F5 261-318 mph Incredible tornado. The maximum wind speeds of tornadoes are not expected to reach the F6 wind speeds. Tornado Frequency - Tornadoes have occurred in every state of the U.S. except Alaska. Because they are associated with strong thunderstorms, the frequency of tornadoes is closely related to the areas that have lots of thunderstorms, but also occur predominantly in the plains states where cold air descending from above the Rocky Mountains encounters warm air moving northward from the Gulf of Mexico. It is these contrasting air masses and their common paths of circulation which give rise to both the thunderstorms and the tornadoes.
Tornadoes are most common during the spring and summer months.
They often occur as swarms associated with cold fronts that move across the U.S. from west to east. One such swarm occurred between April 3rd and 4th, 1974. In 16 hours over 147 tornadoes touched down between Mississippi and Pennsylvania, including six tornadoes of F-5 intensity. Note the generally northeast trending tracks of the tornadoes, which is typical of most tornadoes in the U.S. Tornado Damage - Tornado damage is caused by the high wind speed and high difference in atmospheric pressure between the tornado and its surroundings. The rotating winds can knock down weaker structures, and the extremely low pressure inside the tornado generates strong pressure differences between the inside and outside of buildings.