When is tornado season and why?

When and why?

The months with the greatest number of tornadoes overall are April, May, and June, but tornadoes can and do occur during any month of the year. Tornado seasons vary in different parts of the United States.
In the Southeast, the peak season for tornadoes is February through April. In the northern Plains, tornadoes are most likely to develop from June through August.
Generally, tornado frequency is high in the South in late winter and early spring; and in the Plains, Midwest, and Ohio Valley from early spring through summer.
The reason for this is that low-level heat, moisture, and instability necessary for tornado formation are usually confined to southern regions early in the year, and only reach northern sections with regularity late in the spring.
Also, the jet stream (associated with the upper-level weather systems that can contribute to thunderstorm and tornado formation) migrates northward from winter to summer.

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