Flash flooding is rapid flooding of low-lying areas, rivers and creeks that is caused by the intense rainfall associated with a thunderstorm, or multiple training thunderstorms. Flash flooding occurs when the ground under a storm becomes saturated with water so quickly that it cannot be absorbed. The runoff collects in low lying areas and flows rapidly downhill. As a result, anything in its path is suddenly in rising water. These most often occur in dry areas that have recently received precipitation, but the flash flood can flow for a long distance as long as there is a path downhill.
Flash floods can also be caused by other influences. Ice jams can block the normal course of a river, leading to flooding. In addition, dam breaks have been known to cause flash flood conditions. The Johnstown, Pennsylvania dam break of 1889 is an example of a devastating flash flood.
The U.S. National Weather Service gives the advice "Turn Around Don't Drown" regarding flash floods; that is, it recommends that people get out of the area of a flash flood, rather than trying to cross it, even if they think that the water looks easy to cross. This is because people in general underestimate the dangers of flash floods. Flash floods are extremely dangerous because of their sudden nature. Being in a vehicle does not provide protection against being swept away; it can actually make people more overconfident and less likely to avoid the flash flood; more than half of the fatalities attributed to flash floods are of people swept away in vehicles when trying to cross flooded intersections [1] (http://www.nws.noaa.gov). More people die yearly in floods (127 on average) than by lightning (73), tornadoes (65), or hurricanes (16).
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