National Weather Service United States Department of Commerce



A.    Red Flag Event. Red flag events are defined by critical weather and fire danger conditions that could lead to extensive wildfire occurrences and/or control problems on existing wildfires or prescribed burns.               
B.     Fire Weather Watch. A fire weather watch is issued to advise user agencies of the possible development of red flag conditions in the near future, generally the next 24 to72 hours. Under certain conditions a watch can be issued for the first 12 hours. 
C. Red Flag Warning. A red flag warning is issued to advise user agencies of occurring or imminent red flag conditions, generally within the next 12 to 24 hours.
2.   RED FLAG CRITERIA. Red flag events normally require the combination of critical fuel conditions and critical weather conditions. Several combinations of fuels and weather conditions may combine to produce red flag conditions. Fuel conditions are considered critical when fuel characteristics are favorable for large fire growth, as determined by the land management agencies. NWS forecasters determine when weather conditions are critical.
A. Primary Red Flag Criteria: 
1)   Relative humidity of 15% or less combined with sustained surface winds, or frequent gusts, of 25 mph or greater. Both conditions must occur simultaneously for at least 3 hours in a 12 hour period.
2)   Widely scattered (or more) dry thunderstorms, 15% or more coverage, constituting a Lightning Activity Level (LAL) 6. A thunderstorm is considered “dry” if it produces less than 0.10 inch rainfall.
B. Contributing Red Flag Factors: 
1)   First significant lightning occurrence after a hot and dry period.   This includes “wet” or “dry” thunderstorms, widely scattered (15%) coverage or more. After a hot and dry period, the first occurrence of thunderstorms can readily start wildfires. The RH does not need to meet the criteria listed above.
2) Significant cold frontal passage, expected to cause strong sustained and gusty winds, and an abrupt wind shift. Of heightened concern are dry cold fronts that have the above characteristics, but little or no rainfall, and expected when there are on-going wildfires or prescribed burns. The RH does not need to meet the criteria listed above.
3)   Any combination of weather and fuels conditions that would create a critical fire control situation or extensive wildfire outbreak. These may include: long term drought, much higher than normal maximum temperatures coupled with very low humidity, low fuel moisture, poor nighttime RH recovery, high Energy Release Component (ERC) or Burning Index (BI), a Haines Index of 5 or 6, etc.