(AP) – The devastating and rapidly moving flood that surged through the Hill Country of Texas on Friday has claimed the lives of at least 24 individuals, with more still unaccounted for. This tragic event, affecting areas including a summer camp where over 20 girls are missing, was a flash flood, the deadliest type of storm-related flooding in the nation.

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

Flash floods form rapidly

The National Weather Service defines a flash flood as one that begins within six hours, often occurring in as little as three hours, following heavy rainfall.

Due to the quick rise of water levels, people are often caught by surprise, according to the weather service. Many encounter danger while traveling, and for those at home or work, the water can surge so swiftly that they find themselves trapped before they have a chance to consider an escape route.

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.

There was some 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.

Lt. Gov. Dan Patrick said the potential for heavy rain and flooding covered a large area and “everything was done to give them a heads-up that you could have heavy rain.”

However, when asked about how people were notified in Kerr County so that they could get to safety, Judge Rob Kelly, the county’s chief elected official, said: “We do not have a warning system.”

When reporters pushed on why more precautions weren’t taken, Kelly responded: “Rest assured, no one knew this kind of flood was coming.”

Flash floods can be 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. Flood deaths affect all age groups.

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.

Hurricanes, tropical storms, non-cyclonic weather systems carrying heavy rain and dam failures are some weather events that can cause flash flooding. They can happen in rural areas or in large cities.