Sky-darkening wildfires that took at least six lives and forced tens of thousands of people from their homes blazed throughout California on Friday as firefighting resources strained under the vastness of the infernos authorities were trying to control.

Three major complexes encompassing dozens of fires chewed through a combined 780 square miles (2,020 square kilometers) of forests, canyons and rural areas flanking San Francisco on three sides.

Statewide, nearly 12,000 firefighters are battling blazes that have scorched more than 1,200 square miles (3,120 square kilometers) in California, said Daniel Berlant, assistant deputy director for the state Department of Forestry and Fire Protection, known as Cal Fire.

Crews from Oregon, Idaho and Arizona have arrived to relieve local firefighters, he said, with engines on their way from as far away as Maryland and New Jersey.

Tens of thousands of homes were threatened by flames that drove through dense and bone-dry trees and brush. Many of the fires were sparked by lightning strikes from brief thunderstorms — nearly 12,000 since last weekend — as a high-pressure area over the West brought a dangerous mix of triple-digit weather and monsoonal moisture pulled from the south.

Some fires doubled in size within 24 hours, fire officials said.

And while some evacuations were lifted in the small city of Vacaville, between San Francisco and Sacramento, other areas expanded their evacuation areas. The University of California, Santa Cruz, was evacuated, and a new fire burning near Yosemite National Park also prompted evacuations.

Santa Cruz itself, a coastal city of 65,000, wasn’t affected. But Mayor Justin Cummings urged residents Thursday evening to be prepared to evacuate by gassing up their vehicles and packing important documents, medicines and other belongings.

“Prepare early so that you are ready to go at a moment’s notice,” Cummings said.

More than 64,000 people have been ordered to evacuate in San Mateo and Santa Cruz counties, which make up part of Silicon Valley and hug the coast south of San Francisco.

With firefighting resources tight, homes in remote, hard-to-get-to places burned unattended. Cal Fire Chief Mark Brunton pleaded with evacuees to quit battling fires on their own, saying that just causes more problems for professionals.

“We had last night three separate rescues that pulled our vital, very few resources away,” he said.

An anxious Rachel Stratman, 35, and her husband, Quentin Lareau, 40, waited for word Friday about their home in the Forest Springs community of Boulder Creek after evacuating earlier this week. She knows one house has burned but has received conflicting information about the rest of the neighborhood.

“It’s so hard to wait and not know,” she said. “I’m still torn if I want people to be going back to the area and videotaping. I know they cause the firefighters distraction, but that’s the only way we know.”

The couple are in a San Jose hotel with medication she needs after undergoing a transplant surgery last month. She collected her mother’s ashes and some clothes while her husband closed windows and readied the home before they evacuated Tuesday.

“I kept looking at things and kept thinking I should grab this or that, but I just told myself I needed to leave. I didn’t bring any official documents and I didn’t bring my house deed or car title. No passport,” she said.

The ferocity of the fires was astonishing so early in the fire season, which historically has seen the largest and deadliest blazes when gusty, dry winds blow in the fall.

But the death toll already had reached at least six since the majority of blazes started less than a week ago, with four deaths claimed by fires burning in wine country north of San Francisco.

The bodies of three people were found in a home that burned in Napa, Henry Wofford, spokesman for the Napa County Sheriff’s Office, told the San Francisco Chronicle. In Solano County Sheriff Thomas A. Ferrara reported the death of a male resident there.

Separately, a Pacific Gas & Electric utility worker was found dead Wednesday in a vehicle in the Vacaville area Wednesday. In central California, a pilot on a water-dropping mission in western Fresno County died Wednesday morning when his helicopter crashed.

At least two other people were missing and more than 30 civilians and firefighters have been injured, authorities said.

Smoke and ash billowing from the fires also fouled the air throughout California’s scenic central coast and in San Francisco. The fires have destroyed at least 175 buildings.

Tim and Anne Roberts had gone to the beach with their two children on Monday to avoid the smoke at their home in Boulder Creek in Santa Cruz County. They packed a change of clothes, their children’s school supplies and their passports — just in case.

They learned Wednesday that their house had burned. Birth certificates, legal documents and family heirlooms are gone. But in photos of the ruins, they were surprised by how many redwoods, oaks and fruit trees were still standing.

“It’s a strange sort of comfort,” Tim Roberts said.

The good news for Brookdale resident Larissa Eisenstein Thursday afternoon was that her five chickens, Kelly and The Nuggets, had been safely relocated into a stranger’s yard in a safer, neighboring community.

The chicken evacuation came a day after Eisenstein, a Silicon Valley tech worker, had been forced to leave them behind during an overnight evacuation. She fled with her cats Mochi and Mini, driving from one hotel to the next only to find they were full before landing in a safe place where they could get some rest.

The bad news Thursday was that the fire was burning down her wooded street as she adjusted to the idea that her worldly possessions may now be limited to photos of her parents, some jewelry she had grabbed, and fresh tomatoes from her garden.

“After I got the cats, I realized there was very little important to me, and the priority is to try to remember how lovely things can be,” she said. “I’ve had a wonderful garden this year.”

Although temperatures were predicted to ease slightly on Friday, they were also expected to be hot enough so that firefighters will not be able to count on cool evening weather aiding them. Erratic winds also could drive the fires unpredictably in multiple directions, state fire officials said.

Winds gusting to 20 mph (32 kph) over ridge tops could challenge overnight firefighting efforts in Santa Cruz and San Mateo counties, said the incident commander, Cal Fire Assistant Chief Billy See. More firefighters were sent to battle the complex of fires, but “it’s still not enough,” See said.

“We’re still drastically short for a fire of this size,” he said.