After hours of sweltering heat, the sky darkens to a charcoal gray. Suddenly it feels as if thunderstorms and heavy rain are imminent. An alert appears on your phone or in the news ticker at the bottom of your television screen: A flash flood watch or warning has been posted for your area.
But what exactly does that mean? And what should you do to avoid being caught in fast-rising waters, which the National Weather Service says are responsible for an average of 88 deaths each year in the United States?
Flash flooding gets its name because of the sudden deluge after a heavy rainfall, which the Weather Service says is the most common cause. The flooding begins within six hours and often within three hours of an intense rainfall, though 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, 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. For evidence of this, one need look no further than social media, where commuters share videos of flooded subway stations and roads.
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 flooding, and that it is a possibility.
Flash flooding is distinct from flooding, such as the extraordinary floods that have been blamed for the deaths of more than 180 people in Germany and Belgium.
The Weather Service defines a flood as the 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.
But disaster preparedness experts emphasized that flash flooding can still be 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 organization said.
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.
To raise awareness about the dangers of floodwaters, the Weather Service created a public safety campaign called Turn Around, Don’t Drown. It cautions that roadbeds may be washed away beneath floodwaters, making it dangerous to try to drive through them.
More people die each year in the United States as a result of flash flooding than they do 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.
People who are ordered to evacuate their homes should not try to return until the authorities tell them that it is safe to do so, disaster preparedness experts said.