A Flood Watch is a National Weather Service (NWS) alert that conditions are favorable for flooding, but the flooding is neither certain nor imminent. Since November 2021 it has been the main watch for flash flooding from excessive rainfall as well as for slower areal and river flooding, typically issued 6 to 48 hours ahead of the expected event.
How confident the watch is
The national directive for NWS water products, NWS Instruction 10-922, lists, as one of its issuance criteria, roughly a 50 to 80 percent chance that conditions will lead to flooding within 48 hours. It describes the usual window as 6 to 48 hours before the event.
When flooding becomes imminent or begins, the office typically moves to a warning. That may be a Flash Flood Warning for fast-rising water, or a Flood Warning for slower flooding.
One watch for rain-driven floods
Service Change Notice 21-56, effective on or after 11/04/21, consolidated Flash Flood Watches and Flood Watches into one Flood Watch when the immediate cause is excessive rainfall. A separate Flash Flood Watch now covers only rare cases such as a possible dam failure or rain on a burn scar.
The same change put the Flood Watch in a WHAT, WHERE, WHEN format, so the text states which kind of flooding is possible. The Honolulu office, for example, describes its Flood Watch as meaning heavy rain leading to flash flooding is possible, issued up to 48 hours ahead.
Watches for river forecast points
Some offices also issue a Flood Watch for Forecast Points, an optional product for specific gauges on rivers and streams. The Quad Cities office says it issues one when forecasters have 50 to 80 percent confidence that a river point will rise above flood stage, and upgrades it to a Flood Warning when confidence is high.
What a watch does not tell you
The watch does not pin down where the heaviest rain will fall. The Weather Prediction Center's Excessive Rainfall Outlook and short-term rainfall estimates help narrow that down as the event approaches.