Thunderstorms develop in warm, moist unstable air.
Unstable air means that the air temperature cools rapidly with height. Severe thunderstorms develop in a myriad of different places and are intensified by cold fronts, drylines, upslopes and the like. These thunderstorms often produce large hail and strong winds, and some ultimately produce tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern.