Supercell
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Supercell thunderstorms are the largest, most severe class of single-cell thunderstorm. They have significant vertical "wind shear" (wind speed and/or direction changes with increasing height) resulting in their downdraft being separate from the updraft (i.e. if the circulation of air was viewed from the side, it would be seen to be almost circular—up, across and over, then down), and have a strong rotating updraft (a mesocyclone). However, it should be noted that mild windshear can also lead to supercell formation when combined with sufficient CAPE.
Supercells are usually widely separated giving them a large volume of air to feed off, and can last many hours—they are steady-state storms. Due to the rotating mesocyclone, they tend to track to the right of the mean wind.
Supercells can be any size—large or small, low or high-topped. Usually they produce copious amounts of hail, torrential rainfall, strong winds, substantial downdrafts, and 30% of supercells produce tornadoes within the mesocyclone.
Supercells are usually seen in the Midwestern United States, but they can occur anywhere. The first storm to be so identified was the Wokingham storm over England which was studied by Keith Browning and Frank Ludlam in 1962.
References
- Structure and Dynamics of Supercell Thunderstorms - NWS (http://www.crh.noaa.gov/lmk/soo/docu/supercell.htm)