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.
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
Aleutian Current
An eastward flowing North Pacific Ocean current which lies north of the North Pacific Current.
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).
Antilles Current
A current which originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current.
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-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.
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.
Azores Current
One of the currents of the North Atlantic subtropical gyre.
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.
The flow, or movement, of a fluid (e.g., water or air) in or through a given area or volume.
A cirriform cloud characterized by thin, white patches, each of which is composed of very small granules or ripples. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).
Crepuscular Rays
The alternating bands of light and dark (rays and shadows) seen at the earth's surface when the sun shines through clouds.
Cumulus clouds - Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus.
Cubic Feet per Second
(Abbrev. CFP) - In hydrologic terms, a unit expressing rates of discharge. One cubic foot per second is equal to the discharge through a rectangular cross section, 1 foot wide by 1 foot deep, flowing at an average velocity of 1 foot per second. It is also approximately 7.48 gallons per second.
Cumulus Fractus
Descriptive of all clouds with vertical development in the form of rising mounds, domes, or towers
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.
(Abbrev. CU) - detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus.
Cumulus Buildups
Clouds which develop vertically due to unstable air. Characterized by their cauliflower-like or tower-like appearance of moderately large size
Cumulus Congestus
A large, towering cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a cumulonimbus.
A horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the sun, moon, and earth and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called TIDE. Tidal currents are periodic with a net velocity of zero over the tidal cycle. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. The SET of a current is the direction toward which it flows; the DRIFT is its speed.
Current Meter
In hydrologic terms, device used to measure the water velocity or current in a river.
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.
In hydrologic terms, from passing through a dam’s foundation material. An impervious construction or material which reduces seepage or prevents it.
Cutoff Low
A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft (i.e., retrogression).

"Cutoff low" and "closed low" often are used interchangeably to describe low pressure centers aloft. However, not all closed lows are completely removed from the influence of the basic westerlies. Therefore, the recommended usage of the terms is to reserve the use of "cutoff low" only to those closed lows which clearly are detached completely from the westerlies.
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.
Density Current
In hydrologic terms, a flow of water maintained by gravity through a large body of water, such as a reservoir or lake, and retaining its unmixed identity because of a difference in density.
Depletion Curve
In hydrologic terms, the part of the hydrograph extending from the point of termination of the Recession Curve to the subsequent rise or alternation of inflow due to additional water becomming available for stream flow.
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.
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.
Ebb Current
The movement of a tidal current away form the coast or down an estuary.
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.
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.
In solar-terrestrial terms, a bright region of the photosphere seen in white light, seldom visible except near the solar limb.
Flood Frequency Curve
In hydrologic terms,
(1) A graph showing the number of times per year on the average, plotted as abscissa, that floods of magnitude, indicated by the ordinate, are equaled or exceeded.
(2) A similar graph but with recurrence intervals of floods plotted as abscissa.
Flow Duration Curve
In hydrologic terms, a cumulative frequency curve that shows the percentage of time that specified discharges are equaled or exceeded.
A cumulus cloud presenting a ragged, shredded appearance, as if torn.
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.
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
Inches of Mercury
(or in Hg) Unit of atmospheric pressure used in the United States. The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. See barometric pressure. First divised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

To convert millibars (mb) to inches of mercury (in Hg), divide the millibar reading by 33.86:
in Hg = mb / 33.86
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.
In hydrologic terms, the curved surface of the liquid at the open end of a capillary column
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.
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.
The smallest particle of a substance that retains the properties of the substance and is composed of one or more atoms.
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.
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.).
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.
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.
Used especially in reference to the recurrence of physical parameters every 27 days (the rotation period of the sun)
Rip Current
A relatively small-scale surf-zone current moving away from the beach. Rip currents form as waves disperse along the beach causing water to become trapped between the beach and a sandbar or other underwater feature. The water converges into a narrow, river-like channel moving away from the shore at high speed.
Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.
A headline within National Weather Service high seas forecasts transmitted via the GMDSS to indicate that no hurricane or hurricane force winds are forecast.
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.
Steering Currents
Same as Steering Winds; a prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
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.
Towering cumulus clouds
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.
Towering Cumulus
A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU.)
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.
In hydrologic terms, a current below the upper currents or surface of a fluid body.

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