Flash Floods: How They Form, Warning Signs, and Safety Steps

Flash floods are among the most lethal weather hazards because they leave little time to react. The US National Weather Service (NWS) glossary defines a flash flood as "a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event," such as intense rainfall, a dam failure or an ice jam.

How flash floods form

The NWS says the most common cause is heavy rain from thunderstorms, especially when storms move slowly or several systems pass over the same area in succession, overwhelming streams and turning small waterways into fast-moving torrents. Other triggers listed by the NWS include ice or debris jams and the failure of a levee or dam.

Urban areas face added risk, according to the NWS, because concrete and asphalt prevent rainwater from soaking into the ground. This impervious surface increases immediate runoff, and when heavy downpours exceed the capacity of storm drains, urban flash flooding results even far from rivers.

Dam failures are a less frequent but severe cause. The NWS notes that more than 80,000 dams exist in the United States, roughly a third of which are classified as posing significant hazards if they fail. Failures occur through overtopping (water rising above the dam), excessive seepage, or structural failure, and can stem from prolonged rainfall, landslides, earthquakes, erosion, poor maintenance or negligent operation.

Warning signs to watch for

According to NWS regional guidance, flash floods can develop in minutes and are capable of rolling boulders, uprooting trees and destroying buildings with little or no advance notice. Signs that water is approaching include a roaring sound coming from upstream, a sudden change from clear to muddy water, a sudden rise or fall in a stream's level, and movement of fences, utility poles, boulders or trees in the water.

The NWS cautions that flooding can occur even where it is not currently raining, if heavy rain has fallen upstream. One regional NWS office states plainly: "When a flash flood is headed your way, you may only have seconds to save your life."

Safety precautions: before, during, after

The NWS's long-standing slogan, "Turn Around, Don't Drown," sums up its central warning: never walk, swim or drive into flood water. The agency says six inches of fast-moving water can knock a person off their feet, one foot of moving water will float many cars, and two feet of rushing water can carry away most vehicles, including SUVs and trucks.

1. Before: Know whether your home or route is in a flood-prone area, keep a NOAA Weather Radio or local alert app active, and keep vehicle fuel tanks and devices charged, the US Centers for Disease Control and Prevention (CDC) and NWS advise.
2. During a watch/warning: Move to higher ground immediately; avoid dips, canyons, washes and drainage ditches; never camp or park beside a stream when heavy rain threatens, per the NWS.
3. If driving: If you see a flooded road ahead, turn around and take another route — do not try to judge the depth or drive through, the NWS says.
4. If floodwaters rise around your car: Abandon the vehicle and move to higher ground on foot if it is safe to do so, according to NWS guidance.
5. At home: Turn off gas, electricity and water before evacuating, and unplug appliances to avoid electrical shock, the CDC says.
6. After: Do not enter buildings with standing floodwater until local officials confirm it is safe; floodwater can contain sewage, chemicals, sharp debris or wildlife such as snakes, the NWS and CDC note.
7. Cleanup: Wear rubber boots and gloves, discard items that cannot be disinfected, and clean hard surfaces with a mixture of one cup of bleach per gallon of water, per CDC guidance.

Why flash floods are so deadly worldwide

Flash floods account for roughly 85% of flooding incidents and carry the highest mortality rate of any flood type, causing more than 5,000 fatalities globally each year, the WMO says. The organization links rising flash-flood risk in many regions to rapid population growth in urban areas and the continued expansion of settlements into floodplains.

To cut the death toll, WMO and partners including the US National Weather Service built the Flash Flood Guidance System, a forecasting partnership formed in 2007 that supplies national weather agencies with rainfall and hydrological data to issue earlier warnings. By the end of 2019, the WMO says the system supported warning services covering more than 3 billion people across 67 countries.