"Severe weather most often occurs during the spring, when warm air begins to invade from lower latitudes while cold air is still pushing from the polar regions. Flooding is also most common in and near mountainous areas during this time of year because of snowmelt, many times accelerated by warm rains. In the United States, Tornado Alley is most active by far this time of year, especially since the Rocky Mountains prevent the surging hot and cold air masses from spreading eastward and instead force them directly at each other. Besides tornadoes, supercell thunderstorms can also produce dangerously large hail and very high winds, for which a severe thunderstorm warning or tornado warning is usually issued. Even more so than winter, the jet streams play an important role in severe weather in the springtime."

— From Wikipedia.org