The thunderstorms that produced all the damaging tornadoes across Alabama on Wednesday, April 27, 2011, were supercell thunderstorms. The following information on supercell thunderstorms comes from the National Weather Service's JetStream - Online Weather School page.
Supercell thunderstorms are a special kind of single cell thunderstorm that can persist for many hours.
They are responsible for nearly all of the significant tornadoes produced in the U.S. and for most of the hailstones larger than golf ball size. Supercells are also known to produce extreme winds and flash flooding.
Supercells are highly organized storms characterized updrafts that can attain speeds over 100 miles per hour, able to produce extremely large hail and strong and/or violent tornadoes, downdrafts that can produce damaging outflow winds in excess of 100 mph - all of which pose a high threat to life and property.
Dynamically, all supercells are fundamentally similar. However, they often appear quite different visually from one storm to another depending on the amount of precipitation accompanying the storm and whether precipitation falls adjacent to, or is removed from, the storm"s updraft.
Based on their visual appearance, supercells are often divided into three groups;
In LP supercells the updraft is on the rear flank of the storm, a barber pole or corkscrew appearance of updraft is possible, precipitation sparse or well removed from the updraft, often is transparent and you can"t see it, and large hail is often difficult to discern visually. Also, there is no "hook" seen on Doppler radar.
HP supercells will have...
Beneath the supercell, the rotation of the storm is often visible as well. The wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would usually do so within the wall cloud.
Wall clouds are isolated lower clouds below the rain-free base and below the main storm tower. Wall clouds are often located on the trailing flank of a storm. With some storms, such as high precipitation supercells, the wall cloud area may be obscured by precipitation or located on the leading flank of the storm.
Wall clouds associated with potentially severe storms can: