hazards of thunder storms
temperature and humidity
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
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
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.
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.
- [Slang], frequent (often
continuous or nearly continuous), localized lightning discharges
occurring from within a thunderstorm anvil.
- 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.
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