A Flood Warning is a National Weather Service (NWS) alert that flooding is imminent or occurring and is not covered by a Flash Flood Warning. It comes in two main forms: an areal Flood Warning for flooding that develops more gradually over an area, and a Flood Warning for a river forecast point that is expected to rise above its flood stage.
Areal versus river warnings
An areal Flood Warning covers flooding that builds more slowly, usually from prolonged moderate to heavy rain. Fast-rising water from intense rain goes under a Flash Flood Warning instead, which keeps the two time scales separate.
A river Flood Warning, known in NWS product names as a Flood Warning for Forecast Points, is tied to a specific gauge. The Birmingham, Alabama office describes it as issued when a river is forecast to go above its designated flood stage at that forecast point.
How river warnings are built
Flood stage is a set gauge height at which rising water begins to affect lives, property or commerce. The NWS forecasts the river level at each designated forecast point, and the NWS describes flood severity with minor, moderate and major categories.
Moderate flooding involves some inundation of structures and roads near the stream, while major flooding brings extensive inundation and significant evacuations. The thresholds for each category are set gauge by gauge.
The tier below and the tier above
A Flood Advisory is the lower tier, for flooding that is not considered a significant threat to life or property. In November 2021 the NWS folded five advisory headlines, including the Urban and Small Stream Flood Advisory, into a single Flood Advisory.
If conditions worsen quickly, an advisory may be upgraded to a Flash Flood Warning.
What the warning does not tell you
A river warning describes conditions at gauges, and flooding between gauges can differ. Rain still to come, levee performance and debris can all change the crest, so check updates rather than relying on the first forecast.