NORTHERN CALIFORNIA — 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 miles per hour in the North Bay hills, 35 miles per hour in the Santa Cruz Mountains and 23 to 30 miles per hour 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 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 miles per hour.
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 UC-Los Angeles.
“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, 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 winds typically blow from the 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. They blow from the east, often in the early morning, when fire is least expected. In southern California’s they’re called the Santa Anas; in the San Francisco Bay Area, they’re the Diablos.
That’s because the jet stream shifts in the autumn. Its powerful air currents dip down from Canada and the Pacific Northwest and create a high pressure system over Utah and Colorado’s Great Basin.
Meanwhile, there’s a low pressure system sitting off the California coast. (That’s different than in the summer, when a high pressure ridge is common.)
This weekend, especially, our coast will experience strong and short wave “troughs” of low pressure air.
Here’s what happens: Air always wants to flow from high pressure to lower pressure. So fierce hot north easterly winds cascade from the Great Basin’s deserts down the western slopes of the Sierra Mountains — into the dusty Central Valley and through our dry coastal hills.
The air compresses – so is heated — as it sinks from the higher elevation Great Basin to the lower elevation California coast, said Null.
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 increased as it is channeled over ridges and down canyons.
Meteorologists can forecast the power of the Diablo Winds by calculating the “pressure gradient” – the difference in pressure — between the city of Winnemucca in northern Nevada and San Francisco, said Null.
Meanwhile, 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.
The story behind the naming of “Diablo Winds”
The phrase “Diablo Winds” dates back to the 1991 Tunnel Fire, which devastated a large area of the Oakland and Berkeley hills. Weather forecaster Jan Null and colleague John Quadros, both working at the National Weather Service’s Redwood City office, decided that calling these wind events “the northern California version of Santa Ana winds” was awkward and meteorologically fuzzy.
“We somehow fell upon the name Diablo Winds as a double entendre — because they generally blow from the direction of Mt. Diablo in the far East Bay, and “diablo” translates from Spanish as “devil”; thus, devil winds,” said Null.
Wind and California’s Worst Fires
Fires linked to strong Diablo winds: Camp (2017), Tubbs (2017), Tunnel (1991), Nunns (2017), Jones (1999), Atlas (2017), city of Berkeley (1923) and Redwood Valley (2017).
Fires linked to strong Santa Ana winds: Cedar (2003), Witch (2007), Woolsey (2018) and Thomas (2017)
Source: Warren Blier, National Weather Service in Monterey.
