Flash floods like in Texas are the U.S. top storm-related killer. Here’s why

The flash flood that tore through central on Friday is a grim example of why flash floods are the nation’s deadliest weather hazard, killing at least 24 and leaving many missing.

The monstrous, swift-moving flood that swept through the Hill Country of Texas on Friday, killing at least 24 people and leaving more people missing, was a flash flood, the nation’s top storm-related killer. Among those missing are more than 20 girls attending a summer camp.

Authorities said more than 230 people had been rescued as of late Friday, including more than 160 by helicopter.

According to the National Weather Service, a flash flood is flooding that begins within six hours, and often in as little as three hours, of heavy rainfall.

Waters rise so quickly that people are caught off guard, according to the weather service. Many people run into trouble while traveling. If at home or work, the water can rise so quickly people are trapped before they have time to think about escape.

That is just what happened to residents along the Guadalupe River in Kerr County and the surrounding area after at least 10 inches (25 centimeters) poured from the sky early Friday morning.

Fast-moving waters along the river rose 26 feet (8 meters) in just 45 minutes before dawn Friday, washing away homes and vehicles. The danger was not over as more heavy rains were expected Saturday and flash flood warnings and flood watches remained in effect for parts of central Texas.

On Thursday afternoon, the National Weather Service issued a flood watch, estimating rising water of up to 7 inches (17 centimeters) in spots. A “watch” means conditions are favorable for a flood and people should be prepared, but hazardous conditions might not develop.

But the watch was upgraded to a flood warning overnight, a notice that impacted 30,000 people. A warning is issued when flooding is imminent or occurring, the weather service says.