A Red Flag Warning is a National Weather Service (NWS) warning that weather and fuel conditions meeting the local office's Red Flag criteria are occurring or expected soon. The NWS issues it in conjunction with land management agencies to flag an ongoing or imminent critical fire weather pattern.
What goes into the criteria
The national directive for fire weather products, NWS Instruction 10-401, says Red Flag criteria combine fuel dryness with weather. The weather side can include lightning episodes, a significant dry frontal passage, strong winds, very low relative humidity and dry thunderstorms.
Each office or group of offices works out its criteria with the land management users in its fire weather area. Those criteria should be documented in interagency Annual Operating Plans and the office's Station Duty Manual.
Why the numbers differ by office
The NWS Chicago office, for example, lists sustained 20-foot winds of 20 mph or more, afternoon relative humidity below 25 percent and 10-hour fuel moisture of 8 percent or less. The Albuquerque office lists sustained 20-foot wind of 20 mph or more with relative humidity of 15 percent or less.
The Honolulu office adds a drought index, the Keetch-Byram Drought Index (KBDI), and issues its warnings island by island. Local climate, vegetation and partner needs all shape where each office draws the line.
Confidence and timing
Under the directive, a Red Flag Warning means high confidence that conditions consistent with local criteria will occur within 48 hours or less. Some office pages describe a shorter working window, such as the next 24 hours at Chicago or generally 12 to 24 hours at Grand Junction.
Forecasters are expected to coordinate with local fire and land managers and with Predictive Services at the National Interagency Fire Center or the regional coordination centers. The final call to issue still rests with the NWS.
What the warning does not tell you
A Red Flag Warning describes the environment, not an active fire. It says ignitions could spread readily, not that a fire has started or where one will.
For the bigger picture, the Storm Prediction Center's Fire Weather Outlook shows the national pattern, while the local warning reflects the office's own criteria and its partners' read on fuels.