While we prepare most for tornadoes, the combination of flash floods and river floods have accounted for more deaths and almost as much damage as tornadoes. During the months of February and March, this product contains information on the potential for flooding from the spring snow melt. Flash flooding usually develops within six hours of heavy rainfall, while flooding is a longer-term event, usually taking more than six hours to develop. Also, flash flooding is usually associated with strong thunderstorms that produce heavy rainfall over a short period of time, while flooding generally occurs when the heavy rainfall has ended and light to moderate rain continues. The two key elements are rainfall intensity and duration.
Intensity is the rate of rainfall, and duration is how long the rain lasts.
Topography, soil conditions, and ground cover, also play an important role. Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Most flash flooding is caused by slow-moving thunderstorms or thunderstorms repeatedly moving over the same area.