Red flag warning
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A red flag 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.
The weather criteria for fire weather watches and red flag warnings vary with each Weather Service office’s warning area based on the local vegetation type, topography, and distance from major water sources. They usually include the daily vegetation moisture content calculations, expected afternoon high temperature, afternoon minimum relative humidity and daytime wind speed.
Outdoor burning bans may also be proclaimed by local law and fire agencies based on red flag warnings.
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.
Example of a red flag warning for extremely hot weather
URGENT - FIRE WEATHER MESSAGE NATIONAL WEATHER SERVICE SEATTLE WA 348 PM PDT SAT JUL 25 2009 ...ONE OF THE MOST CRITICAL FIRE WEATHER SETUPS IN YEARS FOR WESTERN WASHINGTON... .AN UPPER LEVEL LOW PRESSURE CENTER CONTINUES TO SPIN OVER CENTRAL WASHINGTON THIS AFTERNOON. WAZ653-654-656>659-661-662-261200- /O.CON.KSEW.FW.W.0003.000000T0000Z-090726T1200Z/ STRAIT OF JUAN DE FUCA AND NORTHWEST INTERIOR LOWLANDS- CENTRAL AND SOUTH PUGET SOUND LOWLANDS- WEST SLOPES OF THE NORTH CASCADES- WEST SLOPES OF THE CENTRAL CASCADES-NORTH CASCADES- CENTRAL CASCADES-EAST PORTION OF THE OLYMPIC MOUNTAINS- EAST PORTION OF NORTH CASCADES NATIONAL PARK/LAKE CHELAN NATIONAL RECREATIONAL AREA- 348 PM PDT SAT JUL 25 2009 ...RED FLAG WARNING REMAINS IN EFFECT UNTIL 5 AM PDT SUNDAY... A RED FLAG WARNING REMAINS IN EFFECT UNTIL 5 AM PDT SUNDAY. AN UPPER LEVEL LOW PRESSURE CENTER CONTINUED TO SPIN OVER CENTRAL WASHINGTON ON SATURDAY AFTERNOON. THUNDERSTORMS HAD ALREADY DEVELOPED OVER SOUTHERN BRITISH COLUMBIA AND WILL MOVE SOUTHWEST INTO WESTERN WASHINGTON LATER THIS AFTERNOON AND TONIGHT AS THEY TRACK AROUND THE PERIPHERY OF THE UPPER LOW. THE STORMS WILL FIRST REACH THE NORTH CASCADES AND NORTH INTERIOR...THEN SPREAD SOUTHWEST AND SOUTH THIS EVENING THROUGH THE CENTRAL CASCADES AND THE PUGET SOUND LOWLANDS. THUNDERSTORMS COULD ALSO CLIP THE EAST SLOPES OF THE OLYMPIC MOUNTAINS. SOME OF THESE THUNDERSTORMS ARE EXPECTED TO BE UNUSUALLY PROLIFIC LIGHTNING PRODUCERS. GIVEN THE EXPECTED ABUNDANCE OF LIGHTNING AND CRITICALLY DRY FUEL STATES...MULTIPLE FIRE IGNITIONS ARE EXPECTED. ONE THING THAT MAKES THIS ROUND OF LIGHTNING SO CRITICAL IS THAT IT WILL BE FOLLOWED UP ON SUNDAY AND MONDAY BY A BUILDING AND PROLONGED HEAT WAVE. UNLIKE WESTERN WASHINGTON`S TYPICAL LIGHTNING PATTERN IN WHICH LIGHTNING IS FOLLOWED BY COOL MARINE AIR...THIS LIGHTNING PATTERN WILL BE FOLLOWED INSTEAD BY HOT...DRY AND INCREASINGLY UNSTABLE WEATHER ON SUNDAY...MONDAY AND TUESDAY. THIS WILL MAKE HOLDOVER AND SLEEPER FIRES A GREAT CONCERN IN THE DAYS TO COME. PLEASE RELAY THIS MESSAGE TO FIRE AND ENGINE CREWS IN THE FIELD. PRECAUTIONARY/PREPAREDNESS ACTIONS... PEOPLE WHO WILL BE OUTDOORS FROM THIS AFTERNOON THROUGH TONIGHT SHOULD BE AWARE OF THE SEVERE LIGHTNING THREAT AND THE ENHANCED THREAT OF WILDFIRES. MAKE AN EXTRA EFFORT TO FULLY EXTINGUISH CAMPFIRES AND REPORT ANY NEW WILDFIRES WHICH YOU MIGHT SEE. FIREFIGHTING AGENCIES SHOULD BE READY FOR SEVERAL BUSY DAYS OF INITIAL ATTACK. A RED FLAG WARNING IS ISSUED WHEN DRY LIGHTNING OR THE COMBINATION OF STRONG WINDS AND LOW HUMIDITIES ARE IMMINENT OR OCCURRING. && $$ HANER WEATHER.GOV/SEATTLE