DISCUSSION: On the night of October 23, 2019, a fire broke out in Sonoma County in California near the town of Geyserville. The cause of the fire is still yet to be determined but it has grown to 25,000 acres by October 26 before exploding to over 75,000 acres over the weekend. The fire was officially contained on November 6 after burning 77,000 acres. Among the main reasons the fire grew rapidly was due to a ridge of high pressure that was dominant over the Pacific Coast which brought dry and warm air into Northern and Central California. In addition to the dry warm air, the fire was aided by strong winds from the mountainous regions of the Sierra Nevada due to the sinking air that is often affiliated with ridges of high pressure.
The fire led Pacific Gas & Electric (PG&E) to implement power outages in the mountainous and hilly regions as a precaution due to prior fires involving their electric lines. PG&E took these steps as the National Weather Service (NWS) in Monterey issued red flag warnings over most of the Bay Area due to wind speeds that were expected to be up to 30 miles per hour with gusts up to 55 mph in some spots as well as the lingering dry and warm air. A red flag warning is issued to alert fire agencies that the weather will likely help spark and develop fires due to wind, low humidity and high temperatures.
The winds decreased over the weekend and the temperature followed suit during the week, however, the humidity remained low due to a trough of low pressure that came in from the polar regions. The smoke of the fire also has led to the air quality in the Bay Area to worsen that Spare the Air alerts have been issued by the Bay Area Air Quality Management District since the smoke reaching and remaining in the lower levels of the atmosphere due to the winds. The decrease in wind also helped with the containment of the fire. In California, major fires often occur during the fall season with recent ones including the October 2017 outbreak with the Nuns, Tubbs, Atlas and Redwood Valley Complex fires and the Camp Fire in November 2018.
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© 2019 Meteorologist JP Kalb