Thunderstorms feed on warm, moist, unstable air and are triggered by lift; the latter is provided by an upper level low or an approaching cold front. Such conditions begin to develop along the Gulf Coast States in February and the zone of thunderstorm frequency gradually shifts to the north and west over the ensuing months.
April marks the beginning of the severe thunderstorm season in the Midwest, which will continue through June. As the jet stream moves farther to the north, the Gulf of Mexico "opens up," allowing warm, moist air to invade the Heartland. This provides fuel for the thunderstorms which develop in advance of Pacific cold fronts. In this most common scenario, the storms cluster in swaths, oriented southwest to northeast, and move northeastward ahead of the front. This "training" produces recurrent, heavy rains along the swath, inducing floods; on the other hand, compacted storms tend to be less severe, since rain and "outflow" from the preceding storm cool the air and reduce instability.
The most severe thunderstorms, which often produce tornadoes, are generally isolated "supercells," which form along the "dry line;" this moving barrier separates the warm, humid air ahead of the storm from the cool, dry air behind the front. Energized by the jet stream, these monsters begin to rotate as the southeast flow in the "warm sector" collides with strong west winds at the dry line. Such conditions classically develop across "tornado alley," on the Great Plains, from mid spring through early summer.