How tornadoes begin

Tornadoes are usually associated with heavy thunderstorm activity and occur only under very special environmental conditions.
For a twister to form the surrounding air must be moist enough to support the building thunderstorm; the lower layers of air must slowly be turning or rotating; the surrounding atmosphere must be highly unstable; and there should be a strong jet stream aloft.
When all of these factors combine in just the right sequence, a tornado may form.
Tornadoes can also accompany tropical storms and hurricanes as they move onto land.