October 24, 2019

Extreme Fire Weather Conditions Will Exist Across Parts Of California On Thursday



Strong winds and low humidity will create favorable conditions for wildfires across portions of California on Thursday. An intense fire had already broken out in Sonoma County on Wednesday night, where winds are gusting as high as 76 MPH. More fires could ignite before weather conditions slowly begin improve on Thursday night.

The combination of strong winds, low humidity, and dry vegetation will allow dangerous fire weather conditions to persist through the day on Thursday across southern California, with the the greatest risk existing in the Transverse Ranges surrounding the Los Angeles area. Critical fire weather conditions will also exist on Thursday in portions of northern California, including the northern Coast Ranges and the Chico area.
An intense fire broke out Wednesday night near Geyserville in Sonoma County, about 20 miles north of Santa Rosa on Highway 101. The Kincade Fire had consumed 1,000 acres in just a couple of hours, with no containment as of the publication of this post around midnight Pacific Time. Several folks published webcam snapshots of the fire on Twitter, showing a sea of flames on the nearby hillsides.

As we see so often during the autumn months, the greatest fire risk today will come from a Santa Ana wind event in southern California. Santa Anas develop when northeasterly winds blow across southern California. Winds cresting the mountains speed up as they race downslope toward the coast. Since sinking air compresses as it descends, these winds grow very hot and very dry by the time they reach lower elevations. This leads to an extended period of strong winds—often in excess of 40-50 MPH—hot temperatures, and bone-dry humidity levels, all three conditions necessary for explosive fire development.

More than 180,000 people in northern California were without power late Wednesday night as part of PG&E's Public Safety Power Shutoff program, designed to prevent wildfires if strong winds damage power equipment. Investigators found that PG&E's power lines sparked the November 2018 Camp Fire, which killed 85 people and destroyed most of the town of Paradise, California.

SoCal Edison warns that 308,000 customers could lose power as a safety precaution when the Santa Ana winds crank up on Thursday.

Conditions should improve on Thursday evening, though critical fire weather conditions could extend into Friday across parts of southern California.

The Storm Prediction Center issues fire weather outlooks much as they issue severe weather outlooks. The three categories—elevated, critical, and extremely critical—convey how favorable conditions are for the development and spread of wildfires. Fires can spark more easily and grow more quickly as you get into "critical" and "extremely critical" territory.

Forecasters issue the "extremely critical" category when sustained winds of 30+ MPH, very low humidity, and warm temperatures are all expected to coincide for three or more hours across an area with dry vegetation. The SPC has an extended rubric on its website explaining the criteria necessary for a forecaster to issue each fire weather category.


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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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