For abreviations or contractions which appear in forecasts
discussions, visit NCEP's contractions
page
ADIABATIC: Occurring without loss or gain of heat.
i.e., "adiabatic expansion of a body of air."
ADVECTION: The horizontal transport of air or atmospheric
properties. Commonly used with temperatures, i.e., "warm air
advection".
ADVISORY: Advisories are issued for weather situations
that cause significant inconveniences but do not meet warning
criteria and, if caution is not exercised, could lead to life-threatening
situations. Advisories are issued for significant events that
are occurring, are imminent, or have a very high probability
of occurrence.
AIR MASS: A large body of air having similar horizontal
temperature and moisture characteristics.
ALERT: Automated Local Evaluation in Real Time. Network
of automatic rain gauges that transmit via VHF radio link
when precipitation occurs. Some sites are also equipped with
other sensors such as temperature, wind, pressure, river stage
or tide level.
ALTOCUMULUS (AC): Mid-altitude clouds with a cumuliform
shape.
ALTOSTRATUS (AS): Mid-altitude clouds with a flat
sheet-like shape.
ANEMOMETER: An instrument that measures wind speed.
ANTICYCLONE: A large area of high pressure around
which the winds blow clockwise in the Northern Hemisphere.
ASOS: Automated Surface Observing System. Observes
sky conditions, temperature and dewpoint, wind direction and
speed, and barometric pressure.
AVN: "Aviation" numerical weather prediction model generated at the National Centers for Environmental Prediction (NCEP). No longer called the Aviation Model (AVN) but rather GFS (Global Forecasting System).
AWIPS: Advanced Weather Information Processing System.
New NWS computer system integrating graphics, satellite and
radar imagery.
BACKING WINDS: A counterclockwise change in wind
direction. Backing winds with height are indicative of cold
air advection (CAA).
BLOWING DUST: Reduction of visibility by strong winds
blowing across dry ground with little or no vegetation. Visibilities
of 1/8 mile or less over a widespread area are criteria for
a Blowing Dust Advisory.
BROKEN CLOUDS: Clouds which cover between 6/10 and
9/10 of the sky. See
Sky Terminology.
CAA: Cold Air Advection
CEILING: The height of the lowest layer of clouds,
when the sky is broken or overcast.
CHANCE: A 30, 40 or 50 percent chance of occurrence
of measurable precipitation.
CIRRUS: High clouds, usually above 18,000 feet, composed
of ice crystals.
CLEAR: Sky condition of less than 1/10 cloud coverage.
CLIMATE: The historical record of average daily and
seasonal weather events.
CLOSED LOW: See Cutoff Low below.
COASTAL FLOODING: The inundation of land areas along
the coast caused by sea water above normal tidal actions.
COASTAL FORECAST: A forecast of wind, wave and weather
conditions between the coastline and 60 miles offshore.
COLD FRONT: The boundary between a cold air mass
that is advancing and a relatively warmer airmass. Generally
characterized by steady precipitation followed by showery
precipitation.
COMBINED SEAS: The combined height of swell and wind
waves.
CONDENSATION: The process of gas changing to liquid.
CONTINENTAL AIR MASS: A dry air mass originating
over a large land area.
CUMULONIMBUS CLOUD (CB): A vertically developed cloud,
often capped by an anvil shaped cloud. Also called a thunderstorm
cloud, it is frequently accompanied by heavy showers, lightning,
thunder, and sometimes hail or gusty winds.
CUMULUS CLOUD (CU): A cloud in the shape of individual
detached domes, with a flat base and a bulging upper portion
resembling cauliflower.
CUT OFF LOW: An area of low pressure cut off from
its associated jet stream.
CWA: County Warning Area. Each forecast office has
a CWA. A CWA is comprised of all counties for which a forecast
office has forecast and warning responsibility.
CYCLONE: An area of low pressure around which winds
blow counterclockwise in the Northern Hemisphere. Also the
term used for a hurricane in the Indian Ocean and in the Western
Pacific Ocean.
DENSE FOG ADVISORY: Issued when fog reduces visibility
to 1/8 mile or less over a widespread area. Most common in
California's central valley. See tule fog.
DEWPOINT: The temperature to which the air must be
cooled for water vapor to condense.
DIABLO WIND: Similar to Santa Ana winds in southern
California. These winds occur below canyons in the East Bay
hills (Diablo range) and in extreme cases can exceed 60 mph.
They develop due to high pressure over Nevada and lower pressure
along the central California coast.
DOPPLER RADAR: A type of weather radar that determines
whether atmospheric motion is toward or away from the radar.
It uses the Doppler effect to measure the velocity of particles
suspended in the atmosphere.
DOWNBURST: A severe localized downdraft from a thunderstorm.
DRIZZLE: Small, slowly falling water droplets, with
diameters between .2 and .5 millimeters.
ECMF: European Centre for Meteorology Forecast model.
EL NINO: A major warming of the equatorial waters
in the Pacific Ocean. El Nino events usually occur every 3
to 7 years, and are characterized by shifts in "normal" weather
patterns.
ENSO: El Nino-Southern Oscillation.
ETA: "Eta" (from Greek) model generated every 12
hours by NCEP.
FAIR: Less than 4/10 opaque cloud cover, no precipitation,
and no extremes in temperature, visibility or winds.
FETCH: 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.
FLASH FLOOD: A flood that occurs within a few hours
(usually less than six) of heavy or excessive rainfall, dam
or levee failure.
FLOOD: High flow, overflow or inundation of a normally
dry area which causes or threatens damage.
FLOOD STAGE: The level of a river or stream at which
considerable inundation of surrounding areas will occur.
FOG: The visible aggregate of minute water droplets
suspended in the atmosphere near the earth's surface. Essentially
a cloud whose base is at the earth's surface, limiting visibility.
FREEZING LEVEL: The altitude in the atmosphere where
the temperature drops to 32F.
FREEZING RAIN: Rain which falls as liquid then freezes
upon impact, resulting in a coating of ice on exposed objects.
FRONT: The transition zone between two distinct airmasses.
The basic frontal types are cold fronts, warm fronts and occluded
fronts.
FROST: The covering of ice that is formed on exposed
surfaces whose temperature falls below freezing.
FUNNEL CLOUD: A rotating, cone-shaped column of air
extending downward from the base of a thunderstorm. When it
reaches the ground it is called a tornado.
GALE: Wind speeds from 39 to 54 mph (34 to 47 knots).
GEOSTATIONARY SATELLITE: A satellite that rotates
at the same rate as the earth, remaining over the same spot
above the equator.
GFS: Global Forecasting System. Numerical Weather Prediction model run every 12 hours at the National Centers for Environmental Prediction (NCEP). The GFS is run four times per day and produces forecasts out to 384 hours (16 days).
GOES: Geostationary Operational Environmental Satellite
GROUND FOG: Fog produced over the land by the cooling
of the lower atmosphere as it comes in contact with the ground.
Also known as radiation fog, and in parts of California as
tule fog.
GUST: A brief sudden increase in wind speed. Generally
the duration is less than 20 seconds and the fluctuation greater
than 10 mph.
GUST FRONT: The leading edge of the downdraft from
a thunderstorm.
HAIL: Precipitation in the form of balls or irregular
lumps of ice.
HAZE: Fine dry or wet dust or salt particles in the
air that reduce visibility.
HIGH: The center of an area of high pressure, usually
accompanied by anticyclonic and outward wind flow. Also known
as an anticyclone.
HIGH WIND: Sustained winds greater than or equal
to 40 mph or gust greater than or equal to 58 mph.
HUMIDITY: The amount of water vapor in the atmosphere.
(See relative humidity).
INSIDE SLIDER: A term used to describe a weather system
that moves into California from the northwest, with the bulk
of the system's energy moving inland toward the Sierra Nevada
and the Great Basin. These systems usually bring cool, breezy
weather to Northern and Central California, but since the
associated winds don't pass over the ocean, an inside slider
normally produces only light precipitation, if any. On some
occasions after the slider passes out of our region, surface
high pressure builds strongly into the Great Basin, causing
an offshore weather pattern to develop over our area, sometimes
with unseasonably warm temperatures and strong, dry winds
from the east or northeast.
INVERSION: An increase in temperature with height.
The reverse of the normal cooling with height in the atmosphere.
ISOBAR: A line of equal barometric pressure on a
weather map.
ITCZ: Inter-tropical Convergence Zone. The region
where the northeasterly and southeasterly tradewinds converge,
forming an often continuous band of clouds or thunderstorms
near the equator.
JET STREAM: Strong winds concentrated within a narrow
band in the atmosphere. The jet stream often "steers" surface
features such as front and low pressure systems.
KNOT: One nautical mile per hour (1.15 mph).
LAND BREEZE: A wind that blows from the land towards
a body of water. Also known as an offshore breeze.
LA NINA: A cooling of the equatorial waters in the
Pacific Ocean.
LAPSE RATE: The change in temperature with altitude
in the atmosphere.
LIKELY: In probability of precipitation statements,
the equivalent of a 60 or 70 percent chance.
LOW: The center of an area of low pressure, usually
accompanied by cyclonic and inward wind flow. Also known as
a cyclone.
MARITIME AIR MASS: Moist air mass originating over
the ocean.
MILLIBAR (MB): A unit of atmospheric pressure. 1
mb = 100 Pa (pascal). Normal surface pressure is approximately
1013 millibars.
MOS: Model Output Statistics.
MRF: "Medium Range Forecast" model generated at the National Centers for Environmental Prediction (NCEP). No longer called the MRF but rather GFS (Global Forecasting System).
NCEP: National Centers for Environmental Prediction.
Central computer and communications facility of the National
Weather Service; located in Washington, DC.
NEXRAD: NEXt Generation RADar. A NWS network of about
140 Doppler radars operating nationwide.
NGM: Nested Grid Model generated every 12 hours by
NCEP.
NOGAPS: Navy Operational Global Atmospheric Prediction
System. A global meteorological model operated by the Department
of Defense.
NUMERICAL WEATHER PREDICTION (NWP): Forecasting weather
by the use of numerical models, run on high speed computers.
Most of the NWP for the National Weather Service is done at
the National Centers for Environmental Prediction (NCEP).
NVA: Negative Vorticity Advection.
OCCLUDED FRONT: A complex frontal system that occurs
when a cold front overtakes a warm front. Also known as an
occlusion.
OFFSHORE BREEZE: A wind that blows from the land
towards a body of water. Also known as a land breeze.
OMEGA: A term used to describe vertical motion in
the atmosphere. The "omega equation" used in numerical weather
models is composed of two terms, the "differential vorticity
advection" term and the "thickness advection" term. Put more
simply, omega is determined by the amount of spin (or large
scale rotation) and warm (or cold) advection present in the
atmosphere. On a weather forecast chart, high values of omega
(or a strong omega field) relate to upward vertical motion
in the atmosphere. If this upward vertical motion is strong
enough and in a sufficiently moist airmass, precipitation
results.
ONSHORE BREEZE: A wind that blows from a body of
water towards the land. Also known as a seabreeze.
OROGRAPHIC UPLIFT: The vertical forcing of air by
terrain features such as hills or mountains. This can create
orographic clouds and/or precipitation.
OVERCAST: Sky condition when greater than 9/10 of
the sky is covered. See
Sky Terminology.
PACIFIC HIGH: A semipermanent anticyclone located
in the Eastern North Pacific.
PARTLY CLOUDY: Sky condition when between 3/10 and
7/10 of the sky is covered.
POPs: Probability of Precipitation. (see Precipitation
Terminology)
PRECIPITATION: Liquid or solid water molecules that
fall from the atmosphere and reach the ground.
PRESSURE: The force exerted by the interaction of
the atmosphere and gravity. Also known as atmospheric pressure.
PVA: Positive Vorticity Advection.
QPF: Quantitative Precipitation Forecast
QUASI-GEOSTROPHIC APPROXIMATION: An approximation
to the complete equations describing atmospheric motion in
which only the terms most important for the growth and decay
of synoptic scale extratropical weather systems (i.e., the
large areas of high and low pressure seen on weather maps)
are retained.
RADAR: An instrument used to detect precipitation
by measuring the strength of the electromagnetic signal reflected
back. (RADAR= Radio Detection and Ranging)
RADIATION FOG: See ground fog.
RADIOSONDE: An instrument attached to a weather balloon
that transmits pressure, humidity, temperature and winds as
it ascends.
RELATIVE HUMIDITY: The amount of water vapor in the
air, compared to the amount the air could hold if it was totally
saturated. (Expressed as a percentage).
REX BLOCK: A blocking pattern where there is an upper
level high located directly north of a closed low.
RFC: River Forecast Center. The California/Nevada
River Forecast Center is located in Sacramento.
RIDGE: An elongated area of high pressure in the
atmosphere.
RUC: Rapid Update Cycle model generated every 3 hours.
SANTA ANA WINDS: Relatively warm, dry winds that
blow into Southern California coastal areas from an anticyclone
located over the high deserts of California or Nevada. The
warmth and dryness are due to compressional heating. See also
Diablo winds.
SCATTERED CLOUDS: Sky condition when between 1/10
and 5/10 are covered. See
Sky Terminology.
SEA BREEZE: A wind that blows from a sea or ocean
towards a land mass. Also known as an onshore breeze.
SEA SURFACE TEMPERATURE (SST): Surface temperature
data collected using IR satellite imagery, buoy and ship data.
SEVERE THUNDERSTORM: A strong thunderstorm with wind
gusts in excess of 58 mph (50 knots) and/or hail with a diameter
of 3/4" or more. A thunderstorm with winds greater than 39
mph and/or hail greater than « inch is defined as approaching
severe.
SHOWER: Precipitation that is intermittent, both
in time, space or intensity.
SLEET: A type of frozen precipitation, consisting
of small transparent pellets.
SLIGHT CHANCE: In probability of precipitation statements,
usually equivalent to a 20 percent chance.
SMALL CRAFT ADVISORY: Winds in excess of 22 knots
(25 mph), and less than 34 knots (39 mph), that may cause
hazardous conditions for operators of small vessels.
SNOW: Frozen precipitation composed of ice particles
in complex hexagonal patterns.
SNOW ADVISORY: An advisory issued when snow is expected
to create hazardous or restricted travel conditions, but not
as severe as expected with a winter storm.
SNOW FLURRIES: Light snow showers, usually of an
intermittent nature with no measurable accumulation.
SPC: Storm Prediction Center. Located in Norman,
OK. This office is responsible for monitoring and forecasting
severe convective weather in the continental U.S. This includes
the issuance of Tornado and Severe Thunderstorm Watches.
SQUALL LINE: A non-frontal band, or line, of thunderstorms.
STATIONARY FRONT: A transition zone between airmasses,
with neither advancing upon the other.
STORM: In marine usage, winds 48 knots (55 mph) or
greater.
STRAIGHT LINE WINDS: Thunderstorm winds most often
found with the gust front. They originate from downdrafts
and can cause damage which occurs in a "straight line", as
opposed to tornadic wind damage which has circular characteristics.
STRATUS: Low-level clouds with a rather uniform base
SUBSIDENCE: Sinking air that is associated with warming
air and little cloud formation.
SUBTROPICAL JET: The branch of the jet stream that
is found in the lower latitudes.
SUSTAINED WINDS: The wind speed obtained by averaging
the observed values over a one minute period.
SWELLS: Ocean waves of regular and longer duration
than wind waves.
THICKNESS: The difference in height between two atmospheric
pressure levels. The thickness is directly related to the
average temperature in the column of air between the pressure
levels (thus, higher thicknesses typically mean warmer temperatures).
The most common thickness used is for the 1000-500mb layer.
Forecasters also look at other thickness layers such as 1000-850
mb layer (sometimes called low level thickness). Thicknesses
are measure in decameters (1 decameter=10 meters).
THUNDERSTORM: A storm with lightning and thunder,
produced by a cumulonimbus cloud, usually producing gusty
winds, heavy rain and sometimes hail.
TORNADO: A violent rotating column of air, in contact
with the ground, pendant from a cumulonimbus cloud. A tornado
does not require the visible presence of a funnel cloud.
TROPICAL DEPRESSION: Tropical mass of thunderstorms
with a cyclonic wind circulation and winds between 20 and
34 knots.
TROPICAL DISTURBANCE: An organized mass of tropical
thunderstorms, with a slight cyclonic circulation, and winds
less than 20 knots.
TROPICAL STORM: An organized cyclone in the tropics
with wind speed between 35 and 64 knots.
TROUGH: An elongated area of low pressure at the
surface or aloft.
TULE FOG: Radiation fog in the Central Valley. It
forms during night and morning hours in late fall and winter
months following the first significant rainfall. A leading
cause of weather related casualties in California.
TURBULENCE: Disrupted flow in the atmosphere that
produces gusts and eddies.
UKMET: United Kingdom forecast model.
UPWELLING: The rising of cold water from the deeper
areas of the ocean to the surface. This phenomena often occurs
along the California coast during the spring and summer.
VEERING WINDS: A clockwise change in wind direction.
Veering winds with height are indicative of warm air advection
(WAA).
VIRGA: Precipitation falling from the base of a cloud
and evaporating before it reaches the ground.
VISIBILITY: The horizontal distance an observer can
see and identify a prominent object.
VORTICITY: A measure of the amount of "spin" (or
rotation) in the atmosphere.
WAA: Warm Air Advection
WARM FRONT: A boundary between a warm airmass that
is replacing a cooler airmass.
WARNING: Forecast issued when a particular hazard
is "imminent" or already occurring (e.g., tornado warning,
flash flood warning).
WATCH: Forecast issued well in advance to alert the
public of the possibility of a particular hazard (eg. tornado
watch, flash flood watch).
WATERSPOUT: A column of rotating air over a body
of water (ie. a tornado over the water).
WIND ADVISORY: Sustained winds 25 to 39 mph and/or
gusts to 57 mph. Issuance is normally site specific. However,
winds of this magnitude occurring over an area that frequently
experiences such winds (e.g., the normal strong summertime
winds near the San Francisco Bay, would not require the issuance
of this product).
WIND SHEAR: The change of wind speed or direction
with distance, usually vertical.
WIND WAVES: Short period and irregular waves created
by the flow of air over the water.
WINTER STORM: A heavy snow event. In the Sierra Nevada
below 7000 feet, a snow accumulation of >6 inches/12 hrs or
>12 inches/24 hrs; and above 7000 feet, >8 inches/12 hrs or
>18 inches/24 hrs.
WSFO/WFO: Weather Service Forecast Office / Weather
Forecast Office. The San Francisco Bay Area WFO is located
in Monterey. Other WFOs for California are located in Eureka,
Sacramento, Hanford, Oxnard and San Diego.
|