After such a horrendous fire season this summer for much of western North America, many Pacific Northwesterners welcomed the onset of autumn with open arms. But unfortunately, as bad as the fires were all this summer throughout the West, the worst fires of the year would flare up in California during the second week of November. Two fires in particular—the Woolsey Fire near Los Angeles and the Camp Fire in Butte County in Northern California—collectively scorched 250,000 acres, incinerated almost 19,000 buildings, and caused billions of dollars in damage this past month. With 83 confirmed fatalities and around 600 people still missing as of 11/21/2018, the Camp Fire is the deadliest fire in the U.S. since the Cloquet Fire in Northern Minnesota of October 1918
The Woolsey Fire was fanned by the “Santa Ana” winds that are notorious for bringing abnormally warm temperatures, very low dew points and very high fire danger to Southern California. The Camp Fire was spread by the lesser-known “Diablo Wind” that flows down the slopes of the Sierra Nevada into the Sacramento Valley and San Francisco Bay Area.
Both the Santa Ana and Diablo Winds are specific examples of “foehn” winds: strong, warm, dry downslope winds on the lee side of a mountain range. The term originates from the warm, dry wind in the leeside of the Alps that warmed Central Europe while the windward sides of the Alps experienced the brunt of powerful extra-tropical cyclones coming off the Atlantic into the region.
Even before these fires, California had seen a much more active-than-normal fire season. Fires ravaged the state in July and August as dry, hot weather persisted throughout the West, creating tinder-dry fuels and extreme fire danger. The combination of dry fuel and strong Santa Ana and Diablo winds led to exceptionally high fire danger that allowed these blazes to grow in size.
The Camp Fire began when a transmission line owned by Pacific Gas and Electric (PG&E), a California Utility, collapsed in the strong winds. Hindsight is 20/20 of course, but given the critical fire danger, PG&E should have de-energized these lines. The cause of the Woolsey Fire is still unknown.
Santa Ana and Diablo winds are much more common in the autumn than the summer. This is because you need high pressure over the Great Basin to drive these offshore winds, and the summertime heat over the Great Basin and Desert Southwest tends to create a thermally induced area of low pressure over the Intermountain West, resulting in onshore flow for most of coastal California.
Though you can still get devastating Santa Ana and Diablo winds in the winter (December 2017 saw strong Santa Ana winds and numerous fires over Southern California), fire danger is generally much less due to decreasing temperatures and increasing precipitation over the area. Whatever happens this month, let’s hope for fewer fires in 2019!
Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist. Check out his weather website at charlie.weathertogether.net.