Ablation: Depletion of snow and ice by melting and evaporation.

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 Seepage:   Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.

ACCAS:  An acronym for Altocumulus Castellanus.  See Altocumulus Castellanus.

Accessory CloudA 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.

AccuracyDegree of conformity of a measure to a standard or true value.

Achluophobia:  The fear of darkness.

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.
Active Conservation Storage:  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 (Usable) Storage Capacity:  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.

A/D ConverterAnanlog-to-digital converter. The electronic device which converts the radar receiver analog (voltage) signal into a number (or count or quanta)

Adirondack Type Snow Sampling Set:  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.

Administrative and Forest Fire Information Retrieval and Management System (AFFIRMS):  A United States Department of Agriculture (USDA) Forest Service interactive computer program used on a time-share computer to collect and disseminate fire weather observations and forecasts, and to compute fire danger indices of the National Fire Danger Rating System.

Administrative Messages (ADM):  Meteorological Operations Division (MOD) is responsible for issuing routine and special administrative messages that provide information to the field and outside users.  These messages contain the following information:  1) the current status of the model run cycles (e.g., any delays in model guidance); 2) upper-air sounding data that were edited or deleted before their use by the models; 3) delays in the creation and/or distribution of MOD products; and 4) delays in the creation and/or distribution of data sets processed on the Hydrometeorological Prediction Center (HPC).  At a minimum, this administrative message is issued once every model cycle at approximately 9:45 AM/PM EST (10:45 AM/PM EDT) by the Senior Duty Meteorologist.  Additional messages are issued as required.
Advection: The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture.

Advection Fog: It is formed as warmer, moist air moves over a cold ground. The air is cooled to saturation by the loss of heat to the cold ground. Unlike radiation fog, advection fog may form under cloudy skies and with moderate to strong winds. Initial stability is relatively unimportant since low level cooling makes the air unstable near the ground.

ADVIS:  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: 1) 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.  2) 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.

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.

Aerophobia:  The fear of drafts, air swallowing, or air bourne noxious substances.

AFOS:  Automation of Field Operations and Services.  This system was installed in the early to mid 1980s and it is being replaced by Advanced Weather Interactive Processing System (AWIPS).
Afterbay:  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.
Agglomerate:  An ice cover of floe formed by the freezing together of various forms of ice.
AHOS:  Automatic Hydrologic Observing System
AHOS-S:  Automatic Hydrologic Observing System - Satellite
AHOS-T:  Automatic Hydrologic Observing System - Telephone
Airborne Snow Survey Program:  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.
Airborne Snow Water Equivalent Measurement Theory A theory based on the fact that natural terrestrial gamma radiation is emitted from the potassium, uranium, and thorium radioisotopes in the upper eight inches of the soil. The radiation is sensed from low flying aircraft 500 feet above the ground. Water mass in the snow cover attenuates the terrestrial radiation signal. The difference between airborne radiation measurements made over bare ground and snow-covered ground can be used to calculate a mean areal snow water equivalent value with a root mean square error of less than a half inch.

Air Mass: A large body of air that has nearly uniform conditions of temperature and humidity.

Air Mass ThunderstormGenerally, 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; therefore, the term is somewhat controversial and should be used with discretion. Also, see Popcorn Convection and Single Cell Thunderstorm.

AIRMET (AIRman's METeorological Information):  This NWS aviation product advises of weather that maybe hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots. However, operators of large aircraft may also be concerned with these phenomena. The items covered are:

     In the Airmet Sierra bulletin:
          Ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time.
          Extensive mountain obscuration
     In the Airmet Tango bulletin:
          Moderate turbulence
          Sustained surface winds of 30 knots or more at the surface
     In the Airmet Zulu bulletin:
          Moderate icing
          Freezing levels

These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

AIRMETs are routinely issued for 6 hour periods beginning at 0145 UTC during Central Daylight Time and at 0245 UTC during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancelation of a SIGMET .

Air Quality Statement(AQI) This National Weather Service product is issued to relay air pollution information and issue Ozone Action Days.

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 AdvisoryThis 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 Transportatable 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.

Albedo:  The portion of incoming radiation which is reflected by a surface.
ALERT Flood Warning System A cooperative, community-operated flood warning system; the acronym stands for Automated Local Evaluation (in) Real Time.

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".

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.

AlgorithmA 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.

AliasingThe process by which frequencies too high to be analyzed with the given sampling interval appear at a frequency less than the Nyquist frequency.

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.

Altocumulus (Ac): These clouds are composed of mainly water. They appear as white or gray colored roll like elements or bands. The individual elements are large and darker than in cirrocumulus clouds. These clouds from between 6,500 and 23,000 feet.

Altocumulus Castellanus (ACCAS): They are middle level convective clouds and possibly they should be classified as clouds with extensive vertical development. They are composed of mainly water vapor. They are characterized by their billowing tops and comparatively high bases. These clouds from between 6,500 and 23,000 feet.  These clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.

Altocumulus Standing Leticular (ACSL): These clouds are formed on the crests of waves crested by barriers in the wind flow. The clouds show little movement hence the name standing. Wind, however, can be quite strong blowing through the cloud. They are characterized by their smooth, polished edges. These may also form on wave crests. They are composed of mainly water vapor and they are generally duller than Cirrocumulus Standing Leticular (CCSL). These clouds from between 6,500 and 23,000 feet.

Altostratus (As): It is a bluish veil or layer of clouds having a fibrous appearance. The outline of the sun may show dimly as through frosted glass. It often merges gradually into cirrostratus. As with cirrostratus, it often is part of a cloud shield associated with a front. This type of cloud is composed of mainly water vapor and result from lifting a layer. These clouds form between 6,500 and 23,000 feet.

Alluvial An adjective referring to alluvium.
Alluvium:  Sediments deposited by erosional processes, usual by streams.

AmplitudeThe maximum magnitude of a quantity.

Anabranch:  A diverging branch of a river which re-enters the main stream.

AnalogClass of devices in which the output varies continuously as a function of the input.

AncraophobiaThe fear of the wind.  See Anemophobia.
Anchor Ice:  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.

AnemophobiaThe fear of air drafts or wind.  See Ancraophobia.

Aneroid Barometer: An instrument designed to measure atmospheric pressure. It contains no mercury.

Angels: Radar echoes caused by birds, insects, and localized refractive index discontinuities.

Angular Area of SphereEquals 4*pi steradians.

Annual Flood:  The maximum discharge peak during a given water year (October 1 - September 30).

Anomalous Propagation (AP): Radio wave propagation that occurs due to non-standard atmospheric conditions. Most common usage refers to ground returns detected by downward bending of the radar beam.

Antecedent Precipitation Index (API): A measure of how much moisture in the top layer of soil within a drainage basin. In other words, this is a soil moisture index.

Antecedent Precipitation Index (API) Method: 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.

AntennaA transducer between electromagnetic waves radiated through space and electromagnetic waves contained by a transmission line.

Antenna Gain:  The measure of effectiveness of a directional antenna as compared to an isotropic radiator; maximum values called 'antenna gain' by convention. Gain can be defined as: (power at point with antenna) / (power at same point with isotropic antenna).

Antenna ReflectorThe portion of an antenna system which reflects the energy from the radiating element into a focused beam; generally circular parabolas for weather radars.

Attenuation:  Any process in which the flux density (power) of a beam of energy is dissipated.

Anticipated Convection (AC):  The Day 1 and Day 2 Convective outlooks issued by the Storm Prediction Center in Norman, Oklahoma.  The term originates from the header coding (ACUS1) of the transmitted product.

Anticyclone: An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. At the center of the circulation, there is sinking air. Generally, this sinking air provides clear skies.

Anticyclonic RotationRotation in the opposite sense as the Earth's rotation.  In the Northern Hemisphere, this would be clockwise as would be seen from above.

Antlophobia:  The fear of floods.

AnvilThe flat, spreading top of a Cumulonimbus Cloud (Cb). Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind (See Back-Sheared Anvil).

Anvil Crawler: Slang for 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 Mesoscale Convective System (MCS).

Anvil DomeA large overshooting top or penetrating top on the top of a Cumulonimbus Cloud (Cb).

Anvil RolloverSlang 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. See cumuliform anvil, knuckles, and mushroom.

Anvil ZitsSlang for frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil.

AOAAn acronym for "At or Above".

AOBAn acronym for "At or Below".

Apparent Temperature: The apparent temperature is a measure of human discomfort due to combined heat and humidity.  It was developed by Dr. R. G. Steadman in 1979 and is based on studies of human physiology and textile (clothing) science.  The apparent temperature is designed so that apparent temperature exceeds the actual air temperature when the humidity is relatively high.  The apparent temperature then measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities.  The apparent temperature is less than the actual air temperature when the humidity is relatively low and that the apparent temperature indicates the reduced stress and increased comfort associated with the higher rate of evaporative cooling of the skin.

Apparent temperatures greater than 80 degrees F (27 degrees C) are likely to produce some discomfort.  Values in excess of 105 degrees F (41 degrees C) may be dangerous and even life-threatening, with severe heat exhaustion or heat stroke possible if the exposure is prolonged or physical activity is high.  The degree of stress may vary with age, health, and body characteristics.

The apparent temperature does not consider the effects of air movement (wind speed) or exposure to sunshine on the degree of discomfort or stress.  The apparent temperature is determined by the following Quasi-empirical equation: Apparent Temperature = 1.03T + T (exp((DP-59)/17)-1)/19-3 where T = Observed Temperature (degree F) DP = Dew Point (degree F)  

General Heat Stress Index

Apparent Temperature

Danger Category

Heat Symptoms

80 to 90 degrees F Caution Fatigue possible with prolonged exposure and physical activity
90 to 105 degrees F Extreme Caution Sunstroke, Heat Cramps, and Heat Exhaustion possible with prolonged exposure and physical activity
105 to 130 degrees F Danger Sunstroke, Heat Cramps, and Heat Exhaustion likely.  Heat Stroke possible with prolonged exposure and physical activity
greater than 130 degrees F Extreme Danger Heat Stroke or Sunstroke imminent

  Approaching Severe LevelsA thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter. See Severe Thunderstorm.

Apogee: The farthest distance between the moon and earth or the earth and sun.

Aquiclude A formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring.
Aquifer:  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:  A geologic formation which has no interconnected openings and cannot hold or transmit water.
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.
Arcus (Arc Cloud) A dense, arched-shaped, menacing-looking accessory cloud to a cumulonimbus that can occur along the leading edge of a thunderstorm's gust front as the consequence of uplift of stable warm air.  Same as a shelf cloud.

Area Forecast Discussion (AFD):  This National Weather Service product is almost identical to the State Forecast Discussion; however, it deals with a much smaller area (just the county warning area of the NWFO), and it may or may not have the non-convective watches listed at the end of the product.  Eventually, this product will replace the State Forecast Discussion.
Area of Influence:  The area covered by the drawdown curves of a given pumping well or combination of wells at a particular time.
Area-Capacity Curve:  A graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
Area Wide Hydrologic Prediction System (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.
Arid:  An adjunctive applied to regions where precipitation is so deficient in quantity, or occurs at such times, that agriculture is impracticable without irrigation.
Arroyo:  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.
Artesian Well:  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:  A weir or other man-made structure which serves as the control for a stream-gaging station.

ASAP:  AHOS SHEF Automatic Processing System
ASAPTRAN:  The software component of ASAP.

AscopeA deflection-modulated display in which the vertical deflection is proportional to target echo strength and the horizontal coordinate is proportional to range.

ASOSAn acronym for Automated Surface Observing System.  See Automated Surface Observing System.

AstraphobiaThe fear of thunder and lightning.  See Astrapophobia, Brontophobia, Ceraunophobia, Keraunophobia, and Tonitrophobia

Astrapophobia:  The fear of thunder and lightning.  See Astraphobia, Brontophobia, Ceraunophobia, Keraunophobia, and Tonitrophobia

ATAD:  An acronym for Automated Telephone Answering Device.  A recorded telephone message giving current and forecast weather information.
Atmospheric Pressure: The pressure exerted by the atmosphere at a given point.

Atmospheric Pressure (Air Pressure): The weight of air pushing down on a unit area of a planet's surface.

Attenuation: a) Radar definition: 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. b) Hydrological definition: The process where the flood crest is reduced as it progresses downstream.

ATTM:  An acronym for "at this time".

Aurora: A glowing light display in the nighttime sky cause by excited gases in the upper atmosphere giving off light. In the Northern Hemisphere, it is called the aurora borealis (northern lights). In the Southern Hemisphere, it is called aurora australis (southern lights).

AuroraphobiaThe fear of the Northern Lights.

Autocorrelation:  A measure of similarity between displaced and undisplaced (in time, space, etc.) versions of the same function.

Automated Event-Reporting Gage:  River stage gages, IFLOWS pressure transducer type gages can be programmed to report if water surface rises or falls by a predetermined amount. See Tipping Bucket Rain Gage.

Automated Local Evaluation in Real Time (ALERT): A local flood warning system where river and rainfall data area collected via radio signals in real-time at an ALERT base station.

Automatic Gain Control (AGC)Any method of automatically controlling the gain of a receiver, particularly one that holds the output level constant regardless of the input level.

Automatic Radio Theodolite Master Control Unit (ART): This unit is used to automatically track radiosondes.
Automatic Surface Observing System (ASOS)The ASOS program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). When installation is completed in the mid-1990s, the ASOS systems will 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.

Automated Tone Dial Telephone Data Collection System (ATDTDCS)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.

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 AdvisoryA preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.

Average Power:  Pulsed radars transmit over a very low duty cycle; i.e., many intense but short and widely separated pulses. The average power is a radar's peak power "its PRF" its pulse length (duration).

Aviation Area Forecast (FA):  This NWS aviation product is a forecast of Visual Flight Rules (VFR) clouds and weather conditions over an area as large as the size of several states. It must be used in conjunction with the Airmet Sierra bulletin for the same area in order to get a complete picture of the weather. The area forecast together with the Airmet Sierra bulletin are used to determine forecast enroute weather and to interpolate conditions at airports which do not have terminal forecasts (FT's) issued. A map displaying the domestic FA areas is available by clicking here (approx. 50k file). FAs are issued 3 times a day by the Aviation Weather Center in Kansas City for each of 6 areas in the contiguous 48 states. In Alaska, FAs are issued by the Weather Service Forecast Office (WSFO's) in Anchorage, Fairbanks, and Juneau for their respective areas (Alaska map ~50k). The WSFO in Honolulu issues FAs for Hawaii (Hawaii map ~50k).

Each FA consists of a 12 hour forecast plus a 6 hour outlook. All times are Coordinated Universal Time (UTC). All distances except visibility are in nautical miles. Visibility is in statute miles.

The FA consists of a:

    1) synopsis section which is a brief summary of the location and movement of fronts, pressure system, and circulation patterns for an 18 hour period.

    2) VFR clouds and weather section which is a 12 hour forecast, in broad terms, of clouds and weather significant to flight operations plus a 6 hour categorical outlook. This section is usually several
    paragraphs. Airmet Sierra supplies information regarding Instrument Flight Rule (IFR) conditions. The breakdown may be by states, by well known geographical areas, or in reference to location and movement of a pressure system or front. A categorical outlook, identified by OTLK, is included for each area breakdown.

Amendments to the FA are issued as needed. An amended FA is identified by:  AMD

a Corrected FA by:  COR

and a delayed FA is identified by:  RTD

Aviation Low Level Significant Weather ForecastsA low-level graphics product is a forecast of aviation weather hazards, primarily intended to be used as a guidance product for briefing the VFR pilot.
The forecast domain covers the 48 contiguous states, southern Canada and the coastal waters for flight levels below 24,000 ft.  Each chart is a four panel product. The upper two panels depict freezing levels, turbulence, and a depiction of low cloud ceilings and/or restrictions to visibility (shown as contoured areas of MVFR and IFR conditions). The lower two panels, provided by NCEP/HPC, consist of graphical displays of the fronts and precipitation areas.  Low altitude Significant Weather charts are issued four times daily and are valid at specific fixed times: 0000, 0600, 1200, and 1800 UTC. Each chart is divided on the left an right into 12 and 24 hour forecast intervals (based on the current ETA model available).

Aviation High Altitude Significant Weather ForecastsIn accordance with the World Meteorological Organization (WMO) and the World Area Forecast System (WAFS) of the International Civil Aviation Organization (ICAO) high altitude Significant Weather (SIGWX) forecasts are provided for the en-route portion of international civil aeronautical operations. The AWC fulfills the role of the Washington Regional Area Forecast Center (RAFC) by providing a suite of SIGWX forecast products. These products are used directly by airline dispatchers for flight planning and weather briefing before departure, and by flight crew members during the flight.

High altitude SIGWX charts are valid at specific fixed times: 0000, 0600, 1200, and 1800 UTC. They show significant en-route weather phenomena over a range of flight levels from 250 to 600, and associated surface weather features.  The significant weather elements are defined by WMO and ICAO, and include:

     thunderstorms and cumulonimbus clouds
     tropical cyclones
     severe squall lines
     moderate or severe turbulence
     moderate or severe icing
     widespread sand storms and dust storms
     well-defined surface convergence zones
     surface fronts with speed and direction of movement
     tropopause heights
     volcanic eruptions

AViatioN model (AVN)One of the operational forecast models run at NCEP. The AVN is run four times daily, at 0000, 0600, 1200, and 1800 GMT.  As of fall 1996, forecast output was available operationally out to 72 hours only from the 0000 and 1200 runs. At 0600 and 1800, the model is run only out to 54 hours.  It is one of the oldest operational models used by forecasters. The AVN model was developed primarily to aid in forecasting for aviation. The AVN gives short range forecasts like the NGM and ETA models do, but it also forecasts well into the medium range with forecasts up to 72 hours into the future. The resolution of the AVN model is about 100 km, which is not as good as the NGM or ETA models, but it still provides valuable insight into the future state of the atmosphere. The AVN also has its own set of statistical equations that use the AVN model output. The output from the AVN statistical equations is known as AVN MOS or FAN Guidance.

Aviation Weather Center (AWC)One of the National Centers for Environmental Prediction.  The Aviation Weather Center (AWC), located in Kansas City, Mo., enhances aviation safety by issuing accurate warnings, forecasts and analyses of hazardous weather for aviation interests. The Center identifies existing or imminent weather hazards to aircraft in flight and creates warnings for transmission to the aviation community. The Center also orginates operational forecasts of weather conditions that will affect domestic and international aviation interests out to two days. The Center collaborates with universities, governmental research laboratories, Federal Aviation Administration facilities, international meteorological watch offices and other National Weather Service components to maintain a leading edge in aviation meteorology hazards training, operations and forecast techniques development. These functions were formerly handled by three collaborating National Weather Service offices.

Warnings of flight hazards, such as turbulence, icing, low clouds and reduced visibility remain most critical for the protection of life and property over the United States from the earth's surface up to 24,000 feet. Above 24,000 feet, the AWC provides warnings of dangerous wind shear, thunderstorms, turbulence, icing and volcanic ash for the Northern Hemisphere from the middle of the Pacific Ocean eastward to the middle of the Atlantic Ocean. Additionally, above 24,000 feet, the AWC forecasts jet stream cores, thunderstorms, turbulence and fronts for the Northern Hemisphere from the east coast of Asia eastward to the west coast of Europe and Africa. Through international agreement, the Center also has responsibility to back up other World Area Forecast Centers with aviation products distributed through the World Area Forecast System.

The AWC supports requirements for products and services established by national and international agreements. The Center coordinates closely with the aviation community to identify new standards which support national requirements of the Federal Aviation Administration and international requirements of the International Civil Aviation Organization. An Experimental Forecast Facility funded by the Federal Aviation Administration and located at the AWC brings new techniques for identifying aviation hazards into operational forefront.

The Center trains forecasters in the latest meteorological theories, develops and improves forecast techniques and evaluates data source and technology. Operational support within the AWC includes 24-hour on-call problem identification and resolution services and electronic and workstation support. This internal support is in part an extension of the NCEP Central Operations.

AWIPS:  An acronym for Advanced Weather Interactive Processing System. This system is replacing Automation of Field Operations and Services (AFOS). This system allows the operator to overlay meteorological data from a variety of sources.

AWOS:  An acronym for Automated Weather Observation System.

Azimuth: A direction in terms of a 360o compass. North is at 0o. East is at 90o. South is at 180o. West is at 270o.  

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