The monstrous, swift-moving flood that swept through the Hill Country of west-central Texas Friday, killing at least 13 and leaving nearly two dozen missing, was a flash flood, the nation’s top storm-related killer.

Flash floods form rapidly

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.

There was advance notice in 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.

Flash floods are deadly

Last year, 145 people died in flash floods, according to the weather service. On average over the past 30 years, floods have claimed 127 lives annually.

Floods trap people in vehicles

Nearly half of all flood-related fatalities involve vehicles.

Many people don’t realize that a car becomes difficult to control in just 6 inches (15 centimeters) of water and can be swept away in as little as 18 inches (46 centimeters). So instead of finding a detour, too often people try to drive through water at underpasses or other low-lying areas.

Flash floods can happen anywhere

It can happen anywhere, according to the weather service, which says that “the normally tranquil streams and creeks in your neighborhood can become raging torrents if heavy rain falls overhead.”

An area can be flooded even without rain — if it’s downstream of a torrential rainstorm and a swollen stream heads its way.