Thunderstorms develop in warm, moist air most often in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds and tornadoes. Occasionally, large outbreaks of tornadoes occur along strong frontal systems that form in the central states and move east. Several states may be affected by numerous severe thunderstorms and tornadoes.
Tornadoes occasionally accompany tropical storms and hurricanes that move over land. They are most common to the right and ahead of the path of the storm center as it comes ashore. Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.