Red Flag Criteria Adjusted

On April 15, 2010, the National Weather Service in San Diego made a change to the criteria used to issue Red Flag Warnings.  The change involves the removal of the long duration, low relative humidity criterion: relative humidity equal or below 10% for at least 10 hours.  This change impacts all coastal, valley and mountain zones within the National Weather Service San Diego's area of responsibility.

The main criteria will remain in force:  relative humidity at or below 15% and winds sustained at 25 mph or gusts to 35 mph for at least six hours in duration.

Why the change?  Steven Vanderburg, forecaster and fire weather specialist, conducted a study that analyzed all three of the major wildfires in our region in the last decade.  He found that in nearly all cases, the fires spread rapidly only if there was some strong wind component in addition to very low relative humidity.  He also found several cases where fires that did start in very dry conditions did not spread rapidly because strong or gusty winds were not present.  Local fire fighting agencies agreed that to remove this dry-without-wind criterion would be an improvement.  Over the past year the final approvals were made to make this change to the criteria.

       

Entity	Highest Threat Level	Circumstances	Actions
National Weather Service	Red-Flag Warning	Relative humidity less than 15 percent with sustained winds greater than 25 mph and/or frequent gusts greater than 35 mph for more than six hours.	Advisory issued to local fire agencies
U.S. Forest Service	Critical*	A forest fire has been confirmed or the weather conditions – particularly wind strength – are extremely dangerous, and the service also lacks resources to promptly battle fires.	Severely restrict public access or close roads, campgrounds and other areas.
Cal Fire	High/ Extreme	Extreme fire conditions determined by fire-behavior analysts.	A ban on outdoor burns and an advisory not to use mechanical equipment after 10:00 a.m.
SDG&E	Red-Flag Warning	Same standard as the National weather Service.	A ban on outdoor burns and an advisory not to use mechanical equipment after 10:00 a.m.