A Red Flag Warning (RFW) can be issued either by the state or the federal government to assist regional firefighting and land management agencies in making decisions on limiting the use of fire for a particular period of days or fighting fire when conditions suggest increased chances. Conditions that increase the chance of wildland fire starts and fire spread are monitored and a RFW is issued when a high degree of confidence that Red Flag conditions will occur within 24 hours of issuance.
So, a RFW is usually issued during a drought and when the relative humidity is very low. High winds and dry lightning strikes can become exacerbating factors and are included in some state and federal agencies that calculate their own warning data. These agencies will alter their staffing and equipment resources according to the data. 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. Extreme care should be taken and the uses of outside fire suspended. Here is an example of a RFW issued by the Colorado Forest Service during the latest Colorado fire crisis:
Event: Red Flag Warning
...RED FLAG WARNING IN EFFECT FROM 11 AM TO MIDNIGHT MDT TONIGHT
FOR DRY THUNDERSTORMS AND GUSTY OUTFLOW WINDS FOR FIRE WEATHER
ZONES 201...203...207...290...291...292 AND 293...
.ANOTHER ROUND OF ISOLATED TO SCATTERED THUNDERSTORMS IS EXPECTED
TODAY AS HOT TEMPERATURES CONTINUE. WHILE THE CONVECTIVE POTENTIAL
REMAINS...MOISTURE WILL BEGIN TO GRADUALLY DECREASE AS HIGH
PRESSURE STARTS TO NUDGE INTO NEW MEXICO. THIS WILL RESULT IN THE
MAJORITY OF STORMS PRODUCING MINIMAL RAINFALL AMOUNTS AT THE
...RED FLAG WARNING REMAINS IN EFFECT FROM 11 AM THIS MORNING TO
MIDNIGHT MDT TONIGHT FOR DRY THUNDERSTORMS AND GUSTY OUTFLOW
WINDS FOR FIRE WEATHER ZONES 201...203...207...290...291...
292 AND 293...
* AFFECTED AREA...
FIRE WEATHER ZONE 201 ROUTT FORECAST AREA...
FIRE WEATHER ZONE 203 GRAND JUNCTION FORECAST AREA...
FIRE WEATHER ZONE 207 DURANGO FORECAST AREA...
FIRE WEATHER ZONE 290 PARADOX VALLEY FORECAST AREA...
FIRE WEATHER ZONE 291 NORTHERN SAN JUAN FORECAST AREA...
FIRE WEATHER ZONE 292 NORTH FORK FORECAST AREA...AND
FIRE WEATHER ZONE 293 GUNNISON BASIN FORECAST AREA.
* THUNDERSTORMS...ISOLATED TO SCATTERED THUNDERSTORMS WILL
DEVELOP FIRST OVER THE MOUNTAINS AND THEN MOVE TO THE EAST AND
NORTHEAST. THE MAJORITY OF STORMS WILL BE DRY...THOUGH SOME
WILL PRODUCE MARGINAL WETTING RAINFALL.
* OUTFLOW WINDS...ALL THUNDERSTORMS WILL BE CAPABLE OF PRODUCING
ERRATIC OUTFLOW WIND GUSTS UP TO 40 MPH.
* IMPACTS...ANY FIRE IGNITION WILL BECOME DIFFICULT TO CONTROL
WITH VERY HIGH RATES OF SPREAD POSSIBLE.
Instructions: A RED FLAG WARNING MEANS THAT CRITICAL FIRE WEATHER CONDITIONS ARE EITHER OCCURRING NOW...OR WILL SHORTLY. A COMBINATION OF STRONG WINDS...LOW RELATIVE HUMIDITY...AND WARM
TEMPERATURES WILL CREATE EXPLOSIVE FIRE GROWTH POTENTIAL. PLEASE ADVISE THE APPROPRIATE OFFICIALS AND AFFECTED FIELD PERSONNEL OF THIS RED FLAG WARNING.
Red Flag criteria also occurs whenever a region (usually a state) has been in a dry spell for a week or two, or for a shorter period under duress. It usually happens either before spring green-up or after fall color in the East or during the hot, windy summer in the West. The National Fire Danger Rating System (NFDRS) is usually high to extreme and the following forecast weather parameters are forecasted to be met:
1) a sustained wind average 15 mph or greater 2) relative humidity less than or equal to 25 percent and 3) a temperature of greater than 75 degrees F. In some states, dry lightning and unstable air are criteria. A Fire Weather Watch may be issued prior to the Red Flag Warning.