Weather Forecast Terminology Glossary

Many of these definitions have been simplified somewhat, but this glossary may help users who have little or no meteorology background to better interpret the reasoning behind the current forecast as expressed by the forecaster in his/her Area Forecast Discussion. The below listed definitions are those used primarily in the MEG (Memphis, TN.) CWA.

500MB - The 500 millibar pressure surface. This is a common level for analyzing upper level weather features. It generally occurs around 18,000 feet above sea level, higher over warm air masses and lower over cold air masses. The term HEIGHTS ("HEIGHTS RISING AS RIDGE BUILDS") usually refers to the height of the 500MB pressure level. For perspective 1000MB is near sea level, 850MB is around 5000 feet, 700MB is around 9000 feet and 250MB is around 30,000 feet, where the jet stream is found.

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.

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 mix down to the surface and lead to rapid development of afternoon 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.

ACSL - Altocumulus Standing Lenticular, flying saucer shaped clouds over mountains indicative of strong winds aloft.

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).

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.

ADVECTION - (Abbrev. ADVCTN) The transfer of a weather element by air movement. Simplified it means cooler air (Cold Advection CAA) or warmer air (Warm Advection WAA) moving into a region.

AMPLITUDE - The maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.

BACKING - (abbrev. BCKG)- A counterclockwise shift in wind direction (for example, south winds shifting to the east).

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.

BAROCLINIC BAND - an area of clouds and/or precipitation caused by a difference in temperature between two air masses. A front is an example of this.

CAA - Cold Air Advection. This will often stabilize an air mass, and can cause windy conditions.

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.) No CAPE, no thunderstorms. Lots of CAPE means a strong thunderstorm potential.See also CIN.

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)

CAVU - Clear or Scattered Clouds (visibility greater than 10 mi.)

CB - Cumulonimbus cloud, characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead."

CIN - Convective INhibition. A measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of the cap. They are obtained on a sounding by computing the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is cooler than the former. (This area sometimes is called negative area.) See CAPE.

CIRRUS - (abbrev. CI) High-level clouds (16,000 feet or higher), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.

CLOSED LOW - A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see Cutoff Low).

CONVECTION - Generally, transport of heat and moisture by the movement of a fluid.

In meteorology, the term is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS clouds all are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.

CONVECTIVE TEMPERATURE - The temperature that daytime heating needs to cause at the surface in order for convection to occur.

CONVERGENCE - Air molecules come together. The convergent area becomes too crowded with air molecules and they must go up or down depending on what level of the atmosphere convergence occurs at. Low level convergence causes lift. Upper level convergence causes sinking in the atmosphere.

CU - Cumulus clouds.

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.

CWA - County Warning Area.

DENDRITIC - In hydrologic terms, the form of the drainage pattern of a stream and it's tributaries when it follows a treelike shape, with the main trunk, branches, and twigs corresponding to the main stream, tributaries, and subtributaries, respectively, of the stream.

DEFORMATION - The change in shape of a fluid mass by variations in wind, specifically by stretching and/or shearing energy aloft creating clouds or precipitation. Deformation is a primary factor in frontogenesis (evolution of fronts) and frontolysis (decay of fronts).

DEEPENING - A decrease in the central pressure of a surface low pressure system. The storm is intensifying.

DIFFLUENCE - Usually used in context of upper level flow. Diffluent flow splits apart and causes air from below to rise up and produce lift in the atmosphere.

DIVERGENCE - Opposite of Convergence. Air molecules become too sparse in a divergent area and need to be replenished by having other air molecules rise up from below or sink from above. Upper level divergence causes air from below to rise into the divergent area, causing lift. Low level divergence causes air from above to sink into the divergent area, stabilizing the atmosphere.

DOWNSTREAM - In the same direction as a stream or other flow, or toward the direction in which the flow is moving.

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. Strong dynamics usually means plenty of energy in the atmosphere to produce precipitation.

ECMWF - European Center for Medium-Range Weather Forecasts. Operational references in forecast discussions typically refer to the ECMWF's medium-range numerical forecast model, which runs out to 10 days.

FA - Forecast Area. Same as CWA.

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.

FETCH - A: The area in which ocean waves are generated by the wind. Also refers to the length of the fetch area, measured in the direction of the wind. B: In hydrologic terms, The effective distance which waves have traversed in open water, from their point of origin to the point where they break. 2. The distance of the water or the homogenous type surface over which the wind blows without appreciable change in direction.

FLOW - (abbrev. FLW) Wind. In meteorology, a qualitative reference of an air parcel(s) with respect to its direction of movement, sometimes specified at a certain height or pressure elevation, e.g. westerly flow at 500 mb. In hydrology, the volumetric flow of water past a given point on a stream or river, usually in cubic feet per second (cfs).

FRONT - A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.

GFS - (Global Forecast System) One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 384 hours.

GOES - Geostationary Operational Environmental Satellite - Satellites orbiting at 22,370 miles above the Equator with the same rotational velocity as the Earth; therefore, the satellite remains over the same location on the Earth 24 hours a day. Besides sending back satellite pictures to earth, it also relays the DCPs river and rainfall data back to the ground.

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.

HEIGHT - In meteorology, usually a reference to Geopotential Height; roughly the height above sea level of a pressure level. For example, if a station reports that the 500mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.

HIGH PRESSURE ALOFT - (or high pressure at the surface) - Dry air masses are found under an upper level ridge. Generally fair weather is found in a surface high pressure ridge.

INVERSION - (abbrev. INVRN) Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap. A layer of warm air over a layer of cooler air. This warm layer acts as a lid on the atmosphere, preventing interaction between the air above and below the inversion.

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.

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.

JET - A fast-moving wind current surrounded by slower moving air.

JETSTREAM - (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.

KT - (Knot)- Unit of speed used in navigation, equal to 1 nautical mile (the length of 1 minute latitude) per hour or about 1.15 statute miles per hour, or 0.5 meters/sec).

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.

LIFTED INDEX (LI) - A scientific measure of instability in the atmosphere. A "LI" above zero is stable. A below zero "LI" implies an unstable atmosphere.

LIKELY - (abbrev. LKLY) In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

LOW LEVEL JET - (abbrev. LLJ)- A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).

The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

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.

MAV - AVN MOS Guidance

MESOSCALE - A small scale weather system, covering a portion of a state or region, often caused by local terrain.

MIXING - The process of mixing air from aloft down to the surface and vice versa. Can cause gusty conditions as stronger winds aloft mix down to the surface. Usually caused by daytime heating of the surface by the sun which leads to rising and sinking air currents.

MOS - Model Output Statistics - the Hydrometeorological Center (HPC) produces a short range (6 to 60 hours) MOS 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.

MOSTLY CLOUDY - When the 6/8th to 7/8ths of the sky is covered by with opaque (not transparent) clouds. Same as Considerable Cloudiness.

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.

NEGATIVELY TILTED - Usually used in referring to an upper level trough. The base of the trough moves out ahead of the rest of the trough. This is a good pattern for severe thunderstorms.

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.

NVA - Negative Vorticity Advection; A kind of transfer of vorticity that under certain conditions causes sinking of air in the atmosphere and stabilization (see VORTICITY).

OBS - Observation(s)

OMEGA BLOCK - A huge, slow moving upper level ridge shaped like a Greek letter Omega. They are very hard to get rid of and usually block weather systems from moving through, sometimes for weeks.

OROGRAPHIC - Pertains to mountains and how they influence the weather.

PARTLY CLOUDY - Between 3/8 and 5/8 of the sky is covered by clouds.

POP(S) - Probability of Precipitation.

PRECIPITABLE WATER - The amount of precipitation an air mass could produce if there was lift available to squeeze all the water out of it. High precipitable water means a moist air mass.

PROGGED - Forecasted

PVA - Positive Vorticity Advection; A kind of transfer of vorticity that under certain conditions will cause lift in the atmosphere.

QPF/QPS - Quantitative Precipitation Forecast. Quantitative Precipitation Forecast. A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.

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.

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.

REX BLOCK - A split flow pattern aloft featuring a ridge to the north over an upper level low pressure to the south.

RIDGE - 1) An elongated area of relatively high atmospheric pressure; the opposite of trough. or 2) In hydrologic terms, a line or wall of broken ice forced up by pressure. May be fresh or weathered.

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.

RAOB - Radiosonde Observation (Upper-Air Observation). The balloon sounding of the atmosphere taken at 12Z (morning) and 00Z (late afternoon) every day.

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."

SFC - Surface

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. As it pertains to thunderstorms, high shear can mean stronger and possibly severe thunderstorms.

SHORT WAVE - 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. A weak and often fast moving upper level trough or ridge.

SHSN - Snow showers

SNOW FLURRIES - Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).

SOUNDING - A set of data measuring the vertical structure of an atmospheric parameter (temperature, humidity, pressure, winds, etc.) at a given time. See RAOB

SPLIT FLOW - An upper level flow pattern where energy splits between a northern track and a southern track.

STABLE - An atmospheric state with warm air above cold air which inhibits the vertical movement of air.

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.

SUBSIDENCE - A descending motion of air in the atmosphere occurring over a rather broad area.

SYNOPTIC - Large scale weather systems. Upper level ridges and troughs, large highs and lows.

TCU - Towering Cumulus clouds. If they keep growing they become Cumulonimbus (CB).

THERMAL TROUGH - A surface low pressure trough caused by surface heating which produces rising air and thus lower pressure. Usually an axis of warmest temperatures (THERMAL AXIS). Also called a THERMAL RIDGE which is technically more correct since thermally the temperatures are highest here. THERMALLY INDUCED PRESSURE TROUGH is the proper label, but seldom used because it is too cumbersome to type out.

THETA-E - Equivalent potential temperature. A scientific way of combining moisture and heat which are some of the ingredients needed for convection to occur.

TROUGH - (abbrev. TROF) Opposite of a RIDGE. An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. Storms form just ahead of an upper level trough. Cooler air masses are generally found under an upper level trough. Fronts are found in a surface pressure trough.

UPPER LEVEL - In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.

VEERING - A clockwise shift in wind direction (for example, south winds shifting to the west). Opposite of BACKING.

VORTICITY - A scientific term for measuring the turning or spin around a point in space. Look at a satellite loop and you will see a lot of turning and spinning in the cloud masses. This is vorticity, and certain relationships between vorticity and atmospheric flow produce lift in the atmosphere.

VORT MAX/ VORT SPOKE/ VORT CENTER/ VORT LOBE - All "pieces" of vorticity that are often the cause of regions of lift and thus clouds and precipitation. 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. WAA into an atmospheric layer not only warms the temperature, but can cause upward motion for clouds and precipitation.

WIND GUST - Rapid fluctuations in the wind speed with a variation of 10 knots or more between peaks and lulls. The speed of the gust will be the maximum instantaneous wind speed.

XBT - Expendable Bathythermograph

XCITED - Excited

XCPT - Expecting

XPC - Expect

XSEC - Cross Section

ZL - Freezing Drizzle

ZNS - zones

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.

Zulu (Z) Time - For practical purposes, the same as Coordinated Universal Time (UTC). The notation formerly used to identify time in Greenwich MeanTime. The word "Zulu" is notation in the phonetic alphabet corresponding to the letter "Z" assigned to the time zone on the Greenwich Prime Meridian.