From the National Weather Service: The National Weather Service (NWS) issues Red Flag Warnings & Fire Weather Watches
to alert land management agencies about the onset, or possible onset,
of critical weather and fuel moisture conditions that could lead to
rapid or dramatic increases in wildfire activity. This could be due to
low relative humidity, strong winds, dry fuels, or any combination
thereof. You may have noticed that the Watch/Warning/Advisory map on our
homepage turns red or orange when a Red Flag Warning or Fire Weather
Watch is issued.
From Wikipedia: A Red Flag Warning, more appropriately known as a Fire Weather Warning, is a forecast warning issued by the United States National Weather Service
to inform area firefighting and land management agencies that
conditions are ideal for wildland fire combustion, and rapid spread.
After drought conditions, when humidity
is very low, and especially when there are high or erratic winds which
may include lightning as a factor, the Red Flag Warning becomes a
critical statement for firefighting agencies. These agencies often alter
their staffing and equipment resources dramatically to accommodate the
forecast risk. To the public, a Red Flag Warning means high fire danger
with increased probability of a quickly spreading vegetation fire in the
area within 24 hours.
A separate but less imminent forecast may include a Fire Weather Watch,
which is issued to alert fire and land management agencies to the
possibility that Red Flag conditions may exist beyond the first forecast
period (12 hours). The watch is issued generally 12 to 48 hours in
advance of the expected conditions, but can be issued up to 72 hours in
advance if the NWS agency is reasonably confident. The term “Fire
Weather Watch” is headlined in the routine forecast and issued as a
product. That watch then remains in effect until it expires, is
canceled, or upgraded to a Red Flag Warning.
