by David Murphy
A supercell thunderstorm is the strongest of all thunderstorms on earth. Supercells produce drenching rain, strong damaging winds, frequent lightning, and large hail. The strongest tornadoes are also produced by supercell thunderstorms.
Supercells can survive on the ground for hours, especially in the Midwest where the dynamics are most favorable for large, lengthy storm episodes. These storms can travel quickly, moving across the surface at 40 miles per hour or more.
A supercell requires a combination of several factors in order to form. First, a surface wind from the south or southeast must be present, drawing humid, moist air into a region. Second, winds above the surface must be shearing, or turning clockwise with height so that eventually, they are flowing out of the southwest. This creates a turning or coiling in the atmosphere that encourages a twisting of winds. Also, some dry air being drawn into the storm's middle layer by a west wind is important, because it encourages evaporation inside the storm which cools the middle layer and makes the hot surface air rush even faster up through the storm's core. Third, there must be strong upper level winds flowing more or less from southwest to northeast. Usually, a strong portion of the jet stream known as a jet streak is required above the storm. This helps suck the air rising up through the storm even faster to the top.
All of these factors combine to create an immense updraft of air, rotation, and a pinching of surface winds that can encourage the formation of a strong tornado or tornadoes.