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Supercell Thunderstorms:
Supercell thunderstorms refer to thunderstorms which 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 scale of the storm (usually from a few to tens of kilometers horizontally and five to twelve kilometers vertically).
   Besides careful study 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 above. Doppler weather radars like those used by the National Weather Service can detect a supercell's rotation.
   Supercells are generally associated with severe weather and often produce hail, very strong winds, torrential rains, and occasionally tornadoes. They 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.).
   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. The first seven photos above depict examples of LP supercells. In contrast, HP supercells 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. The photo referenced by WxTorn01a_03 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.
 
Suggested Reading:
   Cotton, W.C., 1990: Storms. ASTeR Press Inc.
   Bluestein, H.B., 1999: Tornado Alley. Monster Storms of the Great Plains. Oxford University Press