A Fire Weather Watch is a warning issued by the National Weather Service (NWS) when the combination of dry fuels and weather conditions support extreme fire danger. The watch is designed to alert land and fire managers to the high potential for development of Red Flag criteria in the next 12-72 hours. The NWS does not make any decisions about fire suppression or management, but rather provides information to land and fire managers to help them make informed decisions.
A Red Flag Warning is issued when the conditions above are expected to occur or are occurring within the next 24 hours. It is a more urgent warning than a Fire Weather Watch and is issued when there is high confidence that Red Flag criteria will be met within the next 24 to 48 hours, or when those criteria are already being met or exceeded. The warning may be issued for all or portions of a fire weather zone or region, and zones impacted by the event will be listed within the Red Flag Warning product.
The criteria for a Fire Weather Watch and a Red Flag Warning vary by region and are determined by each NWS office. However, some common criteria include:
- Frequent gusts of 25 mph or greater
- Relative humidity of 15% or less
- Dry thunderstorms (15% coverage or more, constituting an LAL 6)
- Haines Index of 5 or 6, indicating a moderate or high potential for large, plume-dominated fire growth
- Wind shifts associated with frontal passages
- First significant lightning event (wet or dry) after an extended hot and dry period
- Poor relative humidity recovery overnight (RH remains at 40% or lower)
- Any combination of weather and fuel moisture conditions which, in the judgment of the forecaster, would cause extensive wildfire occurrences
In summary, a Fire Weather Watch is a warning issued by the NWS to alert land and fire managers to the high potential for development of Red Flag criteria in the next 12-72 hours, while a Red Flag Warning is a more urgent warning issued when the conditions above are expected to occur or are occurring within the next 24 hours. The criteria for these warnings vary by region and are determined by each NWS office.
