Although heavy rains occur in tropical storms, hurricanes and even ordinary thunderstorms, most flash floods are caused by slow moving thunderstorms, or thunderstorms that repeatedly move over the same area, which is called “training.”
Just 18 inches of water can move a vehicle.
In the Spring, snow melt combined with heavy rains from thunderstorms can cause floods in northern states.
Radar View of Thunderstorm on May 20, 2015 in North Florida