Wintertime weather is driven largely by the parade of upper-level weather systems carried rapidly eastward by the high level winds known as the jet stream.
The jet stream is a ribbon of high-speed wind (located at nearly 6 miles above the surface) that is related to the strength, and location, of the pole-to-equator temperature contrast.
As we all know too well after this past winter, cold polar air can spill as far south as Mexico during our winter. This drags the jet stream as far south as southern Texas during the depth of winter.
As we enter the middle of spring, the southern latitudes of our country begin to warm up rapidly and our attention turns to the return of severe weather season.
Though severe weather includes more than just tornadoes, these storms are the season’s highest-profile events. But the climatology of tornadoes varies depending on geography.
We are already past the tornado frequency maximum over the Gulf Coast states (Louisiana, Alabama and Mississippi) that occurs in mid- to late March. From April to May, the tornado frequency maximum occurs over the Southern Plains states of Oklahoma, Texas and Kansas. By June through August, it moves to the Great Lakes states and High Plains.
This northward migration of tornadoes reflects the northward migration of the jet stream. Since the jet stream is tied to the southern edge of the cold air centered on the Pole, as the seasons change from late winter to late summer, the polar cold air retreats to the north dragging the jet stream, and the tornado season, with it.