Severe Thunderstorm (Hail, Lightning, and Wind)
Severe Thunderstorm -
A thunderstorm which produces tornadoes, hail 0.75 inches or more in
diameter, or winds of 50 knots (58 mph) or more. Structural wind
damage may imply the occurrence of a severe thunderstorm.
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.
Thunderstorms may
produce none of the above conditions but by definition produces
lightning.
Severe thunderstorms
may also be a supercell. A supercell is - A thunderstorm with
a persistent rotating updraft. Supercells are rare, but are
responsible for a remarkably high percentage of severe weather
events - especially tornadoes, extremely large hail and damaging
straight-line winds. There are three predominate types of
supercells: High Precipitation, Low Precipitation, and
Classic.
They frequently travel
to the right of the main environmental winds (i.e., they are right
movers). Radar characteristics often (but not always) include a hook
or pendant, bounded weak echo region (BWER), V-notch, mesocyclone,
and sometimes a TVS. Visual characteristics often include a
rain-free base (with or without a wall cloud), tail cloud, flanking
line, overshooting top, and back-sheared anvil, all of which
normally are observed in or near the right rear or southwest part of
the storm. Storms exhibiting these characteristics often are called
classic supercells; however HP storms and LP storms also are
supercell varieties.
High Precipitation
(HP) - 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 events.
Low Precipitation (LP)
- LP Storm (or LP Supercell) - Low-Precipitation storm (or
Low-Precipitation supercell). A supercell thunderstorm characterized
by a relative lack of visible precipitation. Visually similar to a
classic supercell, except without the heavy precipitation core. LP
storms often exhibit a striking visual appearance; the main tower
often is bell-shaped, with a corkscrew appearance suggesting
rotation. They are capable of producing tornadoes and very large
hail. Radar identification often is difficult relative to other
types of supercells, so visual reports are very important. LP storms
almost always occur on or near the dry line, and thus are sometimes
referred to as dry line storms.
Hail - formed when
water droplets are carried into the storm multiple times. Each
time a new layer of water freezes to the stone, resulting in an
increasing hail stone size. Eventually the hail stone becomes
too heavy and falls to the ground. Large hail is dangerous to
property and animals, especially when driven by strong winds.
Lightning - produced
when an electrical discharge occurs within convection.
Lightning is typically characterized as cloud to cloud, cloud to
ground, and in-cloud. Cloud to ground is most hazardous to
animals. A lightning bolt can have a positive or negative
charge.
Wind (convective) -
produced by thunderstorm outflow or downdraft. There are also
Derechos, Derecho - (Pronounced deh-REY-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 across.
Strong winds can also occur ahead of a severe storm or tornado.
This may be in relation to the surface pressure gradient, amplified
by the approaching thunderstorm.
Much of this
information is taken from:
Severe Weather Glossary - Produced by Mike Branick
