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.
Adiabatic Process
A process which occurs with no exchange of heat between a system and its environment.
AFSS
Automated Flight Service Station
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.
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.
ASSOCIATED PRINCIPAL USER
A Principal User with dedicated communications to a WSR-88D unit.
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.
BANDPASS FILTER
A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
Barometric Pressure
The pressure of the atmosphere as indicated by a barometer.
Bergeron Process
The process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.
Brightness
A basic visual sensation describing the amount of light that appears to emanate from an object, or more precisely, the luminance of an object
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.
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.
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.
Cone of Depression
In hydrologic terms, the depression, roughly conical in shape, produced in a water table, or other piezometric surface, by the extraction of water from a well at a given rate. The volume of the cone will vary with the rate of withdrawal of water. Also called the Cone of Influence.
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.
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.
Conveyance Loss
In hydrologic terms, the loss of water from a conduit due to leakage, seepage, evaporation, or evapo-transpiration.
Cross-Valley Wind System
A thermally driven wind that blows during daytime across the axis of a valley toward the heated sidewall.
Deep Percolation Loss
In hydrologic terms, water that percolates downward through the soil beyond the reach of plant roots.
Depression
A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation. The term is also sometimes used as a reference to a Tropical Depression.
Depression Storage
In hydrologic terms, the volume of water contained in natural depressions in the land surface, such as puddles.
Dew Point Depression
The difference in degrees between the air temperature and the dew point.
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.
Emissivity
The ability of a surface to emit radiant energy compared to that of a black body at the same temperature and with the same area.
Energy Dissipator
In hydrologic terms, a structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.
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.
Excess Rain
In hydrologic terms, effective rainfall in excess of infiltration capacity.
Excessive Heat
Excessive heat occurs from a combination of high temperatures (significantly above normal) and high humidities. At certain levels, the human body cannot maintain proper internal temperatures and may experience heat stroke. The "Heat Index" is a measure of the effect of the combined elements on the body.
Excessive Heat Outlook
This CPC product, a combination of temperature and humidity over a certain number of days, is designed to provide an indication of areas of the country where people and animals may need to take precautions against the heat during May to November.
Excessive Heat Warning
Issued within 12 hours of the onset of the following criteria: heat index of at least 105°F for more than 3 hours per day for 2 consecutive days, or heat index more than 115°F for any period of time.
Excessive Heat Watch
Issued by the National Weather Service when heat indices in excess of 105ºF (41ºC) during the day combined with nighttime low temperatures of 80ºF (27ºC) or higher are forecast to occur for two consecutive days.
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.
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.
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.
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.
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.
GMDSS
(Global Maritime Distress and Safety System)- The Global Maritime Distress and Safety System (GMDSS) is intended to provide more effective and efficient emergency and safety communications and disseminate Maritime Safety Information (MSI) to all ships on the world's oceans regardless of location or atmospheric conditions. MSI includes navigational warnings, meteorological warnings and forecasts, and other urgent safety related information GMDSS goals are defined in the International Convention for The Safety Of Life At Sea (SOLAS). The National Weather Service participates directly in the GMDSS by preparing meteorological forecasts and warnings for broadcast via NAVTEX and SafetyNET.
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.
Head Loss
In hydrologic terms, the decrease in total head caused by friction
Index of Wetness
The ratio of precipitation for a given year over the mean annual precipitation.
Isobaric Process
Any thermodynamic change of state of a system that takes a place at constant pressure.
Issuance Time
The time the product is transmitted.
Low Pressure System
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.
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.
Massif
A compact portion of a mountain range, containing one or more summits.
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.
Mount Wilson Magnetic Classifications
In solar-terrestrial terms, a classification system for sunspots:
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.
NSSFC
National Severe Storm Forecast Center
NSSL
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.
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.
Pressure
The exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it.
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 Couplet
It is an area where you have a high pressure area located adjacent to a low pressure area.
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 Ice
Floating sea, river, or lake ice that has been deformed, altered, or forced upward in pressure ridges by the lateral stresses of any combination of wind, water currents, tides, waves, and surf.
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 Jump
A sudden, sharp increase in atmospheric pressure, typically occurring along an active front and preceding a storm.
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 Tendency
The character and amount of atmospheric pressure change during a specified period of time, usually 3-hour period preceding an observation.
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.
Prognostic Discussion
This Hydrometeorological Prediction Center (HPC) discussion may include analysis of numerical and statistical models, meteorological circulation patterns and trends, and confidence factors. Reference is usually made to the manually produced 6- to 10-day Northern Hemisphere prognoses for mean 500 millibar heights and mean 500 millibar height anomalies. Discussions may also refer to the method of operational ensemble predictions.
Progressive Derecho
Derecho characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow. Downburst activity occurs along the bulging portion of the line. This type of derecho typically occurs in the warm season (May through August) and is most frequent in a zone extending from eastern South Dakota to the upper Ohio Valley. The severe wind storms typically begin during the afternoon and continue into the evening hours. Several hours typically pass between initial convection and the first wind damage report.
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".
QPF Discussion
This HPC forecast discussion is directed completely to explaining manual forecasts of areas in the contiguous 48 states expected to receive 1/4 inch or more precipitation during a 24-hour period. The manual forecasts are explained in terms of initial conditions and differences and/or similarities in the numerical model forecasts. General confidence in the manual forecast is expressed where it is appropriate and possible alternatives may be offered. This product is issued 3 times a day.
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).
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
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.
Reflectivity Cross Section
This WSR-88D radar product displays a vertical cross section of reflectivity on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Cross Section is similar to the Range Height Indicator (RHI) slices observed on conventional radar, but it is not limited to alignments along the radar radials. Instead the 2 end points are operator selected anywhere within 124 nm of the radar that are less than 124 nm apart. It is used to:
1) Examine storm structure features such as overhang, tilt, Weak Echo Regions (WER), and Bounded Weak Echo Regions (BWER);
2) Estimate height of higher dBZ's and echo tops; and
3) Locate the bright band (where snow is melting and becoming rain).
Retrogression
(or Retrograde Motion) - Movement of a weather system in a direction opposite to that of the basic flow in which it is embedded, usually referring to a closed low or a longwave trough which moves westward.
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.
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.
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.
Snow Data Assimilation System (SNODAS)
A physically based, mass conserving snow water equivalent (SWE) model.
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.
Spectrum Width Cross Section
This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data
2) Estimate vertical extent of turbulence (aviation use).
SS
Sandstorm
SSHS
Saffir/Simpson Hurricane Scale
SST
Sea Surface Temperature
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.
Steepness
In marine terms, on a buoy report, wave steepness is the ratio of wave height to wave length and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio, the wave becomes unstable and begins to break.
Subtropical Depression
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Sunspot Group Classification
Tropical Depression
A tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
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.
Vapor Pressure
The partial pressure of water vapor in an air-water system.
Velocity Cross Section
This WSR-88D radar product displays a vertical cross section of velocity on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. The two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Examine storm structure features such as location of updrafts/downdrafts, strength of storm top divergence, and the depth of mesocyclones;
2) Locate areas of convergence/divergence (when generated along a radial; and
3) Analyze areas of rotation (when generated from one AZRAN to another).
Volume Velocity Processing
A way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.
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.
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
Zurich Sunspot Classification
In solar-terrestrial terms, a sunspot classification system that has been modified for SESC use.

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