The devastating, rapidly advancing flood that struck the Hill Country of west-central Texas on Friday resulted in the deaths of at least 13 individuals and left nearly 24 others unaccounted for. This incident was a flash flood, recognized as the leading cause of storm-related fatalities in the nation.

Flash floods form rapidly

The National Weather Service defines a flash flood as sudden flooding that starts within six, and often as quickly as three, hours of intense rainfall.

The water levels rise so rapidly that individuals are taken by surprise, according to the weather service. Many face dangers while traveling. If they are at home or work, the water can escalate so swiftly, trapping people before they have the chance to devise an escape plan.

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.