When is the severe weather season?
As the threat of winter snow recedes, it is replaced by the threat of severe weather — thunderstorms with hail, damaging winds and tornadoes. The severe weather season, though broadly spanning March to August across the United States, is actually quite regional. It begins in March in the southern states, moves to the southern Plains during April and May, and then further north toward the Great Lakes states during the summer.
An underlying reason for this northward migration of the severe weather threat is the jet stream, a ribbon of rapid wind near the top of the troposphere, about six miles up.
The position of the jet stream is strongly tied to the southern edge of a dome of cold air centered on the North Pole. During the depths of winter, that cold dome expands almost to the Gulf of Mexico. During spring, the hemisphere warms and the dome shrinks until its southern edge is in central Canada by early summer.
The jet stream is associated with vigorous vertical, air movement. Because the upward motion is instrumental in producing thunderstorms, when the jet stream migrates northward as the weather warms, the bulk of severe weather follows does likewise.