A Flash Flood Warning is a National Weather Service (NWS) alert that flash flooding is imminent or already happening in a specific area. It is reserved for short-fused events that call for immediate action to protect life and property, such as small streams, urban streets and low-water crossings rising quickly after intense rain or a sudden release of water.
How it fits the time scale
Flash flooding develops quickly, and NWS terminology describes it as a rapid rise of water beginning within minutes to multiple hours of the cause. Some NWS pages use a working rule of about six hours, the same rule used in the flash flood entry. The Birmingham, Alabama office, for example, says these warnings cover flooding that usually occurs within 6 hours of heavy rain.
Slower, broader flooding goes under a Flood Warning instead. The Flash Flood Warning is the product for water that rises fast enough that waiting is the risk.
What the tags at the bottom mean
Since a 2019 service change, Flash Flood Warnings use an impact-based format with machine-readable tags. A source tag reads either FLASH FLOOD…RADAR INDICATED or FLASH FLOOD…OBSERVED, which separates warnings based on radar evidence from those where flash flooding has been confirmed by spotters, law enforcement or other credible observers.
A damage threat tag sorts warnings into three tiers. Most warnings carry no tag, the base level, meaning flash flood damage is possible.
CONSIDERABLE is used rarely, when flooding of unusual severity is imminent or ongoing. CATASTROPHIC is used exceedingly rarely, and it equates to a Flash Flood Emergency headline.
When phones get the alert
Wireless Emergency Alerts (WEA) are not sent for every Flash Flood Warning. Since early 2020, the NWS sends them only when the damage threat tag is CONSIDERABLE or CATASTROPHIC, and also for follow-up statements that raise the threat to one of those levels.
The NWS made that change partly because it issues more than 4,000 of these warnings a year, and many people felt it was over-alerting. A warning without a phone alert is still a warning.
What the warning does not tell you
A polygon covers an area, not a depth map. Radar rainfall can miss rain in complex terrain, and an observed tag only means flooding was confirmed somewhere in the warning, so read the listed locations and never drive into water of unknown depth.
For the bigger picture, the Weather Prediction Center's Excessive Rainfall Outlook and Mesoscale Precipitation Discussions show where heavy rain may set up before a warning is needed.