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Supercell thunderstorms refer to thunderstorms that are rotating.
Most, but not all, supercells rotate cyclonically (counter-clockwise in the northern
hemisphere). Rotation is produced by wind shear (a change in wind speed and/or
direction with height) and exists for the updraft portion or the entire height of the
storm (usually from a few to tens of kilometers horizontally and five to twelve
Besides careful observation of a storm to perceive the rotation, visible
cues include striations or cloud bands wrapping around the outside edges in a spiral
pattern (imagine a barber pole), a flared or bell-shaped cloud base, and an
overall cylindrical appearance. Note these features in many of the photos in the
gallery. Doppler weather radars like those used by the National Weather Service
can detect a supercell's rotation because of the rain drops within the cloud.
Supercells are generally associated with severe weather and often
produce hail, very strong winds, torrential rains, and occasionally
are commonly found in the Great Plains states of the US but can occur anywhere in the
world except perhaps Antarctica. In the diagram shown here, many features
are not this obvious because precipitation often obscures the features (let alone
trees, hills, etc.).
Supercell types: LP, HP, and classic
Though supercells encompass a spectrum of storm behavior,
chasers tend to distinguish between three classifications of supercells:
"Low Precipitation" (LP), "High Precipitation" (HP), and
"Classical." These classifications are primarily based on radar and visual
observations of the precipitation region (often referred to as the "core")
with respect to the rotation and updraft location. When the precipitation is widely
removed from the updraft and low-level rotation, the storm is LP. Most of these
photos depict examples of LP supercells because they are more photogenic. In contrast,
typically have abundant precipitation embedded within or very near the
rotation and updraft. Many HP supercells produce flash floods and damaging hail as
well as damaging straight-line winds but do not frequently produce tornadoes.
Likewise, LP supercells are not thought to produce tornadoes as often as the classical
types. Classical supercells are characterized by a precipitation core in close
proximity to the strongest low-level rotation. Often a narrow band of
precipitation (sometimes very large but sparse hailstones) exists on the western
and southern perimeter of rotation. This supercell
transitioned from an LP at the time of this photo to a Classical supercell later
and produced a tornado near Elba, Colorado on 30 May 1996.
Visually, we thought this storm was an LP during the tornado phase but later
inspection of radar data and chaser reports from the storm's west side clearly
indicated a classical supercell with large hail falling immediately west of the tornado.