KinwarnWhat does a flash flood warning mean?
A flash flood warning means a fast, dangerous rise of water is happening or about to happen in the warned area. If you are in a place that floods, move to higher ground right away, and do not drive or walk into floodwater.
A flash flood warning is an urgent alert from the National Weather Service. It means fast-rising water threatens lives in the warned area, either now or very soon. Here is what it means, how it differs from similar alerts, and why it calls for extra care at night and on the road.
What a flash flood warning means
The NWS issues a flash flood warning when a rapid, damaging, and life-threatening rise of water into a normally dry area is happening or about to happen. If you are in an area that floods, move to higher ground right away.
Flash floods form fast. They usually develop within 6 hours of the heavy rain or other cause, and intense storms can start one within minutes. Common causes include:
- Heavy rain, especially from slow-moving storms or several storms over the same spot
- A dam or levee failure
- An ice jam or debris jam that breaks and releases trapped water
- Rain on ground burned by wildfire, which lets water run off quickly
Cities flood quickly because pavement and concrete keep water from soaking into the ground. Steep, hilly land speeds runoff too. The NWS notes that a mountain creek that is normally 6 inches deep can rise to 10 feet in less than an hour. A flash flood can even reach places where no rain is falling, as water rushes down from storms upstream.
Flash flood watch, flood warning, and other alerts
| Alert | What it means | What to do |
|---|---|---|
| Flash flood watch | Conditions could lead to flash flooding, but it is not certain | Be prepared |
| Flood advisory | Flooding not bad enough for a warning, but it could turn dangerous if people are not careful | Be aware |
| Flood warning | Flooding is happening or about to, often on rivers and larger streams | Take action |
| Flash flood warning | A fast, dangerous rise of water is happening or about to | Take action now |
| Flash flood emergency | Exceedingly rare, catastrophic flash flooding with a severe threat to life | Get to higher ground immediately |
The biggest difference between a flood warning and a flash flood warning is speed. River flood warnings usually cover flooding that arrives 6 hours or more after the heavy rain, snowmelt, or ice jam that caused it. Flash floods come much faster.
Tags inside a flash flood warning
Flash flood warnings can end with a damage threat tag:
- No tag (base): used most of the time, when flood damage is possible.
- Considerable: used rarely, when flooding of unusual severity is happening or about to, and urgent action is needed to protect lives and property.
- Catastrophic: used exceedingly rarely, when water has risen or will rise to levels rarely if ever seen. This is a flash flood emergency.
Only considerable and catastrophic flash flood warnings trigger Wireless Emergency Alerts on phones in the area. A standard flash flood warning does not. For more on these tags, see what is a tornado emergency.
Why night and driving are the danger
Floodwater on a road is very hard to judge. You cannot always tell how deep it is, or whether the road under it has washed out or collapsed. That is why the NWS says it is never safe to drive or walk into floodwater.
It takes less water than most people think:
- About 6 inches of fast-moving water can knock an adult off their feet.
- About 12 inches of rushing water can carry away most cars.
- About 2 feet of rushing water can carry away SUVs and trucks.
Citing the CDC, the NWS says more than half of flood drownings happen when someone drives into dangerous floodwater. The next largest share involves people walking into or near floodwater.
Darkness makes all of this worse. At night it is hard to see water on the road or judge how deep and fast it is, so the NWS urges extra care after dark. If you must travel during a flash flood warning, take only the trips you truly need, and turn around at any flooded road.
For step-by-step advice, read what to do during a flash flood warning.
See where warnings are now on the flash flood warnings page.