Tornado season begins as early as late February or early March in northern Texas and Oklahoma, the southern portion of Tornado Alley, where the tornado threat remains through most of May. By early June, the greatest threats tend to lie to the north, in Kansas and Nebraska. By July, the peak threat finally reaches the southern Great Lakes states.

This northward march of the tornado threat is most directly related to the northward retreat of the jet stream over the same period. The jet stream is a ribbon of high wind speeds, about six miles above the surface, that is located roughly at the warm edge of polar air at upper levels.

In early spring, the Northern Hemisphere is still rather chilly and the warm edge of the upper level polar air resides over or near the Gulf of Mexico. The polar air gradually retreats to the Great Lakes by late June and even further north later in the summer.

As climate warms, there is some reason to expect a change in the number of tornadoes in some locations in the United States but, since other factors contribute to tornado frequency, it is not clear if the numbers will increase or decrease.

August 18, 2005: A late-season tornado forms a supercell over Stoughton, Wisconsin. Photo: S.V. Medaris

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