Why this weekend’s winds could be very dangerous

On Sunday morning, a warm dawn wind will race through parched Bay Area mountains with such speed that it will almost scream.

“This will be a big event,” said Craig Clements, director of the Fire Weather Research Laboratory, studying a computer screen from his office at San Jose State University. “We’re watching it set up now. Wow.”

Wind speeds are forecast to reach up to 40 to 45 mph in the North Bay hills, 35 mph in the Santa Cruz Mountains and 23 to 30 mph in the East Bay’s Diablo Range.

This doesn’t mean that it’ll be gusty in your yard on a downtown street nestled in a valley.

But it’ll roar through the notches in our wild and high landscapes, already baked by the sun and filled with kindling. This could confound efforts, warn experts, to control the Kincade Fire in Sonoma County and the Tick Fire north of Los Angeles, both spreading dangerously close to highly populated communities.

“It could be similar to what was seen in October 2017’s North Bay fires,” a series of 21 major blazes that burned at least 245,000 acres and killed 44 people, said meteorologist Jan Null of Golden Gate Weather Services.

Autumn is always a transitional season, with a shift in the patterns of air that flow across the surface of our landscape, say meteorologists. These changes are expected to be particularly acute this weekend.

But right now something else is happening high above us. A similar pattern of pressure shifts can be seen high in the atmosphere — at 17,000 feet — where winds could roar 60 mph.

They’re aligned. Together, they’ll pack a double whammy, said Clements.

And this coincides with extremely low humidity, according to Daniel Swain, a climate scientist with the Institute of the Environment and Sustainability at UCLA.

“That’s a robust & seriously concerning signal,” he tweeted on Friday morning.

San Francisco and the surrounding Bay Area is experiencing an unseasonal heat wave, with temperatures this week reaching the 90s. On Wednesday, Los Angeles had its hottest day of the year: 98 degrees.

Fifteen of California’s 20 most destructive wildfires have been associated with these strong wind events, including last year’s Camp Fire, the Tubbs Fire in 2017 and 1991’s Tunnel Fire in the East Bay hills, according to Warren Blier, the science officer for the National Weather Service forecast office in Monterey.

California has winds all year long, of course. But our windstypicallyblowfromthe west, off the Pacific, and are laden with moisture so any fires that start in the hills can be quickly controlled, with rare loss of homes.

Autumn winds turn everything around.

The winds blow from the east because the jet stream shifts in the fall. There’s a powerful high pressure system sitting over Utah and Colorado’s Great Basin. Meanwhile, right now there’s a very low pressure system off the California coast.

Air pressure naturally moves from high to low, creating wind. This wind blows from the high Great Basin desert over the Sierra Nevada into the dusty Central Valley and through our dry coastal hills.

The air compresses, gaining heat, as it cascades down the Sierra toward our coast, said Null. In Southern California the winds are called the Santa Anas; in the San Francisco Bay Area, they’re the Diablos.

The primary impact will be felt over and downwind of the Coast Range and Diablo Range, from about Lake County in the north to San Benito County in the south, he said. The velocity of the wind can be drastically increasedasitischanneledover ridges and down canyons.

Meteorologists can forecast the power of the Diablo winds by calculating the differences in air pressure between the city of Winnemucca in northern Nevada and San Francisco, said Null.

To make matters worse, there’s a similar pressure gradient in the upper atmosphere, causing winds to blow the same direction, said Clements.

“That makes the event stronger,” he said.