According to the National Oceanographic and Atmospheric Administration, tornadoes form when warm air creates a rotating updraft in a powerful thunderstorm. When winds blow in sharply different directions or at different speeds in these storms, they can set up a rotation that feeds on itself, creating a condition called a mesocyclone. When this construct rotates and touches the ground, it becomes a tornado.
Tornadoes can form quickly and without warning, and their destructive nature makes them hard to study. Any time thunderstorms occur in conditions where temperatures at the ground are substantially warmer than those aloft, the storms can be strong enough to create one or more tornadoes.
It can be difficult to forecast a tornado, but Doppler radar systems are a valuable tool for determining where and when one is about to occur. Doppler systems can detect wind shear, or divergences in wind direction and speed. An area where the winds are blowing strongly in different directions creates a distinctive "hook echo" on the radar. This can quickly form the rotation necessary to suck warm, moist air up into the upper layers of the storm, creating the energy necessary to turn the rotation into a mesocyclone and then a tornado.