Tornadoes can occur year-round anywhere over the lower 48 of the United States, but the spring to summer period is prime time for severe thunderstorms that spawn the violent swirling columns of air. Building warmth and surging humidity are key ingredients for thunderstorms which, when severe, can evolve to produce tornadoes.
Severe thunderstorms will typically form when we get warm, moist air from the Gulf of Mexico clashing with cool air from Canada and sometimes dry air from the deserts. We often see the most tornadoes develop from central Texas, northward through the Nebraska and Iowa area, which is referred to as "Tornado Alley".
In order for severe thunderstorms to form, there must be a mechanism for air to rise in vigorous fashion. As a column of air rises through the atmosphere, it cools and causes moisture within to form the towering clouds we see as thunderstorms. The more moisture available and the cooler the air is aloft, the more efficient and potentially the more violent this process is. A push of dry air or cooler air from the outside can also intensify this process.
Any time there is a surge in temperature and humidity levels, combined with the approach of cooler and drier air thunderstorms can erupt. the most dramatic time of the year for this is in the spring to early summer. The peak for severe weather and tornado season is from April through June. Areas of the country that typically warm up the fastest with an ample supply of moisture are usually the most likely areas for early-season severe thunderstorms and tornadoes.