Glossary of NWS Terms, Contractions, Abbreviations, and Acronyms
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L | M | N | O | P | Q | R | S
| T | U | V | W | X | Y | Z
The horizontal movement of an air mass that causes changes in the physical properties of the air such as
temperature and moisture.
A NWS product that means a weather event may threaten life or property if caution is not
A large body of air that has nearly uniform conditions of temperature and humidity.
A low pressure system that moves out of southwest Canada and mainly affects the Plains,
Midwest, and Great Lakes region. Usually accompanied by light snow, strong winds, and
colder temperatures. Another variation of the same system is called a Saskatchewan Screamer.
At This TiMe.
A short to medium range computer weather prediction model run twice daily in Washington,
DC and used by meteorologists to forecat the weather.
Wind-driven snow that significantly reduces surface visibility to less than seven miles.
Sustained wind speeds of 15-25 mph.
A 25-55% probability that one point in a forecast area will have precipitation.
A wispy, cloud that is composed of ice crystals and is formed at altitudes of 20,000
to 40,000 feet above the ground.
A cumulus cloud that is vertically developed and often has an anvil shaped top.
Generally associated with lightning, thunder, heavy showers, and occasionally hail and
A cloud that has a flat base with an upper portion that is billowy or heaping.
CWA, CWFA (County Warning/Forecast Area):
The area of responsibility of a NWS office.
An area of low atmospheric pressure that has a closed circulation. Cyclones (or more
commonly called 'low pressures') usually bring about marked changes in the weather.
Gauges the amount of heating or cooling needed for a building using 65 degrees as a
baseline. To compute heating/cooling degree-days, the average temperature is taken and
referenced to a base line of 65 degrees. An average temperature of 50 yields 15 heating
degree-days while an average of 75 would yield 10 cooling degree-days. Electrical, natural
gas, power, heating, and air conditioning industries utilize heating and cooling degree
information to calculate their needs.
Water droplets that form upon surfaces on or near the ground when air is cooled toward
The temperature to which air must be cooled, at constant pressure and moisture content,
in order for saturation to occur. The higher the dew point, the greater the amount of
water vapor in that vicinity. Dewpoints in the 70's make people feel uncomfortable.
Doppler Weather Radar:
A new Weather Surveillance Radar (WSR-88D) system developed in 1988. About 121 systems
have been installed at Weather Forecast Offices, with an additional 24 systems at
Department of Defense (Air Force Bases) sites. This powerful and sensitive Doppler system
generates many useful products for meteorologists, among them: standard reflectivity
'echoes', wind 'velocity' or atmospheric air motion pictures, and areal 1-hour, 3-hour, or
storm-total precipitation images.
A strong downdraft, initiated by a thunderstorm, that includes an outburst of damaging
winds on or near the ground. Downbursts may last for anywhere from a few minutes in small
scale microbursts on up to 20 minutes in larger, longer lived microbursts. One example of
a downburst, called straight-line winds, can reach speeds of 110-150 mph, or squarely in
the range of a strong tornado. Downburst are further detailed as either: Microburst: a
convective downdraft with an affected outflow area of less than 2.5 miles wide and peak
winds lasting less than 5 minutes. They can create dangerous vertical/horizontal wind
shears which can adversely affect aircraft performance and cause property damage.
Macroburst: a convective downdraft with an affected outflow area of at least 2.5 miles
wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause
Air that descends down a mountain chain or over sloping terrain (pressurized air moving
from high pressure to low pressure), resulting in subsequent drying, and in some cases,
dramatic warming of air that can quickly melt a snowcover. Local names for downslope
winds, or "foehn" winds in the western USA are Chinook Wind, East Winds, North Winds, and
Mono Winds. Usually associated with little or no clouds. On the other hand, upslope flow
is representative of air being lifted by rising terrain and is normally associated with
extensive clouds and/or precipitation.
Water drops that are very small and fine. For the most part, drizzle falls from stratus
clouds and is usually accompanied by low visibility and fog.
Significant warming of the waters in the eastern Pacific Ocean, usually off the coast
of South America, which results in shifts of world-wide weather patterns. Can cause
prolonged periods of drought or floods.
ETA (as in the Greek letter):
A short range computer weather prediction model run twice daily in Washington, DC and
used by meteorologists to forecast the weather.
AVN forecast output (such as high/low temperatures and precipitation) for a specific location;
used by meteorologists as a "first guess".
A unit of length equal to six feet which is used to measure the depth of water.
An area from which waves are generated by a wind that is nearly constant in direction and speed.
A dangerous and sudden flood that threatens lives and property and usually occurs after
heavy rain. May also occur after an ice jam breaks up or after a dam fails.
A rise in water level of a creek, stream, or river that results in water overflowing
into the surrounding basin; usually caused by heavy rain, snowmelt, or a combination of
Light snowfall that generally does not produce measurable accumulation.
A nebulous arc or circle of white or yellowish light sometimes seen in fog.
Freezing Drizzle or Rain:
Describes the effect of drizzle or rain freezing upon impact on objects that have a
temperature of 32 degrees or below.
The point in the atmosphere where temperatures are at 32 degrees.
The boundary between two different air masses, ie. cold front, warm front, stationary
A covering of small ice crystals that forms on or near the ground when temperatures
approach or drop below 32.
A rotating, visible extension of cloud, pendant to a cumulus or cumulonimbus with
circulation not reaching the ground.
A scale used to classify tornadoes based on wind damage and was developed by Theodore
Fujita (University of Chicago).
Fog of little vertical extent, usually 20 feet or less.
The leading edge of a downdraft associated with a thunderstorm which is marked by a
sudden wind shift, sharply falling temperatures and possibly heavy downpours and/or hail.
A small tornado, usually weak and short-lived, that occurs along the gust front of a
thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. It
is not associated with the storm-scale rotation found in severe thunderstorms.
500 mb pressure surface (geopotential Height); about 18,000 feet above sea level.
Precipitation in the form of balls or lumps usually consisting of concentric layers of
ice. A thunderstorm is classified as severe when it produces hail 3/4 of an inch or larger
Rings or arcs that encircle the sun or moon which are caused by refraction of light
through ice crystals that make up cirrus clouds.
Fine particles of dust, smoke or water droplets suspended in the air that reduces
The apparent temperature that describes the combined effect of high temperatures and
high levels of humidity.
Clouds above 20,000 feet; usually cirriform (cirrus).
Sustained winds of 40 mph or higher, or gusts greater than 57 mph (not due to thunderstorms),
that are expected to last for an extended period of time.
Amount of water vapor in the atmosphere.
A dangerous tropical cyclone with winds speeds of 64 knots (74 mph), or higher.
(typhoon in western Pacific)
A freezing rain event that produces damaging ice accumulations of 1/4 inch or greater.
A situation where the temperature increases with height instead of decreasing, which is
usually the case.
Instability (Unstable Air):
A state of atmosphere in which the vertical distribution of temperature allows rising,
warm air to continue to rise and accelerate. This kind of motion is conducive for
Lines of equal barometric pressure as shown on a weather map.
Isolated (ISOLD, ISLD):
Showers or thunderstorms with 1-25% areal coverage.
A concentrated region within the jet stream where the wind speeds are the strongest.
The jet streak sets up unique wind currents in its vicinity which either enhance or
diminish the likelihood of clouds and precipitation. The jet streak will propagate
downstream along the jet stream axis.
A narrow band of strong winds in the atmosphere that controls the movement of high and
low pressure systems and associated fronts. Jet streams meander from time to time. Wind
speeds can reach 200 mph or higher in certain cases. It is usually found at 30,000 to
40,000 feet above the earth's surface. The jet stream owes its existence to the large
temperature contrast between the polar and equatorial regions.
Unit of speed used in aviation and marine activities, which is equal to about 1.15
statue miles an hour.
A lake breeze occurs when prevailing winds blow off the water, while a land breeze
indicates winds blowing from land to sea. Both are caused by the difference in surface
temperature (heating) of the land and water. As a result, a lake breeze occurs during the
day while a land breeze happens at night.
Lake-Effect Snow Squall (Lake Snow):
A local intense, narrow band of moderate to heavy snow that can extend long distances
inland, persist for many hours, and may be accompanied by strong, gusty surface winds and
possibly lightning. Accumulations can be 6 inches or more in 12 hours.
The side of an object away from the direction in which the wind is blowing. On the
other hand, windward is the side of an object facing into the wind.
A sudden visible flash of energy and light caused by electrical discharges from thunderstorms.
A 55-75% probability that one point in a forecast area will have precipitation.
Clouds below 1,000 feet; usually stratiform (stratus).
Unit of atmospheric pressure.
MOS (Model Output Statistics):
NGM forecast output (such as temperatures, winds, and precipitation) for a specific
location; used by meteorologists as a "first guess".
MRF (Medium Range Forecast):
A medium range computer weather prediction model run once a day in Washington, DC and
used by meteorologists to forecast the weather.
Mean Sea Level.
MSO (Meso Eta):
A higher resolution version of the Eta computer weather prediction model.
A unit of distance used in marine navigation and forecasts, equal to 1.15 statute miles.
The waters of oceans, seas or the Great Lakes which extend out to five miles from shore.
An acronym that stands for NEXt generation of weather RADar.
NGM (Nested Grid Model):
A short range computer weather prediction model run twice daily in Washington, DC and
used by meteorologists to forecast the weather.
A strong low pressure system that affects the Mid-Atlantic and New England States. It
can form over land or over coastal waters. It usually produces heavy snows, flooding rains,
strong northeast winds, and coastal flooding, and beach erosion.
Showers or thunderstorms with 55-75% areal coverage.
Negative Vorticity Advection (Anticyclonic Vorticity Advection).
National Weather Service.
Offshore flow occurs when air moves from land to sea, while onshore flow is when air
over the water advances across land. Offshore flow is usually associated with dry weather,
while onshore flow indicates an increase in moisture and resultant precipitation
Offshore (Open) Waters:
The waters extending from the midpoint of a lake, sea, or ocean to within 5 miles of the shore.
Orographic Uplift (Upslope Flow):
Occurs when air is forced to rise and cool due to terrain features such as hills or
mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional
cooling results in rain or snow. It can cause extensive cloudiness and increased amounts
of precipitation in higher terrain.
A nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic
odor like that of weak chlorine. Its formula is O3. It is usually found in trace amounts
in the atmosphere, but is primarily found at 30,000 to 150,000 feet above the ground. Its
production results from a photochemical process involving ultraviolet radiation. Because
it absorbs harmful ultraviolet radiation at those heights, it is a beneficial gas. However,
photochemical processes involving industrial/vehicle emissions can produce ozone near the
ground, in which case it can be harmful to people with respiratory or heart problems.
Probability Of Precipitation.
Positive Vorticity Advection (Cyclonic Vorticity Advection).
Quantitative Precipitation Forecast.
The cooling of the earth's surface. At night, the earth suffers a net heat loss to
space due to terrestrial cooling.
Indicates a nearly steady and uniform fall of liquid precipitation (rain) over an area
for several hours, as opposed to the term "showers", which implies intermittent and
scattered precipitation of a more unstable, convective nature.
An arc that exhibits in concentric bands the colors of the spectrum and is formed
opposite the sun by refraction and reflection of the sun's rays in raindrops.
Relative Humidity (RH):
The ratio of the amount of moisture in the air to the amount which the air could hold
at the same temperature and pressure if it were saturated; usually expressed in percent.
An elongated area of high pressure in the atmosphere; the opposite of trough.
A turbulent cloud formation that resembles a roller. This cloud can be found in the
lee of some mountains. The air in the cloud rotates around an axis parallel to range of
mountains. It is also sometimes found along the leading edge of a thunderstorm cloud;
formed by rolling action in the wind shear region between cool downdrafts and warm updrafts.
Showers or thunderstorms with 25-55% areal coverage.
A standing wave oscillation in any enclosed lake which continues after the forcing
mechanism has ceased. In the Great Lakes, this forcing mechanism may be either strong
winds blowing along the axis of a lake or a pressure jump, or down draft winds associated
with fast moving squall lines over a lake. In either case, water is piled up at one end.
The water then sloshes from one end of the lake to the other causing fluctuations of perhaps
several feet before damping out.
Severe Thunderstorm (SVR TS):
A thunderstorm that produces either of the following: damaging winds of 58 miles an
hour or greater, hail 3/4 of an inch in diameter or larger, or a tornado. Severe
thunderstorms can result in the loss of life and property.
Intermittent rain or snow.
Describes solid grains of ice formed by the freezing of raindrops or the refreezing of
largely melted snowflakes.
Slight Chance (SLGT CHC):
A 1-25% probability that one point in a forecast area will have precipitation.
A steady fall of snowflakes for several hours over the same area.
The combined layers of snow and ice on the ground at any one time. Also called the "snowcover".
Snow that starts and stops suddenly and is characterized by rapid changes in both
intensity and visibility. There is normally measurable accumulation.
A periodic, large scale atmospheric oscillation of the large scale distribution of sea
level pressure, and air and water temperature that originates over the southern hemisphere.
Consequently, there is an associated change in the surface winds, and some storms become
stronger than normal. This oscillation is on the scale of a year or a few years, and has
global implications such as widespread drought or flooding. Oceanic fishing is also
A broken or solid line of thunderstorms that may extend across several hundred miles,
ahead or along an advancing cold front.
Low clouds that are flat and gray, usually covering most of the sky.
Wind speed determined by averaging observed values over a 1-minute period.
A relatively small-scale, rising air current produced when the earth's surface is
heated. Thermals are a common source of low level turbulence for aircraft.
Height difference between two pressure surfaces; corresponds to the temperature in that
layer of the atmosphere.
A violently rotating column of air that is in contact with the ground. Tornadoes
usually develop from severe thunderstorms and can produce winds of 100 to 300 mph.
Tropical or Subtropical Depression:
Cyclones that have maximum sustained winds of 33 knots (38 mph) or less. These are
referred to as low pressure systems in public advisories and statements.
Tropical cyclone that has maximum sustained winds from 34 to 63 knots (39 to 73 mph)
An elongated area of low pressure in the atmosphere; the opposite of a ridge.
A disturbance of the flow pattern in the upper atmosphere, which is usually associated
with clouds and precipitation. This disturbance is characterized by distinct cyclonic
flow, a pocket of cold air; and sometimes, a jet streak. These features make the air
aloft more unstable and conducive to clouds and precipitation.
Wisps or streaks of rain or snow falling out of a cloud, but evaporating before
reaching the ground.
A quantitative measure of "spin" in the atmosphere.
A local, abrupt lowering of a rain-free cumulonimbus base forming a low hanging
accessory cloud that is usually 1 to 4 miles in diameter. The wall cloud is usually
situated in the right-rear quadrant of the cumulonimbus with respect to storm motion,
below an intense updraft associated with a strong or severe thunderstorm. Rotating wall
clouds often precede tornado development.
Indicates that a hazardous weather element is imminent or has a very high probability
Alerts the public to the possibility of severe weather, or some other hazardous weather
element. It is intended to provide enough lead time so those who need to set their plans
in motion can do so.
A violently rotating column of air, usually pendant to a cumulus or cumulonimbus cloud,
over a body of water, with circulation reaching the water.
Wet Bulb Temperature:
The temperature an air parcel would have if cooled to saturation at a constant pressure
by evaporation of water into it.
Waves generated from the action of wind on a water surface, as opposed to swell.
An apparent temperature that describes the combined effect of wind and low air
temperatures on exposed skin.
Sustained wind speeds of 20-30 mph.
Z (Zulu Time):
Greenwich Mean Time (GMT), Universal Coordinated Time (UTC).
A local geographic region of a NWS office's forecast area.
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