Debris blown by tornado winds can pound buildings to pieces. The strongest tornadoes come from the kind of long-lasting, especially fierce thunderstorms known as supercells. As the name implies, these are intense thunderstorms which can produce large hail and downbursts in addition to tornadoes. Some bring heavy rain while others are relatively dry. Supercells are most common on the Plains in the Southeast and across the Midwest, but do occur elsewhere.
Storms form when there is unstable air, a source of lifting, and humidity.
A supercell forms when particular patterns of upper air winds are also present, patterns which cause the storm to last longer than an ordinary thunderstorm. Supercells often develop rotating winds inside them known as mesocyclones, associated with strong tornadoes. However, scientists don't yet understand the connections between mesocyclones and tornadoes that actually reach from them down to the ground. Not all tornadoes come from supercells, but the strongest twisters usually have a supercell as a their parent. Weaker vortices, such as waterspouts like those common in the Florida Keys, can come from cumulus congestus clouds, also known as towering cumulus.