| Flash flood forecasting and warning program in the Western Region - :14084 | National Weather Service (NWS)
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Flash flood forecasting and warning program in the Western Region
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  • Description:
    "A flash flood is distinguished from a river flood by a very rapid rise in runoff water, and is usually defined as a damaging flood occurring within four hours of the causative event. Flash floods in western United States are usually associated with heavy thunder-showers, most of which have their origin in the mountains, so that flooding frequently occurs less than an hour after the downpour. The area affected is very limited, usually confined to a single stream or drainage. Since the heaviest rainfalI frequently occurs in inaccessible wilderness country, instrumentation and human witnesses are often lacking, so flooding may take place with Iittle or no warning. Extensive flash floods often result from a hurricane that has moved inland and turned into an extra-tropical storm in eastern United States, but this type of situation is relatively infrequent in the West. However, tropical moisture, associated with a hurricane off the coast of Mexico which has been moved northward by the general circulation, is a common cause of heavy showers and flash flooding in the southwestern states"--Introduction.

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