wave height to wavelength and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio; the wave typically becomes unstable and begins to break.
Wave Trough
The lowest part of the wave.
Wavelength
Distance between crests or troughs of a wave.
WAVEWATCH III
One of the operational forecast models run at NCEP. The WWIII is run four times daily, with forecast output out to 126 hours.
WC/ATWC
West Coast and Alaska Tsunami Warning Center. The National Weather Service's West Coast and Alaska Tsunami Warning Center in Palmer, AK has a regional tsunami responsibility for the Canadian coastal regions and the ocean coasts of all U.S. States except Hawaii. See also PTWC.
Weak Echo Region
(Abbrev. WER) - A WSR-88D radar product which displays reflectivity for up to 8 elevation angles for a radar operator selected location as a set presentation of a storm. The plains in this product are presented in an ascending order, lowest plain is lowest elevation angle selected. It is used to depict storm tilt and to identify Weak Echo Regions (WER) and Bounded Weak Echo Regions (BWER) in thunderstorms.
Weather
The state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. Weather refers to these conditions at a given point in time (e.g., today's high temperature), whereas Climate refers to the "average" weather conditions for an area over a long period of time (e.g., the average high temperature for today's date).
Weather Forecast Office
(Abbrev. WFO) - this type of National Weather Service office is responsible for issuing advisories, warnings, statements, and short term forecasts for its county warning area
Weatherfax
See RADIOFACSIMILE
Wedge Tornado
Slang for a large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. The term "wedge" often is used somewhat loosely to describe any large tornado. However, not every large tornado is a wedge. A true wedge tornado, with a funnel at least as wide at the ground as it is tall, is very rare.

Wedges often appear with violent tornadoes (F4 or F5 on the Fujita Scale), but many documented wedges have been rated lower. And some violent tornadoes may not appear as wedges (e.g., Xenia, OH on 3 April 1974, which was rated F5 but appeared only as a series of suction vortices without a central condensation funnel). Whether or not a tornado achieves "wedge" status depends on several factors other than intensity - in particular, the height of the environmental cloud base and the availability of moisture below cloud base. Therefore, spotters should not estimate wind speeds or F-scale ratings based on visual appearance alone. However, it generally is safe to assume that most (if not all) wedges have the potential to produce strong (F2/F3) or violent (F4/F5) damage.
WEFAX
System for transmitting weather charts and imagery via satellite. Occasionally used as an abbreviation for radiofacsimile via HF radio.
Weighing-Type Precipitation Gage
A rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance. The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.
West African Disturbance Line
A line of convection about 300 miles long, similar to a squall line. It forms over west Africa north of the equator and south of 15 degrees North latitude. It moves faster than an Easterly Wave between 20 and 40 mph. They move off the African coast every 4 to 5 days mainly in the summer. Some reach the American tropics and a few develop into tropical cyclones.
West Wall
The coast side boundary of the Gulf Stream, typically south of Cape Hatteras. See also North Wall
Wet Bulb Globe Temperature
An indication of expected heat stress on humans taking into account temperature, humidity, wind speed, sun angle and cloud cover (solar radiation).
Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporting water into the air.
Wetland
In hydrologic terms, an area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.
White Light Flare
In solar-terrestrial terms, a major flare in which small parts become visible in white light. Such flares are usually strong X-ray, radio, and particle emitters.
Whitecap
The breaking crest of a wave, usually white and frothy.
Widespread
Areal coverage of non-measurable, non-convective weather and/or restrictions to visibility affecting more than 50 percent of a forecast zone(s).
Wildlands
Any nonurbanized land not under extensive agricultural cultivation, e.g., forests, grasslands, rangelands.
Wind Advisory
This product is issued for strong winds that are not expected to reach warning criteria. Generally for sustained wind speeds of 30 to 39 mph lasting for 1 hour or longer.
Wind Radii
Term used in National Weather Tropical Cyclone Forecast Advisory products (TCM). Wind radii are the largest radii of that wind speed found in that quadrant. Quadrants are defined as NE (0-90), SE (90-180), SW (180-270), and NW (270-0). As an example, given maximum 34 knot radii to 150 nm at 0 degrees, 90 at 120 degrees, and 40 nm at 260 degrees, the following line would be carried in the forecast/advisory: 150NE 90SE 40SW 150NW.
Wind Shear
The rate at which wind velocity changes from point to point in a given direction (as, vertically). The shear can be speed shear (where speed changes between the two points, but not direction), direction shear (where direction changes between the two points, but not speed) or a combination of the two.
Wind Shear Profile
The change in wind speed and/or direction usually in the vertical. The characteristics of the wind shear profile are of critical importance in determining the potential for and type of severe weather.
Wind Waves
Local, short period waves generated from the action of wind on the water surface (as opposed to swell). Commonly referred to as waves. In a National Weather Service Coastal Marine Forecast or Offshore Forecast, wind waves are used when swells are described in the forecast.
or
Waves generated by the local wind blowing at the time of observation.
Windward
The side toward the wind. Compare with leeward.
Winter Storm Warning
This product is issued by the National Weather Service when a winter storm is producing or is forecast to produce heavy snow or significant ice accumulations. The criteria for this warning can vary from place to place.
Winter Storm Watch
This product is issued by the National Weather Service when there is a potential for heavy snow or significant ice accumulations, usually at least 24 to 36 hours in advance. The criteria for this watch can vary from place to place.
Winter Weather Advisory
This product is issued by the National Weather Service when a low pressure system produces a combination of winter weather (snow, freezing rain, sleet, etc.) that present a hazard, but does not meet warning criteria.
Wire Weight Gage
In hydrologic terms, a river gage comprised of a weight which is lowered to the water level. The weight is attached to a cable; and as the weight is lowered, a counter indicates the length of cable released. The stage is determined from the length of cable required to reach the water level.
Wrapping Gust Front
A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.
X-Band
A frequency band of microwave radiation in which radars operate.
X-Ray Background
In solar-terrestrial terms, a daily average background X-ray flux in the 1 to 8 angstrom range. It is a midday minimum designed to reduce the effects of flares.
X-Ray Burst
In solar-terrestrial terms, a temporary enhancement of the X-ray emission of the sun. The time-intensity profile of soft X-ray bursts is similar to that of the H-alpha profile of an associated flare.
X-Ray Flare Class
In solar-terrestrial terms, rank of a flare based on its X-ray energy output. Flares are classified by the order of magnitude of the peak burst intensity (I) measured at the earth in the 1 to 8 angstrom band as follows:

ClassIntensity (in Watts/m2)
BI < 10-6
C10-6 <= I < 10-5
M10-5 <= I < 10-4
XI >= 10-4
X-Rays
Very energetic electromagnetic radiation with wavelengths intermediate between 0.01 and 10 nanometers (0.1-100 Angstroms) or between gamma rays and ultraviolet radiation. Essentially all X-Rays from space are absorbed in the Earth's upper atmosphere.
YDA
Yesterday
Year
The period during which the Earth completes one revolution around the sun.
Zero Datum
In hydrologic terms, a reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the channel) to mean sea level, or to any other recognized datum.
Zonal Flow
Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.
Zone of Aeration
In hydrologic terms, the locus of points just above the water table where soil pores may either contain air or water. This is also called the vadose zone
Zone of Saturation
In hydrologic terms, the locus of points below the water table where soil pores are filled with water. This is also called the phreatic zone
Zoned Embankment Dam
In hydrologic terms, an embankment dam which is comprised of zones of selected materials having different degrees of porosity, permeability and density.
Zurich Sunspot Classification
In solar-terrestrial terms, a sunspot classification system that has been modified for SESC use.
Z\/R Relationship
An empirical relationship between radar reflectivity factor z (in mm^6 / m^3 ) and rain rate ( in mm / hr ), usually expressed as Z = A R^b; A and b are empirical constants.

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