Cool & Calm
Cool & Calm
Cool & Calm
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Weather Glossary Terms & Abbreviations
- Convective outlook issued by the Storm
Prediction Center. Abbreviation for Anticipated Convection; the term originates from
the header coding [ACUS1] of the transmitted product.
- (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
precede the rapid development of thunderstorms.
- 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.
ADVY - Advisory
- Transport of an atmospheric property by
the wind. See cold advection, or warm advection.
AFTN - Afternoon
- Generally, 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.
thunderstorms are associated with some type of forcing mechanism,
synoptic-scale or otherwise, the existence of true air-mass
thunderstorms is debatable.
- Rotation in the opposite sense as the
Earth's rotation, i.e., clockwise in the Northern Hemisphere as
would be seen from above. The opposite of cyclonic
- The flat, spreading top of a Cb
(cumulonimbus), often shaped like an anvil. Thunderstorm anvils
may spread hundreds of miles downwind from the thunderstorm
itself, and sometimes may spread upwind.
- Anvil Crawler
- [Slang], 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
- Anvil Dome
- A large overshooting top or
AOA - At or Above
AOB - At or Below
- (severe levels) - A thunderstorm which
contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2" or
larger but less than 3/4" in diameter.
- A low, horizontal cloud formation
associated with the leading edge of thunderstorm outflow (i.e.,
the gust front). Roll clouds
and shelf clouds both are types of
ATTM - At This Time
- AVN - The Aviation Model (120-hour numerical
model of the atmosphere)
- Back-building Thunderstorm
- A thunderstorm in which new development
takes place on the upwind side (usually the west or southwest
side), such that the storm seems to remain stationary or propagate
in a backward direction.
- Backing Winds
- Winds which shift in a counterclockwise
direction with time at a given location (e.g. from southerly to
southeasterly), or change direction in a counterclockwise sense
with height (e.g. westerly at the surface but becoming more
southerly aloft). The opposite of veering
storm spotting, a backing wind usually refers to the turning of a
south or southwest surface wind with time to a more east or
southeasterly direction. Backing of the surface wind can increase
the potential for tornado development by increasing the
directional shear at low levels.
- A region in which a temperature gradient
exists on a constant pressure surface. Baroclinic zones are
favored areas for strengthening and weakening systems; barotropic systems, on the
other hand, do not exhibit significant changes in intensity. Also,
shear is characteristic of a baroclinic zone.
- A weather system in which temperature and
pressure surfaces are coincident, i.e., temperature is uniform (no
temperature gradient) on a constant pressure surface. Barotropic
systems are characterized by a lack of wind shear, and
thus are generally unfavorable areas for severe thunderstorm
development. See baroclinic zone.
operational meteorology, references to barotropic systems refer to
equivalent barotropic systems -
systems in which temperature gradients exist, but are parallel to
height gradients on a constant pressure surface. In such systems,
height contours and isotherms are parallel everywhere,
and winds do not change direction with height.
As a rule, a true
equivalent barotropic system can never be achieved in the real
atmosphere. While some systems (such as closed lows
or cutoff lows) may reach a state
that is close to equivalent barotropic, the term barotropic system
usually is used in a relative sense to describe systems that are
really only close to being equivalent barotropic, i.e., isotherms
and height contours are nearly parallel everywhere and directional
wind shear is weak.
- BECMG - Becoming
BKN - Broken
BL - Boundary Layer
- A blizzard means that the following
conditions are expected to prevail for a period of 3 hours or
- Sustained wind or frequent gusts to 35
miles an hour or greater; and
- considerable falling and/or blowing
snow (i.e., reducing visibility frequently to less than ¼ mile).
- BLO - Below (refers to cloud coverage)
- Blowing Dust or
- Strong winds over dry ground, that has
little or no vegetation, can lift particles of dust or sand into
the air. These airborne particles can reduce visibility, cause
respiratory problems, and have an abrasive affect on machinery. A
concentration reducing the visibility to ¼ mile or less often
poses hazards for travelers.
- Blowing Snow
- Blowing snow is wind-driven snow that
reduces surface visibility. Blowing snow can be falling snow or
snow that has already accumulated but is picked up and blown by
strong winds. Blowing snow is usually accompanied by drifting
- 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
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 part 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, but again the exact depth is
indeterminate. Like friction, the effects of insolation and
radiational cooling are strongest within this layer.
- Bow Echo
- A radar echo which is linear but bent
outward in a bow shape. Damaging straight-line winds often
occur near the "crest" or center of a bow echo. Areas of
circulation also can develop at either end of a bow echo, which
sometimes can lead to tornado formation - especially in the left
(usually northern) end, where the circulation exhibits cyclonic
C - Celsius
CAA - Cold Air Advection
- (or Capping Inversion) - 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. 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. See CIN.
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
- 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 1,000 joules per
kilogram (j/kg), and in extreme cases may exceed 5,000 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.) See also CIN.
- 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
- Cloud-to-Cloud lightning.
- Convection in
the form of a single updraft, downdraft, or
updraft/downdraft couplet, typically seen as a vertical dome or
tower as in a towering cumulus cloud. A
typical thunderstorm consists of several cells.
The term "cell" also
is used to describe the radar echo returned by an individual
shower or thunderstorm. Such usage, although common, is
- Cloud-to-Ground lightning flash.
CHC - Chance
- Channeled High
- In mountainous areas or in cities with
tall buildings, air may be channeled through constricted passages
producing high winds. Santa Ana winds and winds through passes
from the cold Alaskan interior to the sea are examples of these
winds. Channeled high winds are local in nature but can be
extremely strong. These winds generally occur in well-defined
- Chinook or Foehn
- These are warm, dry winds that occur in
the lee of high mountain ranges. It is a fairly common wintertime
phenomena in the mountainous west and in parts of Alaska. These
winds develop in well-defined areas and can be quite
CI - Cirrus clouds
- 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.
- High-level clouds (16,000 feet or more),
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).
- Cloud Streets
- Rows of cumulus or cumulus-type clouds
aligned parallel to the low-level flow. Cloud streets sometimes
can be seen from the ground, but are seen best on satellite
- Cold Advection
- Transport of cold air into a region by
- A funnel
cloud or (rarely) a small, relatively weak tornado that can
develop from a small shower or thunderstorm when the air aloft is
unusually cold (hence the name). They are much less violent than
other types of tornadoes.
- Cold Pool
- A region of relatively cold air,
represented on a weather map analysis as a relative minimum in
temperature surrounded by closed isotherms. Cold
pools aloft represent regions of relatively low stability, while
surface-based cold pools are regions of relatively stable
- A funnel-shaped cloud associated with
rotation and consisting of condensed water droplets (as opposed to
smoke, dust, debris, etc.).
- A pattern of wind flow in which air flows
inward toward an axis oriented parallel to the general direction
of flow. It is the opposite of difluence.
Confluence is not the same as convergence.
Winds often accelerate as they enter a confluent zone, resulting
in speed divergence which offsets the
(apparent) converging effect of the confluent flow.
- CONUS - Contnental U.S.
- 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,
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.
- The approximate temperature that the air
near the ground must warm to in order for surface-based
convection to develop, based on analysis of a sounding.
Calculation of the
convective temperature involves many assumptions, such that
thunderstorms sometimes develop well before or well after the
convective temperature is reached (or may not develop at all).
However, in some cases the convective temperature is a useful
parameter for forecasting the onset of convection.
- A contraction of a vector field; the
opposite of divergence. Convergence in a
horizontal wind field indicates that more air is entering a given
area than is leaving at that level. To compensate for the
resulting "excess," vertical motion may result: upward forcing if
convergence is at low levels, or downward forcing (subsidence) if
convergence is at high levels. Upward forcing from low-level
convergence increases the potential for thunderstorm development
(when other factors, such as instability,
are favorable). Compare with confluence.
COORD - Coordination
CU - Cumulus clouds
- Cutoff Low
- A closed 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
CWFA - County Warning and Forecast Area
- (Pronounced day-RAY-cho), a widespread
and usually fast-moving windstorm associated with convection. Derechos include any
family of downburst clusters produced by an extratropical MCS, and
can produce damaging straight-line winds over
areas hundreds of miles long and more than 100 miles
- Dew Point
- (or Dew-point Temperature) - A measure of
atmospheric moisture. It is the temperature to which air must be
cooled in order to reach saturation (assuming air pressure and
moisture content are constant).
- (or Diffluence) - A pattern of wind flow
in which air moves outward (in a "fan-out" pattern) away from a
central axis that is oriented parallel to the general direction of
the flow. It is the opposite of confluence.
Difluence in an
upper level wind field is considered a favorable condition for
severe thunderstorm development (if other parameters are also
favorable). But difluence is not the same as divergence.
In a difluent flow, winds normally decelerate as they move through
the region of difluence, resulting in speed convergence which
offsets the apparent diverging effect of the difluent
- Daily; related to actions which are
completed in the course of a calendar day, and which typically
recur every calendar day (e.g., diurnal temperature rises during
the day, and diurnal falls at night).
- The expansion or spreading out of a
vector field; usually said of horizontal winds. It is the opposite
of convergence. Divergence at upper
levels of the atmosphere enhances upward motion, and hence the
potential for thunderstorm development (if other factors also are
- A strong downdraft that induces an
outburst of damaging winds on or near the ground.
- Drifting Snow
- Drifting snow is an uneven distribution
of snowfall/snow depth caused by strong surface winds. Drifting
snow may occur during or after a snowfall. Drifting snow is
usually associated with blowing snow.
- Dry Line
- A boundary separating moist and dry air
masses, and an important factor in severe weather frequency in the
Great Plains. It typically lies north-south across the central and
southern high Plains states during the spring and early summer,
where it separates moist air from the Gulf of Mexico (to the east)
and dry desert air from the southwestern states (to the west). The
dry line typically advances eastward during the afternoon and
retreats westward at night. However, a strong storm system can
sweep the dry line eastward into the Mississippi Valley, or even
further east, regardless of the time of day.
A typical dry line
passage results in a sharp drop in humidity (hence
the name), clearing skies, and a wind shift from south or
southeasterly to west or southwesterly. (Blowing dust and rising
temperatures also may follow, especially if the dry line passes
during the daytime. These changes occur in reverse order when the
dry line retreats westward. Severe and sometimes tornadic
thunderstorms often develop along a dry line or in the moist air
just to the east of it, especially when it begins moving
- A microburst
with little or no precipitation reaching the ground; most common
in semi-arid regions. They may or may not produce lightning. Dry
microbursts may develop in an otherwise fair-weather pattern;
visible signs may include a cumulus cloud or small Cb with a high base
and high-level virga, or perhaps only an orphan anvil
from a dying rain shower. At the ground, the only visible sign
might be a dust plume or a ring of blowing dust beneath a local
area of virga.
- Dry Slot
- A zone of dry (and relatively cloud-free)
air which wraps east- or northeastward into the southern and
eastern parts of a synoptic scale or mesoscale low
pressure system. A dry slot generally is seen best on satellite
- Generally, any forces that produce motion
or affect change. In operational meteorology, dynamics usually
refer specifically to those forces that produce vertical motion in
DVV - Downward Vertical Velocity
DZ - Drizzle
- E - East
- European Center for Medium-Range Weather Forecasting. Operational references in
forecast discussions typically refer to the ECMWF's medium-range
forecast model. See MRF, UKMET.
- See Enhanced Fujita
occurring within an elevated layer, i.e., a layer in which the
lowest portion is based above the earth's surface. Elevated
convection often occurs when air near the ground is relatively
cool and stable, e.g., during periods of isentropic lift, when an
unstable layer of air is present aloft.
In cases of elevated
convection, stability indices based on near-surface measurements
(such as the lifted index) typically will
underestimate the amount of instability
present. Severe weather is possible from elevated convection, but
is less likely than it is with surface-based
- Enhanced-Fujita Scale
- (or E F Scale) - A scale of wind damage intensity in which wind speeds are
inferred from an analysis of wind damage:
||DERIVED EF SCALE
||OPERATIONAL EF SCALE|
||3 Second Gust (mph)
||3 Second Gust (mph)
||3 Second Gust (mph)|
*** IMPORTANT NOTE
ABOUT ENHANCED F-SCALE WINDS: The
Enhanced F-scale still is a set of wind estimates (not
measurements) based on damage. Its uses three-second gusts
estimated at the point of damage based on a judgment of 8 levels
of damage to the 28 indicators listed below. These estimates vary
with height and exposure. Important:
The 3 second gust is not the same wind as in standard surface
observations. Standard measurements are taken by weather stations
in open exposures, using a directly measured, "one minute mile"
- An option used by the SPC in tornado and
severe thunderstorm watches when the potential for strong/violent
tornadoes, or unusually widespread damaging straight-line winds,
statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE
POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado watches
with enhanced wording. Severe thunderstorm watches may include the
statement "THIS IS A PARTICULARLY
DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING
WINDS," usually when a derecho event is occurring or forecast to
- The region upstream from a
wind speed maximum in a jet stream (jet max), in
which air is approaching (entering) the region of maximum winds,
and therefore is accelerating. This acceleration results in a
vertical circulation that creates divergence in
the upper-level winds in the right half of the entrance region (as
would be viewed looking along the direction of flow).
results in upward motion of air in the right rear quadrant (or
right entrance region)
of the jet max. Severe weather potential sometimes increases in
this area as a result. See also exit region,
left exit region.
- (or EL) - On an upper air sounding, the
level above the level of free convection (LFC) at which the
temperature of a rising air parcel again equals the temperature of
The height of the EL is the height at which
thunderstorm updrafts no longer accelerate upward.
Thus, to a close approximation, it represents the height of
expected (or ongoing) thunderstorm tops. However, strong updrafts
will continue to rise past the EL before stopping, resulting in
storm tops that are higher than the EL. This process sometimes can
be seen visually as an overshooting top or anvil dome.
The EL typically is
higher than the tropopause, and is a more accurate
reference for storm tops.
- ERN - Eastern
ETA - The Eta Model (60-hour numerical model of the
- Exit Region
- The region downstream from a wind speed
maximum in a jet stream (jet max), in
which air is moving away from the region of maximum winds, and
therefore is decelerating. This deceleration results in divergence in the upper-level
winds in the left half of the exit region
(as would be viewed looking along the direction of flow).
results in upward motion of air in the left front quadrant (or
left exit region) of the jet max. Severe weather potential
sometimes increases in this area as a result. See also entrance region, right entrance
F - Fahrenheit
- Excessive heat occurs from a combination
of high temperatures (significantly above normal) and high
humidities. At certain levels, the human body cannot maintain
proper internal temperatures and may experience heat stroke. The
"Heat Index" is a measure of the effect of the combined elements
on the body.
FA - Forecast Area
FAN - AVN MOS Guidance (older version)
- FCST - Forecast
- Flash Flood
- A flood which is caused by heavy or
excessive rainfall in a short period of time, generally less than
6 hours. Also, at times a dam failure can cause a flash flood,
depending on the type of dam and time period during which the
- The inundation of a normally dry area
caused by an increased water level in an established watercourse,
such as a river, stream, or drainage ditch, or ponding of water at
or near the point where the rain fell.
- Flood/Flash Flood
- Issued to inform the public that flooding
is imminent or in progress.
- Flood/Flash Flood
- Issued to inform the public and
cooperating agencies that current and developing
hydrometeorological conditions are such that there is a threat of
flooding, but the occurrence is neither certain nor
- FMR - MRF MOS (older version)
- Fog is water droplets suspended in the
air at the Earth's surface. Fog is often hazardous when the
visibility is reduced to ¼ mile or less.
- 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.
FROPA - Frontal Passage
- A freeze is when the surface air
temperature is expected to be 32°F or below over a widespread area
for a climatologically significant period of time. Use of the term
is usually restricted to advective situations or to occasions when
wind or other conditions prevent frost. "Killing" may be used
during the growing season when the temperature is expected to be
low enough for a sufficient duration to kill all but the hardiest
- Freezing Rain or
- This occurs when rain or drizzle freezes
on surfaces, such as the ground, trees, power lines, motor
vehicles, streets, highways, etc. Small accumulations of ice can
cause driving and walking difficulties while heavy accumulations
produce extremely dangerous and damaging situations primarily by
pulling down trees and utility lines.
- Frost describes the formation of thin ice
crystals on the ground or other surfaces in the form of scales,
needles, feathers, or fans. Frost develops under conditions
similar to dew, except the temperatures of the Earth's surface and
earthbound objects falls below 32°F. As with the term "freeze,"
this condition is primarily significant during the growing season.
If a frost period is sufficiently severe to end the growing season
or delay its beginning, it is commonly referred to as a "killing
frost." Because frost is primarily an event that occurs as the
result of radiational cooling, it frequently occurs with a
thermometer level temperature in the mid-30s.
FT - Foot or Feet
- Funnel Cloud
- A condensation funnel
extending from the base of a towering
cumulus or Cb, associated with a rotating column of
air that is not in contact with the
ground (and hence different from a tornado). A condensation funnel
is a tornado, not a funnel cloud, if
either a) it is in contact with the ground or b) a debris cloud or
dust whirl is visible beneath it.
- FWC - NGM MOS Guidance
- Gradient High
- These high winds usually cover a large
area and are due to synoptic-scale, extra-tropical low pressure
- Heavily rimed new snow, often shaped like little Styrofoam balls. Graupel is that Styrofoam ball type of snow that stings your face when it falls from the sky. It forms from strong convective activity within a storm (upward vertical motion) caused by the passage of a cold front or springtime convective showers. The static buildup from all these falling graupel pellets sometimes cause lightning as well.
- Gust Front
- The leading edge of gusty surface winds
from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also gustnado or outflow boundary.
- (or Gustinado) - [Slang], gust front
tornado. 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. Gustnadoes are not associated with storm-scale rotation
(i.e. mesocyclones); they are more likely to be associated
visually with a shelf cloud than with a wall cloud.
- Heavy Snow
- This generally means...
In forecasts, snowfall
amounts are expressed as a range of values, e.g., "8 to 12
inches." However, in heavy snow situations where there is
considerable uncertainty concerning the range of values, more
appropriate phrases are used, such as "...up to 12 inches..." or
alternatively "...8 inches or more...".
- snowfall accumulating to 4" or more in
depth in 12 hours or less; or
- snowfall accumulating to 6" or more in
depth in 24 hours or less.
- A property of a moving fluid which
represents the potential for helical flow (i.e. flow which follows
the pattern of a corkscrew) to evolve. Helicity is proportional to
the strength of the flow, the amount of vertical wind shear, and
the amount of turning in the flow (i.e. vorticity).
is computed from the vertical wind profile in the lower part of
the atmosphere (usually from the surface up to 3 km), and is
measured relative to storm motion. Higher values of helicity
(generally, around 150 m2/s2 or more) favor the development of
mid-level rotation (i.e. mesocyclones). Extreme values can exceed
- High Wind
- Sustained wind speeds of 40 mph or
greater lasting for 1 hour or longer, or winds of 58 mph or
greater for any duration.
- (or Hook Echo) - A radar reflectivity
pattern characterized by a hook-shaped extension of a thunderstorm
echo, usually in the right-rear part of the storm (relative to its
direction of motion). A hook often is associated with a
mesocyclone, and indicates favorable conditions for tornado
- HP Storm
- or HP Supercell - High-Precipitation
storm (or High-Precipitation supercell). A supercell thunderstorm
in which heavy precipitation (often including hail) falls on the
trailing side of the mesocyclone.
Precipitation often totally envelops the
region of rotation, making visual identification of any embedded
tornadoes difficult and very dangerous. Unlike most classic
supercells, the region of rotation in many HP storms develops in
the front-flank region of the storm (i.e., usually in the eastern
portion). HP storms often produce extreme and prolonged downburst
events, serious flash flooding, and very large damaging hail
- Generally, a measure of the water vapor
content of the air. Popularly, it is used synonymously with relative humidity.
- Ice Storm
- An ice storm is used to describe
occasions when damaging accumulations of ice are expected during
freezing rain situations. Significant accumulations of ice pull
down trees and utility lines resulting in loss of power and
communication. These accumulations of ice make walking and driving
extremely dangerous. Significant ice accumulations are usually
accumulations of ¼" or greater.
- See upper
- Incoming solar radiation. Solar heating;
- 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.
See lifted index.
- 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.
- IR - Infrared
- Lifting of air that is traveling along an
upward-sloping isentropic surface.
often is referred to erroneously as overrunning,
but more accurately describes the physical process by which the
lifting occurs. Situations involving isentropic lift often are
characterized by widespread stratiform
clouds and precipitation, but may include elevated convection in
the form of embedded thunderstorms.
- A two-dimensional surface containing
points of equal potential temperature.
- A line connecting points of equal
- A line connecting points of equal dew point temperature.
- A line connecting points of equal
ISOLD - Isolated
- General term for a line connecting points
of equal value of some quantity. Isobars, isotherms, etc. all are examples of
- A line connecting points of equal wind
- A line connecting points of equal
- Jet Max
- (or Speed Max, Jet Streak) -
a point or area of relative maximum wind speeds within a jet stream.
- Jet Streak
- A local wind speed maximum within a jet stream.
- Jet Stream
- 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(S) - Knot(s)
- [Slang], a tornado that does not arise
from organized storm-scale rotation and therefore is not
associated with a wall cloud (visually) or a
mesocyclone (on radar). Landspouts typically are observed beneath
towering cumulus clouds
(often as no more than a dust whirl), and essentially are the
land-based equivalents of waterspouts.
- 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.
- Left Front
- (or Left Exit Region) - The area
downstream from and to the left of an upper-level jet
max (as would be viewed looking along the direction of flow).
Upward motion and severe thunderstorm potential sometimes are
increased in this area relative to the wind speed maximum. See
also entrance region, right rear
LFQ - Left Front Quandrant
LI - Lifted Index
- Lifted Index (or
- A common measure of atmospheric instability. Its value is
obtained by computing the temperature that air near the ground
would have if it were lifted to some higher level (around 18,000
feet, usually) and comparing that temperature to the actual
temperature at that level. Negative values indicate instability -
the more negative, the more unstable the air is, and the stronger
the updrafts are likely to be with any
developing thunderstorms. However there are no "magic numbers" or
threshold LI values below which severe weather becomes
- LLJ - Low Level Jet
- 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
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.
- A convective downdraft with an affected
outflow area of at least 2½ miles wide and peak winds lasting
between 5 and 20 minutes. Intense macrobursts may cause
tornado-force damage of up to F3 intensity.
MAV - AVN MOS Guidance
MB - Millibar
- Mesoscale Convective Complex. A large MCS, generally round
or oval-shaped, which normally reaches peak intensity at night.
The formal definition includes specific minimum criteria for size,
duration, and eccentricity (i.e., "roundness"), based on the cloud
shield as seen on infrared satellite photographs:
- Size: Area of cloud top -32 degrees C
or less: 100,000 square kilometers or more (slightly smaller
than the state of Ohio), and area of cloud top -52 degrees C or
less: 50,000 square kilometers or more.
- Duration: Size criteria must be met for
at least 6 hours.
- Eccentricity: Minor/major axis at least
MCCs typically form during the afternoon and
evening in the form of several isolated thunderstorms, during
which time the potential for severe weather is greatest. During
peak intensity, the primary threat shifts toward heavy rain and
- Mesoscale Convective System. A complex of thunderstorms which
becomes organized on a scale larger than the individual
thunderstorms, and normally persists for several hours or more.
MCSs may be round or linear in shape, and include systems such as
tropical cyclones, squall lines, and MCCs (among others).
MCS often is used to describe a cluster of thunderstorms that does
not satisfy the size, shape, or duration criteria of an
- Medium Range
- In forecasting, (generally) three to
seven days in advance.
- Large-scale atmospheric flow in which the
north-south component (i.e., longitudinal, or along a meridian) is
pronounced. The accompanying zonal (east-west) component often is
weaker than normal. Compare with zonal
- A regional network of observing stations
(usually surface stations) designed to diagnose mesoscale weather features and
their associated processes.
- Size scale referring to weather systems
smaller than synoptic-scale systems but larger than storm-scale
systems. Horizontal dimensions generally range from around 50
miles to several hundred miles. Squall
lines, MCCs, and MCSs are examples of
mesoscale weather systems.
- Mesoscale High
- These high winds usually follow the
passage of organized convective systems and are associated with
wake depressions or strong mesohighs.
MEX - MRF MOS
- A convective downdraft with an affected
outflow area of less than 2½ miles wide and peak winds lasting
less than 5 minutes. Microbursts may induce dangerous
horizontal/vertical wind shears, which can adversely affect
aircraft performance and cause property damage.
- A measure of the degree to which moist
air is converging into a given area, taking into account the
effect of converging winds and moisture advection. Areas of
persistent moisture convergence are favored regions for
thunderstorm development, if other factors (e.g., instability) are
MOS - Model Output Statistics
- Medium-Range Forecast model; one of the
operational forecast models run at NCEP. The MRF is
run once daily, with forecast output out to 240 hours (10
N - North
- National Centers for Environmental Prediction; the modernized version of
NE - Northeast
- An upper
level system which is tilted to the west with increasing
latitude (i.e., with an axis from southeast to northwest). A
negative-tilt trough often is a sign of a developing or
NERN - NortheasternNGM - The Nested Grid Model (48-hour numerical
model of the atmosphere)
NM - Nautical Mile(s)
NOGAPS - Navy Operational Global
Atmospheric Prediction System (120-hour numerical model)
NRN - Northern
NVA - Negative Vorticity
NW - Northwest
NWRN - Northwestern
NWS - National Weather
OBS - Observations
A storm-scale or mesoscale
boundary separating thunderstorm-cooled air (outflow) from the
surrounding air; similar in effect to a cold front, with passage
marked by a wind shift and usually a drop in temperature. Outflow
boundaries may persist for 24 hours or more after the
thunderstorms that generated them dissipate, and may travel
hundreds of miles from their area of origin.
often develop along outflow boundaries, especially near the point
of intersection with another boundary (cold front, dry line, another outflow boundary,
etc.; see triple point).
- An outlook is used to indicate that a
hazardous weather or hydrologic event may develop. It is intended
to provide information to those who need considerable lead time to
prepare for the event.
- OVC - Overcast
- A weather pattern in which a relatively
warm air mass is in motion above another air mass of greater
density at the surface. Embedded thunderstorms sometimes develop
in such a pattern; severe thunderstorms (mainly with large hail)
can occur, but tornadoes are unlikely.
Overrunning often is applied to the case of
warm air riding up over a retreating layer of colder air, as along
the sloping surface of a warm front. Such use of
the term technically is incorrect, but in general it refers to a
pattern characterized by widespread clouds and steady
precipitation on the cool side of a front or other
- (or Penetrating Top) - A dome-like
protrusion above a thunderstorm anvil, representing a very strong
updraft and hence a higher potential
for severe weather with that storm. A persistent and/or large
overshooting top (anvil dome) often is present on a
short-lived overshooting top, or one that forms and dissipates in
cycles, may indicate the presence of a pulse
PAC - Pacific
POP - Probability of
- An upper
level system which is tilted to the east with increasing
latitude (i.e., from southwest to northeast). A positive-tilt
trough often is a sign of a weakening weather system, and
generally is less likely to result in severe weather than a negative-tilt trough if
all other factors are equal.
PROG - Forecast
- Pulse Storm
- A thunderstorm within which a brief
period (pulse) of strong updraft occurs, during and
immediately after which the storm produces a short episode of
severe weather. These storms generally are not tornado producers,
but often produce large hail and/or damaging winds. See also overshooting top.
PVA - Positive Vorticity Advection
PW - Precipitable Water
PWAT - Precipitable Water
QG - Quasigeostrophic
QPF - Quantitative Precipitation
RA - Rain
RAOB - Radiosonde Observation (Upper
- A dimensionless ratio, expressed in
percent, of the amount of atmospheric moisture present relative to
the amount that would be present if the air were saturated. Since
the latter amount is dependent on temperature, relative humidity
is a function of both moisture content and temperature. As such,
relative humidity by itself does not directly indicate the actual
amount of atmospheric moisture present. See dew
- (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.
RH - Relative Humidity
- Right Entrance
- (or Right
Rear Quadrant) - The area upstream from
and to the right of an upper-level jet max (as
would be viewed looking along the direction of flow). Upward
motion and severe thunderstorm potential sometimes are increased
in this area relative to the wind speed maximum. See also exit region, left front
- An elongated area of relatively high
atmospheric pressure; the opposite of trough.
- Right Mover
- A thunderstorm that moves appreciably to
the right relative to the main steering winds and to other nearby
thunderstorms. Right movers typically are associated with a high
potential for severe weather. (Supercells often are right
RRQ - Right Rear Quadrant
- Right Rear
- see Right
- Roll Cloud
- A low, horizontal tube-shaped arcus
cloud associated with a thunderstorm gust front
(or sometimes with a cold front). Roll clouds are relatively
rare; they are completely detached from the thunderstorm base or
other cloud features, thus differentiating them from the more
familiar shelf clouds. Roll clouds usually
appear to be "rolling" about a horizontal axis, but should not be
confused with funnel clouds.
- (or Rope Funnel) - A narrow, often
contorted condensation funnel
usually associated with the decaying stage of a tornado. See rope stage.
- Rope Cloud
- In satellite meteorology, a narrow,
rope-like band of clouds sometimes seen on satellite images along
a front or other boundary. The term
sometimes is used synonymously with rope or rope
- Rope Stage
- The dissipating stage of a tornado,
characterized by thinning and shrinking of the condensation funnel into
a rope (or rope funnel).
Damage still is possible during this stage.
- Rapid Update Cycle,
a numerical model run at NCEP that focuses on short-term (up to
12 h) forecasts and small-scale (mesoscale)
weather features. Forecasts are prepared every 3 hours for the
contiguous United States.
S - South
SCT - Scattered
SAT - Satellite
SE - Southeast
SERN - Southeastern
- Severe Local Storm
- A convective storm that usually covers a
relatively small geographic area, or moves in a narrow path, and
is sufficiently intense to threaten life and/or property. Examples
include severe thunderstorms with large hail, damaging wind, or
tornadoes. Although cloud-to-ground lightning is not a criteria
for severe local storms, it is acknowledged to be highly dangerous
and a leading cause of deaths, injuries, and damage from
thunderstorms. A thunderstorm need not be severe to generate
frequent cloud-to-ground lightning. Additionally, excessive
localized convective rains are not classified as severe storms but
often are the product of severe local storms. Such rainfall may
result in related phenomena (flash floods) that threaten life and
- A thunderstorm that produces a tornado,
winds of at least 58 mph (50 knots), and/or hail at least ¾" in
diameter. Structural wind damage may imply the occurrence of a
severe thunderstorm. A thunderstorm wind equal to or greater than
40 mph (35 knots) and/or hail of at least ½" is defined as approaching severe.
- The relative coverage and/or threat for
severe thunderstorms in a specified area. The following describes
the possible density/risk of severe thunderstorms in an outlook
- A non-severe category that indicates an area of strong
convection; used to highlight areas where strong thunderstorms
are anticipated but not expected to become severe.
- SLIGHT risk
- Severe thunderstorms are expected; the severe storms may not
have a mesoscale organization or may be isolated in areal extent
with between 2-5% aerial coverage.
risk - Severe thunderstorms are expected and are anticipated to
be more organized on the mesoscale. They will be more numerous
or widespread than in the SLIGHT category. The potential for
personal injury and/or significant property damage is
significantly enhanced with between 6 and 10 percent coverage. A
moderate risk indicates the possibility of a significant severe
- HIGH risk -
Severe thunderstorms are expected and are anticipated to be
widespread. A dangerous situation exists with the strong
potential for killer tornadoes, devastating windstorms, and
widespread property damage. This category generally is confined
for use in anticipated tornado outbreaks with more than 10
percent coverage. A high risk is rare and implies the
possibility of a major severe weather outbreak.
SFC - Surface
SHRA - Rain Showers
- Variation in wind speed (speed shear) and/or direction
(directional shear) over a short distance. 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.
- Shelf Cloud
- A low, horizontal wedge-shaped arcus
cloud, associated with a thunderstorm gust front
(or occasionally with a cold front, even in the absence of
thunderstorms). Unlike the roll cloud,
the shelf cloud is attached to the base of the parent cloud above
it (usually a thunderstorm). Rising cloud motion often can be seen
in the leading (outer) part of the shelf cloud, while the
underside often appears turbulent, boiling, and wind-torn.
- Short Term
- A product used to convey information
regarding weather or hydrologic events in the next few
- Sleet is defined as pellets of ice
composed of frozen or mostly frozen raindrops or refrozen
partially melted snowflakes. These pellets of ice usually bounce
after hitting the ground or other hard surfaces. Heavy sleet is a
relatively rare event defined as an accumulation of ice pellets
covering the ground to a depth of ½" or more.
- Smoke in various concentrations can cause
significant problems for people with respiratory ailments. It
becomes a more universal hazard when visibilities are reduced to ¼
mile or less.
- Snow Flurries
- Snow flurries are an intermittent light
snowfall of short duration (generally light snow showers) with no
measurable accumulation (trace category).
- Snow Showers
- A snow shower is a short duration of
moderate snowfall. Some accumulation is possible.
- Snow Squalls
- Snow squalls are intense, but limited
duration, periods of moderate to heavy snowfall, accompanied by
strong, gusty surface winds and possibly lightning (generally
moderate to heavy snow showers). Snow accumulation may be
- Squall Line
- A solid or broken line of thunderstorms
or squalls. The line may extend across several hundred
- Speed Shear
- The component of wind shear which
is due to a change in wind speed with height, e.g., southwesterly
winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000
feet. Speed shear is an important factor in severe weather
development, especially in the middle and upper levels of the
SRN - Southern
- Squall Line
- A solid or nearly solid line or band of
- Generally, any wind that is not
associated with rotation, used mainly to differentiate them from
- Having extensive horizontal development,
as opposed to the more vertical development characteristic of convection. Stratiform clouds
cover large areas but show relatively little vertical development.
Stratiform precipitation, in general, is relatively continuous and
uniform in intensity (i.e., steady rain versus rain
- 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.
- A low, generally gray cloud layer with a
fairly uniform base. Stratus may appear in the form of ragged
patches, but otherwise does not exhibit individual cloud elements
as do cumulus and stratocumulus clouds. Fog
usually is a surface-based form of stratus.
occurring within a surface-based layer, i.e., a layer in which the
lowest portion is based at or very near the earth's surface.
Compare with elevated
- SW - Southwest
- SWEAT Index
- Severe Weather ThrEAT index. A stability index developed by
the Air Force which incorporates instability,
shear, and wind speeds as follows:
- SWEAT=(12 Td 850 ) + (20 [TT-49]) +( 2 f
850) + f 500 + (125 [s+0.2]) where
- Td 850 is the dew point
temperature at 850 mb,
- TT is the total-totals index,
- f 850 is the 850-mb wind speed (in
- f 500 is the 500-mb wind speed (in
- s is the sine of the angle between the
wind directions at 500 mb and 850 mb (thus representing the
directional shear in this layer).
SWEAT values of
about 250-300 or more indicate a greater potential for severe
weather, but as with all stability indices, there are no magic
SWEAT index has the advantage (and disadvantage) of using only
mandatory-level data (i.e., 500 mb and 850 mb), but has fallen
into relative disuse with the advent of more detailed upper air
sounding analysis programs.
SWRN - Southwestern
S/WV - Short Wave
- TCU - Towering Cumulus
- In general, the relationships between
heat and other properties (such as temperature, pressure, density,
etc.) In forecast discussions, thermodynamics usually refers to
the distribution of temperature and moisture (both vertical and
horizontal) as related to the diagnosis of atmospheric instability.
THETAE - Equivalent Potential Temperature
- (or Equivalent Potential Temperature) -
The temperature a parcel of air would have if a) it was lifted
until it became saturated, b) all water vapor was condensed out,
and c) it was returned adiabatically (i.e., without transfer of
heat or mass) to a pressure of 1000 millibars. Theta-e, which
typically is expressed in degrees Kelvin, is directly related to
the amount of heat present in an air parcel. Thus, it is useful in
diagnosing atmospheric instability.
- Theta-e Ridge
- An axis of relatively high values of theta-e. Severe weather and excessive
rainfall often occur near or just upstream from a
- A violently rotating column of air,
usually pendant to a cumulonimbus, with circulation reaching the
ground. It nearly always starts as a funnel cloud and may be
accompanied by a loud roaring noise. On a local scale, it is the
most destructive of all atmospheric phenomena.
- A stability index and severe weather
forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice
the temperature at 500 mb. The total-totals index is the
arithmetic sum of two other indices: the Vertical Totals Index
(temperature at 850 mb minus temperature at 500 mb) and the Cross
Totals Index (dew point at 850 mb minus temperature at 500 mb). As
with all stability indices there are no magic threshold values,
but in general, values of less than 50 or greater than 55 are
considered weak and strong indicators, respectively, of potential
severe storm development.
- A large cumulus cloud with great vertical
development, usually with a cauliflower-like appearance, but
lacking the characteristic anvil of a Cb. (Often shortened
to "towering cu," and abbreviated TCU.)
- Tropical Cyclone
Associated High Winds
- High winds that occur a few hundred miles
or so inland from the coast of a landfalling tropical cyclone.
- Triple Point
- The intersection point between two
boundaries (dry line, outflow boundary, cold front, etc.), often a focus for
thunderstorm development. Triple point also may refer to a point
on the gust front of a supercell, where
the warm moist inflow, the rain-cooled outflow from the forward
flank downdraft, and the rear flank downdraft all intersect; this
point is a favored location for tornado development (or
- The upper boundary of the troposphere, usually
characterized by an abrupt change in lapse rate
from positive (decreasing temperature with height) to neutral or
negative (temperature constant or increasing with height).
- The layer of the atmosphere from the
earth's surface up to the tropopause,
characterized by decreasing temperature with height (except,
perhaps, in thin layers - see inversion, cap),
vertical wind motion, appreciable water vapor content, and
sensible weather (clouds, rain, etc.).
- An elongated area of relatively low
atmospheric pressure, usually not associated with a closed
circulation, and thus used to distinguish from a closed low. The opposite of ridge.
TSRA - Thunderstorms (with
TSTM - Thunderstorm
- A medium-range numerical weather
prediction model operated by the United Kingdom METeorological Agency. (72-hour numerical model of the
- A small-scale current of rising air. If
the air is sufficiently moist, then the moisture condenses to
become a cumulus cloud or an individual tower of a towering cumulus or Cb.
- Upper Level
- A general term for any large-scale or mesoscale disturbance capable of
producing upward motion (lift) in the middle or upper parts of the
atmosphere. This term sometimes is used interchangeably with
impulse or shortwave.
- Upslope Flow
- Air that flows toward higher terrain, and
hence is forced to rise. The added lift often results in
widespread low cloudiness and stratiform
precipitation if the air is stable, or an increased chance of
thunderstorm development if the air is unstable.
- Toward the source of the flow, or located
in the area from which the flow is coming.
- Urban and Small
- Flooding of small streams, streets, and
low-lying areas, such as railroad underpasses and urban storm
drains. This type of flooding is mainly an inconvenience and is
generally not life threatening nor is it significantly damaging to
UTC - Universal Coordinated Time (same as
Greenwich Mean Time)
UVV - Upward Vertical Velocity
- (or UVV) - Upward Vertical Motion (or Velocity).
VCNTY - Vicinity
- Veering Winds
- Winds which shift in a clockwise direction with time at a given
location (e.g., from southerly to westerly), or which change
direction in a clockwise sense with height (e.g., southeasterly at
the surface turning to southwesterly aloft). The latter example is
a form of directional shear which is important for tornado
formation. Compare with backing
- A low-pressure system, usually a closed low or cutoff low, which is not tilted
with height, i.e., located similarly at all levels of the
atmosphere. Such systems typically are weakening and are
slow-moving, and are less likely to produce severe weather than
tilted systems. However, cold pools aloft associated with
vertically-stacked systems may enhance instability
enough to produce severe weather.
- Vertically-Integrated Liquid water. A property computed by
RADAP II and WSR-88D units that takes into account the
three-dimensional reflectivity of an echo. The maximum VIL of a
storm is useful in determining its potential severity, especially
in terms of maximum hail size.
- Video Integrator and Processor, which contours radar
reflectivity (in dBZ) into six VIP levels:
- VIP 1 (Level 1, 18-30 dBZ) - Light
- VIP 2 (Level 2, 30-38 dBZ) - Light to
- VIP 3 (Level 3, 38-44 dBZ) - Moderate
to heavy rain.
- VIP 4 (Level 4, 44-50 dBZ) - Heavy rain
- VIP 5 (Level 5, 50-57 dBZ) - Very heavy
rain; hail possible.
- VIP 6 (Level 6, >57 dBZ) - Very
heavy rain and hail; large hail possible.
- Streaks or wisps of precipitation falling
from a cloud but evaporating before reaching the ground. In
certain cases, shafts of virga may precede a microburst;
see dry microburst.
VIS - Visible
- Volcanic Ash
- A volcanic eruption can send an ash plume
into the atmosphere reducing visibility at the ground and in the
air. The chemical composition and abrasive characteristics of the
particles varies widely and can seriously affect people and
machinery on the ground and aircraft.
- A measure of the local rotation in a
fluid flow. In weather analysis and forecasting, it usually refers
to the vertical component of rotation (i.e., rotation about a
vertical axis) and is used most often in reference to synoptic
scale or mesoscale weather systems. By
convention, positive values indicate cyclonic rotation.
- Vort Max
- (Slang; short for vorticity maximum), a center, or maximum, in the vorticity field of a fluid.
- VAD Wind Profile.
A radar plot of horizontal winds, derived from VAD data, as a
function of height above a Doppler Radar. The display is plotted
with height as the vertical axis and time as the horizontal axis
(a so-called time-height display), which then depicts the change
in wind with time at various heights. This display is useful for
observing local changes in vertical wind shear, such
as backing of low-level winds, increases in speed shear,
and development or evolution of nearby jet streams
(including low-level jets).
W - West
WAA - Warm Air Advection
- Wall Cloud
- A localized, persistent, often abrupt
lowering from a rain-free base. Wall clouds can range from a
fraction of a mile up to nearly five miles in diameter, and
normally are found on the south or southwest (inflow) side of the
thunderstorm. When seen from within several miles, many wall
clouds exhibit rapid upward motion and cyclonic rotation.
However, not all
wall clouds rotate. Rotating wall clouds usually develop before
strong or violent tornadoes, by anywhere from a few minutes up to
nearly an hour. Wall clouds should be monitored visually for signs
of persistent, sustained rotation
and/or rapid vertical motion.
"Wall cloud" also is used occasionally in
tropical meteorology to describe the inner cloud wall surrounding
the eye of a tropical cyclone, but the proper term for this
feature is eyewall.
- Transport of warm air into an area by
horizontal winds. Low-level warm advection sometimes is referred
to (erroneously) as overrunning. Although the two
terms are not properly interchangeable, both imply the presence of
lifting in low levels.
- A warning is issued when a hazardous
weather or hydrologic event is occurring, is imminent, or has a
very high probability of occurring. A warning is used for
conditions posing a threat to life or property.
- A watch is used when the risk of a
hazardous weather or hydrologic event has increased significantly,
but its occurrence, location, and/or timing is still uncertain. It
is intended to provide enough lead time so that those who need to
set their plans in motion can do so.
- In general, a tornado occurring over
water. Specifically, it normally refers to a small, relatively
weak rotating column of air over water beneath a Cb or towering cumulus cloud.
Waterspouts are most common over tropical or subtropical
definition of waterspout is debatable. In most cases the term is
reserved for small vortices over water that are not associated
with storm-scale rotation (i.e., they are the water-based
equivalent of landspouts). But there is
sufficient justification for calling virtually any rotating column
of air a waterspout if it is in contact with a water
- Wind Chill
- Increased wind speeds accelerate heat
loss from exposed skin. No specific rules exist for determining
when wind chill becomes dangerous. As a general rule, the
threshold for potentially dangerous wind chill conditions is about
WRN - Western
WFO - Weather Forecast
OfficeWV - Water Vapor
WX - Weather
- XSEC - Cross Section
ZFP - Zone Forecast
- 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
88D - Doppler Radar
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