The centroid of severe thunderstorm activity changes with the season:
Late winter (Dec-Feb): Gulf Coast / "Dixie Alley"
Mississippi, Alabama, and Louisiana see an early-season spike, sometimes starting as early as February, driven by the jet stream still sitting far enough south to interact with Gulf moisture at these latitudes.
Spring (Mar-May): Southern Plains
Research has indicated that peak tornado activity in this area has shifted about 7 days earlier over the past six decades.1American Geophysical Union (on Long & Stoy, Geophysical Research Letters), "Tornadoes occurring earlier in "Tornado Alley"," news.agu.org, September 2014. https://news.agu.org/press-release/tornadoes-occurring-earlier-in-tornado-alley/
As the jet stream migrates northward, the activity centroid shifts into Texas, Oklahoma, Kansas, and Arkansas, the classic "Tornado Alley" core. April is typically when the year's most violent outbreaks concentrate, since this is when Gulf moisture, Plains instability, and strong jet-driven shear are all simultaneously at their most favorable overlap.
Late spring/early summer (May-Jun): Central/Northern Plains and Midwest
The centroid continues migrating north into Nebraska, Iowa, South Dakota, and the upper Midwest as the jet stream retreats poleward with the advancing season. June is climatologically the peak month for the central/northern Plains specifically.
Summer (Jul-Aug): Upper Midwest / Great Lakes
By mid-to-late summer, the jet stream has retreated well north, and the severe weather focus shifts further into the Dakotas, Minnesota, Wisconsin, and the Great Lakes region, often shifting from tornado-dominant to more damaging-wind-dominant (derecho-prone) storm modes.
Fall: reversal back toward the Gulf Coast/Southeast
As the jet stream migrates south again in autumn, a secondary (smaller) severe weather season redevelops across the Southeast and Gulf Coast.