What Warrants a Red Flag Warning from the National Weather Service?

Another Red Flag Warning was issued for Southern California late last week for very low humidities and strong offshore winds. But what factors actually warrant this official notice from the National Weather Service and what do the associated weather conditions mean?

A Red Flag Warning is used by the National Weather Service to inform area firefighting and land management agencies that conditions are ideal for wildland fire combustion and rapid spread. Specically, the warning denotes a high degree of confidence that weather and fuel conditions meet the ‘Red Flag Event’ criteria in place for a given fire weather zone. These criteria involve low relative humidity, strong winds, dry fuels, or any combination thereof.

According the the NWS, a Red Flag event is verified when the weather and fuel conditions are met simultaneously for any three hour period, and the warning remains in effect until the critical fire weather pattern ends. The characteristics of fire weather zones differ greatly across wildfire-prone areas.  Therefore the specific thresholds needed to meet the warning criteria can vary as well, based on the local vegetation type, topography, and distance from major water sources.  

RFW_map_title.png

 Southern California’s Ventura and Los Angeles Counties had a Red Flag Warning in effect for below 15% RH and gusty winds.

 

Red Flag Warning Criteria From Regions of California.

(source: SoCal GACC) http://gacc.nifc.gov/oscc/predictive/weather/myfiles/Watches_and_Warnings_for_California.htm

Area Description

NWS Fire Weather Zones

Criteria

Southern California desert area excluding the Colorado River Valley

226-228, 230, 232, 260­, 262

Relative Humidity  ≤ 15% and wind gusts GTE 35 mph for 3 hours or more

Colorado River Valley

229,231

Relative Humidity ≤ 15%, with sustained winds (20 foot) ≥ 20 mph and/or frequent gusts ≥ 35 mph for 3 hours or more

Antelope Valley and SE Kern County Deserts

298, 299, 259

Relative Humidity ≤ 15% and sustained (20-foot) winds ≥ 25 mph for a duration of 8 hours or more

Southern California from mountains westward

234-258, 288-297

Either
Relative Humidity ≤15%, with sustained winds ≥ 25 mph and/or frequent gusts ≥ 35 mph (duration of 6 hours or more)

Or
Relative Humidity ≤ 10% (duration of 10 hours or more) regardless of wind

Northern California East of Cascade/Sierra Crest and Western Great Basin including the Modoc Plateau

214, 270-273, 278, 284, 285

Tahoe Management Basin: Three hours of wind gusts ≥ 30 mph and Relative Humidity ≤ 20%

Other Regions:
Three hours of wind gusts ≥ 30 mph and Relative Humidity ≤1

 

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