What Is Flash Flooding and How Can You Stay Safe

Flash floods are so named because sudden deluges can occur after a heavy rainfall, which the National Weather Service says is the most common cause. The flooding begins within six hours of heavy rain and often within three hours of an intense rainfall, although sometimes it can happen within minutes, giving people little time to take precautions.

Flooding occurs in areas where the ground is unable to absorb all of the water that has fallen, according to forecasters, who explained that flash flooding can also be caused by mudslides or breaks in dams or levees.

Urban areas are particularly vulnerable to flash flooding because they have a lot of paved surfaces.

A flash flood warning means that flash flooding is imminent or already happening, under the designations used by the Weather Service. A flash flood watch indicates that conditions are favorable for flash floods and that they are possible.

Flash floods are distinct from floods, which the Weather Service defines as inundation of a normally dry area with rising water from a river or stream. Flooding can last days or weeks, which is much longer than flash flooding.

Flash flooding is dangerous, in part because appearances can be deceiving.

“Six inches of fast, flowing water can knock you over, and two feet is enough to float an entire vehicle,” said Katie Wilkes, a spokeswoman for the American Red Cross. “I think one of the most important things to know is that flash floods are called flash floods for a reason.”

The Red Cross recommends that people closely monitor weather forecasts for flash flooding advisories, keep an emergency kit at hand and develop an evacuation plan. Move immediately to higher ground, and never try to cross floodwaters.

The driver of a vehicle that is on a flooded road should get out and move to higher ground if it is safe, Ms. Wilkes said. The Weather Service cautions that roadbeds may be washed away beneath floodwaters, making them dangerous to drive through.

More people die each year in the United States as a result of flash flooding than from tornadoes, hurricanes or lightning, according to the Weather Service.

Assessing the dangers of floodwaters can be even more difficult at night, according to state and federal public safety agencies, which warn people to avoid camping or parking next to creeks or in other flood-prone areas.

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