Why do tornadoes usually form from severe thunderstorms rather than from ordinary storms?

Tornadoes need a special combination of ingredients: warm, moist air near the ground, colder air aloft, and strong wind shear that changes wind speed or direction with height. In supercell thunderstorms, this can produce a rotating updraft called a mesocyclone. That rotation can tighten and stretch until a tornado forms. Most of the world's strong tornadoes occur in the central United States because the atmospheric setup there can be ideal.