While tornadoes can happen in any state in the nation, and at any time of the day or night, the greater likelihood for tornadoes to occur is on a hot, sticky day during the summertime. It is these conditions, along with daily sunshine, that causes daytime heating to create unstable conditions and massive convective wind currents building vertically into the atmosphere. These vertical wind currents develop into towering cumulus clouds and thunderstorms. While not all thunderstorms are severe, all thunderstorms are dangerous and can develop tornadoes.
Tornadoes may develop from thunderstorms when warm, moist air is in advance of eastward-moving cold fronts. Watch for skies that turn suddenly dark and greenish (a phenomenon caused by hail) and increasing wind speeds. Keep a particular lookout for the formation of funnel clouds.
More information about the science of tornadoes is available from the National Weather Service.
What is a tornado?