There were varying reports on damage. Cal Fire said in a 7 a.m. update that the blaze, burning in an area where PG&E had shut off power Wednesday afternoon due to the onset of gusty winds and plummeting humidity, had damaged two structures. But social media posts from journalists in the area showed residences that had been destroyed in the fire. Parkes said at a noon press conference that an undetermined number of structures have been destroyed, but crews could not yet determine if they were residences or outbuildings.

Beverly Hansel, who evacuated her Alexander Valley home, said she heard news of the fire at about 10 p.m. while watching the news. She said she lost her home, which she shared with her daughter and daughter's boyfriend, to the fire.

"And then as soon as we started packing our stuff, we realized we had to get out now," Hansel said. "We didn't grab any of it. Some of it we got, but not very much. We just have a little vehicle. And we watched the rest of it burn down. It’s heartbreaking. It’s heartbreaking."

Hansel said she wants to leave Sonoma County, because she’s sick of wildfires after losing her home Wednesday night and after being evacuated two years ago, referring to the devastating 2017 North Bay fires that killed 44 and damaged or destroyed 21,000 homes.

Cal Fire responded by rushing bulldozers and strike teams to the area and ordering crews to be held on duty as forces were marshaled to try to slow the spread of the blaze. After first light Thursday, the agency began deploying air tankers to drop fire retardant in an effort to protect structures and slow the fire's spread.

Parkes said approximately 500 personnel from several agencies are battling the blaze.

The Sonoma County Sheriff's Office ordered evacuations for:

All of the town of Geyserville

Pine Flat Road

Geysers Road to Highway 128

Red Winery Road

Alexander Mountain Road

Highway 128 from Geysers Road to River Road including, the River Rock Casino

All roads off River Road

Three evacuation centers were set up:

Healdsburg Community Center, 1557 Healdsburg Ave.

Santa Rosa Veterans Memorial Building, 1351 Maple Ave.

Sonoma County Fairgrounds, 1350 Bennett Valley Rd.

An evacuation center was initially established at Windsor High School, but the location was shut down early Thursday as the school day began and evacuees were sent to the Healdsburg site.

Healdsburg Mayor David Hagele said city officials are doing their best to provide support to residents who fled their homes.

"What we have been trying to do here in Healdsburg is provide a welcoming spot for people that are displaced or that have to evacuate. You can come here and we will do our best to take care of you," Hagele said.

The fire reportedly started near an electrical generating facility near Kincade Road, about 13 miles east of the town of Cloverdale. No cause had been determined by early Thursday.

That area, known as The Geysers, is the site of numerous geothermal generating sites. Calpine, perhaps the biggest operator in the area, said in an email that the blaze had "flashed" through a portion of its facilities late Wednesday.

"We believe there is relatively minor damage to our facilities and further threat has passed," the Calpine statement said. The company said that it had shut down its power lines in the area before the blaze started, "consistent with our fire prevention protocols."

"We do not believe our facilities caused the fire," Calpine said. "There are power lines operated by third parties across The Geysers."