Bad-news weather forecast for Wine Country: High winds returning

Firefighters work to save structures on Soda Canyon Road in Napa, Calif., on Monday, Oct. 9, 2017. Firefighters work to save structures on Soda Canyon Road in Napa, Calif., on Monday, Oct. 9, 2017. Photo: Noah Berger, Special To The Chronicle Photo: Noah Berger, Special To The Chronicle Image 1 of / 80 Caption Close Bad-news weather forecast for Wine Country: High winds returning 1 / 80 Back to Gallery

The offshore gales that propelled massive blazes in Wine Country will return Wednesday night and Thursday, increasing the risk of fires in the North Bay, according to the National Weather Service.

At least 17 people were killed and up to 3,000 structures were leveled after the fires erupted late Sunday, tearing through northern Santa Rosa, the Wine Country and elsewhere in the North State.

ROBERTA GONZALES: What causes the 'Diablo Winds' that spread the explosive fires?

And the ravaged areas are still vulnerable. Forecasters issued a red flag warning for the North Bay hills from 5 p.m. Wednesday to 5 p.m. Thursday, and for the East Bay hills from 11 p.m. Wednesday to 5 p.m. Thursday.

A cold front is bringing winds expected to hit 30 mph or stronger, with gusts up to 45 mph, especially in the hills. The winds will be coming from the northeast rather than the typical onshore pattern off the ocean.

The weather service warned that any fires that ignite will escalate rapidly, and asked people to refrain from outdoor burning. The blustery winds could topple power lines and touch off new fires, the weather service warned.

Winds that hit 70 mph on the higher peaks Sunday night combined with parched air to spread flames across thousands of acres in Napa, Sonoma and other counties. Last year’s rains created a bumper crop of grass and brush after five years of drought, turning acres of land into kindling.

“Fire literally exploded and raced along the landscape,” the weather service said in a bulletin published Monday.

It’s unusual to have such extreme fire weather at night, said UC Berkeley Professor Scott Stephens, who specializes in fire science. Temperatures typically drop after sunset. That makes the land more moist, because cold air holds less water.

But that has changed somewhat in California over the past 15 years, Stephens said. Overnight lows are rising, so the air stays drier after dusk. One result, he said, is that night fires are much more common.

On Sunday night, the humidity hovered around 15 percent in the North Bay, Stephens said — far lower than the 50 to 60 percent norm.

High pressure in the Great Basin and low pressure on the coast created a gradient that drove strong Diablo winds from inland areas, similar to the ones that stoked the Oakland hills fire in 1991, said Neil Lareau, an assistant professor of meteorology at San Jose State University.

With the Tubbs Fire, gusts drove the flames from near Calistoga to Santa Rosa, a distance of 12 miles, in about three hours. Hundreds of homes were destroyed in Santa Rosa’s Coffey Park and Fountaingrove neighborhoods.

Photo: John Blanchard, San Francisco Chronicle Conditions that were too right: In the early hours of Monday...

“When embers land on people’s roofs, they can ignite leaves in the gutters” and set the house ablaze, Lareau said. That, in turn, sends more embers wafting through the wind.

Smoke and particulates from the Wine Country fires billowed for miles on Monday and Tuesday, prompting the Bay Area Air Quality Management District to issue advisories for residents throughout the region.

The San Francisco Unified School District kept students indoors during lunch, recess and P.E., and Mayor Ed Lee encouraged residents to seek respite in four of the city’s public libraries — Civic Center, Chinatown, Mission Bay and Glen Park — which have air filtration systems.

The air district will probably send out advisories again Wednesday, said district spokesman Tom Flannigan.

San Francisco Chronicle

staff writer Evan Sernoffsky contributed to this report.