The power outages began to spread across the region shortly before dusk on Thursday, around 4:30 p.m., PG&E spokeswoman Brandi Ehlers said. In Sonoma County, some of the hardest hit cities were Santa Rosa, with 21,655 outages, Sonoma, with 19,110, and Guerneville, with 4,233.

PG&E called in extra workers to respond to the blackouts. Ehlers said 86 crews were working around the clock to repair downed lines and power poles. By Friday evening, power had been restored to all but 1,985 homes in Sonoma County, she said. That included more than 1,000 homes in Santa Rosa and 236 in Glen Ellen.

Fallen tree limbs and downed power lines closed several major roads in Sonoma County to traffic on Friday morning, including Arnold Drive, between Leveroni Road and Petaluma Avenue, and Trinity Road, about a mile from Highway 12.

The strength of the storm was evident when daylight broke at Santa Rosa Junior College. An iconic oak tree, which stood just behind the school's brick sign on Mendocino Avenue, snapped near its 4.5-foot diameter base.

"These are really important trees," said Carl Dobson, an arborist and the junior college's manager of grounds and recycling. "There is so much history these trees have seen."

The tree was among the 10 oldest trees at SRJC and had stood more than a century before the 115-acre campus was established on land once used for experiments by famed horticulturist Luther Burbank. The tree grew in a "heritage tree protection zone" on campus.

"It is a symbol," Dobson said of the campus' 1,000 oak trees, 150 of which are heritage age. "We really value our trees here."

It was unclear Friday morning what would become of the massive limbs and trunk of the tree. Dobson said school officials would likely transplant an oak sapling to the spot where the heritage tree once stood.

In Kenwood, a large oak tree fell and crushed a building used as an office behind a home on Jessie Street, according to Kenwood Fire Department Capt. Darren Bellach.

The oak crashed into the unoccupied building just after 9 a.m. and destroyed the structure, he said. There was additional damage to items inside the office.

High winds also raked Lake and Mendocino counties, where 18,000 residents lost power, trees were felled, roofs blown away and burn piles were fanned into flames that threatened homes.

In Mendocino County, the rural Potter Valley community was hardest hit.

"The damage is very extensive," said Potter Valley Assistant Fire Chief Dan Thornton.

Wind downed dozens of giant oak trees, one of which fell into a house, trapping its residents inside, he said.

"We had to help them out," Thornton said. Luckily, no one was hurt, he said.

The wind tore the tin roofs from most of the valley's barns and ripped off 20 percent of the shingles at Potter Valley Elementary School, Thornton said. The school also lost windows and portable buildings to flying debris and fallen trees, he said.

"There was debris flying everywhere last night," Thornton said. "If you had a little portable carport or trampoline, those are on your neighbor's property."

The wind gusts caused four separate fires to erupt from smoldering burn piles, he said. One burned 10 acres of grass and threatened structures before it was brought under control.

A 45-acre fire was reported just north of Hopland on Friday but it was 95 percent contained by afternoon, according to CalFire. The cause has yet to be determined.

In Lake County, Clearlake suffered a majority of the damage. More than 7,700 customers were without power at some point Thursday and Friday, said PG&E spokeswoman Ehlers. By Friday afternoon, more than 7,000 had their power restored, she said.

More than 1,000 customers in Lakeport; 1,600 in Nice; almost 600 in Upper Lake; and more than 500 in Kelseyville remained without power Friday afternoon, Ehlers said.

In Mendocino County, 730 homes in Willits and 241 in in the Ukiah Valley remained without power, she said. All of Potter Valley remained in the dark, Thornton said.

Other communities suffered between 15 and 60 outages.

There were 148 separate outages in Lake and Mendocino counties, she said.

Staff Writers Jamie Hansen and Mary Callahan contributed to this report.