issued by the SPC. Abbreviation for Anticipated Convection; the term
originates from the header coding [ACUS1] of the transmitted
product. See SWODY1, SWODY2.
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
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 popcorn convection.
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 analyse 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 rotation.
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
AP - Anomalous Propagation. Radar term for false
(non-precipitation) echoes resulting from non-standard 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 thunderstorm.
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 clouds.
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 72 hours only from the 0000 and 1200 runs. At
0600 and 1800, the model is run only out to 54 hours.