What You Need To Know About a Flash Flood Warning

A flash flood warning is nothing to ignore. Here's how to respond to one to keep you and your family safe.

While melting snow and spring rains signal the end of winter’s frigid grasp, they also make some homeowners nervous, especially in areas prone to springtime flooding. And rightly so.

“We are seeing more flash flooding than in the past,” says Ceil Strauss, flood plain manager for the state of Minnesota. “There’s a slight increase in total precipitation on average each year, but a much higher percentage of water comes in intense storms. There’s a 65% increase in the six-inch or larger precipitation compared with the past.”

These storms don’t always hit in the spring and summer, either. “In Minnesota, we had a big rain last February,” Strauss says. “A few years ago, Illinois had floods in January. Even those who don’t believe in climate change recognize more big storms are a concern, and that people need to be prepared.”

Here’s what to know about flash floods and how to prepare your family and home.

What Is a Flash Flood?

According to the National Weather Service (NWS), “Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam.”

Water rises rapidly over low areas, changing an idyllic stream or creek into a violent, raging river. Flash floods generally develop within six hours after slow-moving thunderstorms pass through an area. They may also result from ice or debris jams, and levee or dam failure.

And flooding may occur far from where heavy rain initially fell. This is common in the western United States, where low areas may be dry one minute and overrun with rushing water the next. A flash flood can even happen in areas where it never rained at all.

Urban areas with large areas of concrete surfaces are especially prone to flash floods. Steep, rocky land with clay soil also hastens flash flood development since it can’t absorb much water. The NWS says narrow valleys can quickly make a six-inch-deep mountain creek swell to a 10-foot depth in less than one hour.

What Is a Flash Flood Warning?

It means a flash flood is imminent or occuring. The NWS says flash floods are the No. 1 storm-related killer in the U.S., so when a warning is issued, take immediate action. Don’t dilly-dally. If you’re in a flood-prone area, move immediately to high ground.

What’s the Difference Between a Flash Flood Warning and a Flash Flood Watch?

A flash flood watch means weather conditions favor flooding. A watch doesn’t guarantee a flash flood will occur, but your community will probably experience severe weather. Prepare to take action.

A flash flood warning means, get to high ground now. A flash flood can develop in minutes; sometimes it happens so fast there isn’t time for a warning.

How To Get Flash Flood Warnings on Your Phone

Protect yourself and your family by signing up for Wireless Emergency Alerts (WEA), sent by local and state public safety agencies, the Federal Emergency Management Agency (FEMA), the Federal Communications Commission (FCC), the Department of Homeland Security (DHS) and the NWS.

Alerts are sent automatically to WEA-capable phones during an emergency. WEA messages include a special tone and vibration, repeated twice.

You can also download apps for additional protection. The Storm Shield app sends severe weather alerts based on specific location. It also offers radar maps and current conditions, as well as hourly and daily forecasts. The Weather Channel app lets you set up severe weather alerts as well.

Monitoring social media before and during a crisis is another way to stay on top of what’s happening. “Sheriff and police offices and emergency management folks use social media to send out warnings, block off roads, find help at community locations, etc.,” Strauss says. “Twitter, Facebook and other social media are used very effectively.”

The NWS continuously broadcasts updated weather warnings and forecasts from National Oceanic and Atmospheric Administration (NOAA) Weather Radios. Average range is 40 miles, depending on topography. Purchase a radio with a battery backup and tone-alert feature that automatically alerts you when a watch or warning is issued.

How To Prepare for a Flash Flood

“The more you do to look at your situation ahead of time, the better,” says Strauss. Learn how to prepare for and stay safe during a flash flood.

Strauss says new homeowners should look up their neighborhood on FEMA official sub-plain maps to judge the flood risks.

“You can see two-foot elevation contours and get an idea where your house is to the surrounding land,” Strauss says. “This is helpful to see where you are relative to a FEMA flood plain. Even if you are not in the official FEMA flood plain, it’s helpful to see what’s your elevation compared to the surrounding area.”

Knowing your elevation can help you plot the best evacuation route. “Most deaths happen when people try to drive through areas they shouldn’t,” Strauss says. Strauss referenced Turn Around, Don’t Drown (TADD), an NWS campaign warning about the dangers of walking or driving through floodwaters.

“When you see water on a roadway, you don’t know when that road has been washed out underneath,” Strauss says.

“Often, streams have blown out smaller culverts or bridges that weren’t built for that kind of flooding. You’ve got a very serious situation there. It only takes six inches of fast-moving water to knock over most adults, and 12 inches of fast-moving water to carry away smaller vehicles.”

Gary Legwold
Gary Legwold is a Minneapolis-based writer, an editor and an award-winning author of five books, including a novel. He is also the humble Lefse King, blogging about all things lefse and some things lutefisk and selling lefse-related products at www.lefseking.com.