There were 6 tornado occurrences in 1998 in the Keys and 2 moved through the Upper Keys during Tropical Storm Mitch in November. Fortunately, there were no deaths which occurred during these tornadoes but the tornado induced by Mitch in Key Largo injured 20 people. Tornadoes form in severe thunderstorms. A key ingredient to tornado formation is wind shear. The greater the wind shear, the greater the potential for tornado formation.
Wind shear causes air near the surface to rotate horizontally.
The updraft, a column of rapidly rising air, of a developing severe thunderstorm can tilt the rotating air from horizontal to vertical inside the thunderstorm. This is a typical way tornadoes form and not all tornadoes form in this manner. It is the opinion of many tornado researchers that much is still to be learned about tornadoes and their formation. Tornado intensity is ranked by the Fujita Scale which is based on wind speed and the damage it can cause. This scale is not accurate for all types of structures.