|Glossay, Terms and definitions for storm spotters.
Severe weather definitions here.
The information below is from NOAA's NWS Glossary and is their expertise and work that has
made this section available. It is provided here to give you accurate and professional
definitions for storm spotters and storm chasers.
Glossary (A-B) Glossary (C-H) Glossary (I-R) Glossary (S-Z)
AC - Convective outlook issued by the SPC. Abbreviation for Anticipated Convection; the
term originates from the header coding [ACUS1] of the transmitted product. See SWODY1,
ACCAS (usually pronounced ACK-kis) - AltoCumulus CAStellanus; mid-level clouds (bases
generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show
cumulus-type development. These clouds often are taller than they are wide, giving them a
turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede
the rapid development of thunderstorms.
Accessory Cloud - A cloud which is dependent on a larger cloud system for development and
continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.
Advection - Transport of an atmospheric property by the wind. See cold advection, moisture
advection, warm advection.
Air-mass Thunderstorm - 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. See
Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale
or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the
term is somewhat controversial and should be used with discretion.
Algorithm - A computer program (or set of programs) which is designed to systematically
solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze
radar data and automatically determine storm motion, probability of hail, VIL, accumulated
rainfall, and several other parameters.
Anticyclonic Rotation - 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
Anvil - 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 (see back-sheared anvil).
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 MCS.
Anvil Dome - A large overshooting top or penetrating top.
Anvil Rollover - [Slang], a circular or semicircular lip of clouds along the underside of
the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See
cumuliform anvil, knuckles, mushroom.
Anvil Zits - [Slang], frequent (often continuous or nearly continuous), localized
lightning discharges occurring from within a thunderstorm anvil.
AP - Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from
nonstandard propagation of the radar beam under certain atmospheric conditions.
Approaching (severe levels) - A thunderstorm which contains winds of 35 to 49 knots (40 to
57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diameter. See severe
Arcus - 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 arcus
AVN - AViatioN model; one of the operational forecast models run at NCEP. The AVN is run
four times daily, at 0000, 0600, 1200, and 1800 GMT. As of fall 1996, forecast output was
available operationally out to 120 hours only from the 0000 and 1200 runs. At 0600 and
1800, the model is run only out to 72 hours.
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 winds.
In 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.
Back-sheared Anvil - [Slang], a thunderstorm anvil which spreads
upwind, against the flow aloft. A back-sheared anvil often implies a
very strong updraft and a high severe weather potential.
Barber Pole - [Slang], a thunderstorm updraft with a visual appearance including cloud
striations that are curved in a manner similar to the stripes of a barber pole. The
structure typically is most pronounced on the leading edge of the updraft, while drier air
from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Baroclinic Zone - 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, wind shear is characteristic of a baroclinic zone.
Barotropic System - 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.
Usually, in 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.
Bear's Cage - [Slang], a region of storm-scale rotation, in a thunderstorm, which is
wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or
mesocyclone, especially one associated with an HP storm.
The term reflects the danger involved in observing such an area visually, which must be
done at close range in low visibility.
Beaver('s) Tail - [Slang], a particular type of inflow band with a relatively broad, flat
appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft
and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or
southeast to northwest. As with any inflow band, cloud elements move toward the updraft,
i.e., toward the west or northwest. Its size and shape change as the strength of the
inflow changes. See also inflow stinger.
Spotters should note the distinction between a beaver tail and a tail cloud. A
"true" tail cloud typically is attached to the wall cloud and has a cloud base
at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not
attached to the wall cloud and has a cloud base at about the same height as the updraft
base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail
cloud forms from air that is flowing from the storm's main precipitation cascade region
(or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front.
Blue Watch (or Blue Box) - [Slang], a severe thunderstorm watch.
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
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 rotation.
Box (or Watch Box) - [Slang], a severe thunderstorm or tornado watch. See blue box, red
BRN - See Bulk Richardson Number.
Bubble High - A mesoscale area of high pressure, typically associated with cooler air from
the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or
outflow boundary separates a bubble high from the surrounding air.
Bulk Richardson Number (or BRN) - A non-dimensional number relating vertical stability and
vertical shear (generally, stability divided by shear). High values indicate unstable
and/or weakly-sheared environments; low values indicate weak instability and/or strong
vertical shear. Generally, values in the range of around 50 to 100 suggest environmental
conditions favorable for supercell development.
Bust - [Slang], an inaccurate forecast or an unsuccessful storm chase; usually a situation
in which thunderstorms or severe weather are expected, but do not occur.
BWER - Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a
thunderstorm characterized by a local minimum in radar reflectivity at low levels which
extends upward into, and is surrounded by, higher reflectivities aloft. This
feature is associated with a strong updraft and is almost always found in the inflow
region of a thunderstorm. It cannot be seen visually. See WER.
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From NOAA Technical Memorandum NWS SR-145 - Michael
Branick NOAA/WFO Norman