Diablo and Santa Ana winds are to descend on California and raise wildfire risk

SAN FRANCISCO — A major "diablo wind" — notorious in autumn for its hot, dry gusts — is expected to whip up across Northern California on Thursday evening, causing humidity levels to drop and raising the risk of wildfires.

Forecasters have issued red flag warnings for fire danger until Saturday from the central coast through the San Francisco Bay Area and into northern Shasta County, not far from the Oregon border.

Sustained winds reaching 35 mph (56 kph) are expected in many areas, with possible gusts topping 65 mph (104 kph) along mountaintops, according to the National Weather Service.

“This could end up being the most significant wind event for this year so far,” said meteorologist Brayden Murdock with the service’s Bay Area office. “We want to tell people to be cautious.”

During a diablo wind, common in the fall, the air is so dry that relative humidity levels plunge, drying out vegetation and making it ready to burn. The name — “diablo” is Spanish for “devil” — is informally applied to a hot wind that blows near the San Francisco region from the interior toward the coast as high pressure builds over the West.

Pacific Gas and Electric Co. said it was prepared to turn off power to a small number of customers in areas where strong gusts could damage electrical equipment and spark blazes.

Targeted power shutoffs were also possible in Southern California, where another notorious weather phenomenon, the Santa Ana winds, are expected Friday and Saturday.

Winds around greater Los Angeles won't be as powerful as up north, with gusts between 25 and 40 mph (40 and 64 kph) possible in mountains and foothills, said Mike Wofford, a meteorologist with the weather service’s Los Angeles-area office.

“I think it's going to be more moderate,” he said Wednesday. “But the risk of fires is still there.”

But snow is in the forecast for the mountaintops around Lake Tahoe, where up to 2 inches (5 centimeters) was forecast by Friday morning, according to the National Weather Service in Reno, Nevada. Winds around Lake Tahoe could gust up to 70 mph (113 kph).

The service also issued its first freeze warning of the season along the Sierra’s eastern front effective from 2 a.m. to 9 a.m. Friday from south of Carson City to the north through Reno into Lassen, Sierra and Plumas counties in California where temperatures could dip into the low 20s Fahrenheit (-5 Celsius).

“Frost and freeze conditions could kill crops, other sensitive vegetation and possibly damage unprotected outdoor plumbing,” the service said.