Flash floods are the single deadliest storm-related weather hazard in the United States. Learn about what they are and what to watch for in case one affects your area.
What are Flash Floods?
As the name implies, flash floods happen suddenly, leaving little time for warning (in fact, flash floods have developed in as little as 58 seconds). They usually occur in small streams, canyons, or dry river beds with less than 20 square miles of drainage area. Whereas “regular” flooding may not occur until days or even weeks following a period of snowmelt or rain, flash flooding typically happens as the source of the rain is still overhead.
Tip: Stay out of flowing water! A mere six inches of moving water can knock you off your feet, while two feet of water will carry away most cars.
Flash Flood “Red Flags”
It’s difficult to predict flash floods, so you should always be aware of the conditions that lead can to them:
- Flash floods occur within six hours of a rain event.
- Listen for news of dam or levee failures.
- Watch for slow-moving storms that repeatedly move over the same area.
- Hurricanes are another big source of intense rain.
- If you see water is collecting in pools, this is a sign the ground is oversaturated with water and that flooding can occur.
- During flood watches, do not park your car near a river or on a street that you know floods. As land turns from fields or woodlands into roads and parking lots, it loses its ability to absorb rainfall.
- Be aware of a “flood watch” or “flood warning” from the National Weather Service.
- Flood Watch: A flood watch is issued when conditions are favorable for flooding. It does not necessarily mean that flooding will occur, but that it is possible.
- Flood Warning: A flood warning is issued when flooding is imminent or occurring. Seek higher ground immediately.
- See more flood safety tips here.
Predicting a Flash Flood
To have any chance of predicting a flash flood, forecasters need to know where the “bullseye” of intense rain is—that is:
- How much rain is falling
- How quickly it’s coming down
- How saturated the soil is
Radar and stream gauges automatically convey this information via phone lines, radio waves, or satellites. “That way, the forecaster can receive [data] as the rain is falling,” says Matthew Kelsch with the Forecast Systems Laboratory, based in Boulder, Colorado.
However, radar is limited in its ability to detect rain in mountainous areas. In addition, the farther away you get from the radar, the less accurate the information.
Another big unknown: How will the water behave once it’s on the ground? “Flash floods don’t always happen in the natural stream channel,” says Kelsch.
To address this, land surface characteristics such as terrain are now included in computer models along with data from radar and gauges, resulting in better predictions about how a certain location will handle a given amount of water.
The most difficult challenge is how to measure how much moisture the ground can soak up: NASA has begun exploring the use of satellites to measure soil moisture over much larger areas than those currently monitored by rain gauges.
One For the Records
Date: July 31, 1976
Forecast: Heavy rain, possible flooding
Fact: Thousands of campers in Colorado’s narrow Big Thompson Canyon ignored warnings of flooding on July 31, 1976, with tragic results. Twelve inches of rain fell in five hours, making the river rise with astonishing speed. A 19-foot wall of water killed 139 people, as it swept away boulders, cars, bridges, even a restaurant with the diners still inside.
Have you ever experienced a flash flood? Tell us about it in the comments below!