Authorities in Northern California found themselves scrambling to get local residents out of the way of fast-moving wildfires this weekend, and are blaming a particular meteorological phenomenon called “Diablo winds” for helping to spread over a dozen blazes in Napa and Sonoma counties.
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The most dangerous wildfires have sprung up in and around the area just north of San Francisco. Tens of thousands of people have been evacuated from the area, with 15 confirmed fatalities and over 100 reported injuries as of Tuesday morning.
At least 1,500 homes and businesses have been destroyed, and California Gov. Jerry Brown has declared a state of emergency.
READ MORE: Wildfires tear through wine country in Northern California, at least 10 killed
The speed and intensity of the fires are indeed unusual. They are being fanned by winds that are hot, dry and strong.
In Southern California these are known as Santa Ana winds, but in northern California they have been dubbed Diablo winds.
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Named for the state’s Mount Diablo, they originate inland in Utah and Nevada.
How the winds work
As air moves from areas of high pressure in those states toward the low-pressure zones along California’s coast, it travels over mountain ranges, heating up, picking up speed and losing humidity as it funnels down steep slopes. Diablo winds are typically strongest atop mountain ridges.
When these powerful winds (some gusting to 115 kilometres per hour) blew into Napa and Sonoma counties this weekend, they encountered the low humidity and dried-out vegetation that are typical of the end of California’s hot summer season.
READ MORE: Wine country fires could mean fewer California wines for years
It was a recipe for disaster, with even the tiniest spark able to grow rapidly into an inferno. Within hours, over a dozen fires were burning. While the Diablo winds didn’t necessarily start the fires, they certainly made them worse.
“It’s just about the worst case weather conditions to spread a wildfire quickly, given the fuel,” Daniel Swain, a UCLA climate scientist, told the Los Angeles Times.
Meanwhile, in Southern California, a Santa Ana wind “event” blew through on Monday and into Tuesday. So far this fall, however, the most destructive wildfires have been in the northern part of the state.
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