Fire weather watch and warning definitions

Red Flag Warning

Definition:  

Informs land management agencies of the imminent or actual occurrence of Red Flag conditions. A Red Flag Warning will be 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. A warning may be issued for all, or portions of a fire weather zone or region. Zones impacted by the event will be listed within the Red Flag Warning product. 

Criteria:  

A combination of weather and fuels conditions (as determined by fire management) for any 3 hours or more in a 12 hour period. These criteria for the forecast area of the Denver/Boulder NWS office are defined as the following:

1. Frequent gusts of 25 mph or greater – AND relative humidity of 15% or less

2. Dry thunderstorms (15% coverage or more, constituting an LAL 6.)

OTHER FACTORS: 
In addition to the basic criteria above, a combination of other elements may result in Red Flag Conditions:

3. Haines Index of 5 or 6, indicating a moderate or high potential for large, plume dominated fire growth.

4. Wind shifts associated with frontal passages.

5. First significant lightning event (wet or dry) after an extended hot and dry period.

6. Poor relative humidity recovery overnight (RH remains at 40% or lower.)

7. Any combination of weather and fuel moisture conditions which, in the judgment of the forecaster, would cause extensive wildfire occurrences.

Fire Weather Watch

Definition:  

Alerts land management agencies to the high potential for development of Red Flag criteria (see above) in the next 12-72 hours. The watch may be issued for all, or portions of a fire weather zone or region. Zones impacted by the event will be listed within the RFW