The Napa Valley town of Calistoga faced one of the biggest threats, and its 5,000-plus residents were ordered to leave their homes as winds picked up and fire crept closer.

Winds of up to 60 miles per house (100 kilometres per hour) and humidity of just 10 per cent will create “critical fire weather conditions” and “contribute to extreme fire behaviour” on Friday afternoon and into Saturday, the National Weather Service said.

A force of 8,000 firefighters was working to reinforce and extend buffer lines across the region where the flames have scorched more than 190,000 acres (77,000 hectares), an area nearly the size of New York City.

With 3,500 homes and businesses incinerated, the so-called North Bay fires have reduced entire neighbourhoods in the city of Santa Rosa to smouldering ruins dotted with charred trees and burned-out cars. California State Parks said it was offering campsites to displaced residents from the affected counties.

At a fairground converted to a shelter in the nearby city of Petaluma, about 250 cots were full by Friday, and people slept in tents in the parking lot as volunteers served porridge and eggs for breakfast.

Twenty-eight year-old Yasmin Gonzalez, her four children and husband, a grape picker, were anxious to leave the shelter and return to their flat in Sonoma.

“It’s horrible to leave your home, and your things and not know what’s going to happen,” Gonzalez said on Friday.

Calistoga Mayor Chris Canning said anyone refusing to heed the mandatory evacuation would be left to fend for themselves if fire approached, warning on Thursday: “You are on your own.”

The cause of the blaze was under investigation, but officials said Pacific Gas and Electric Company power lines toppled by gale-force winds on Sunday may be to blame.

The California Public Utilities Commission on Thursday directed PG&E to preserve all evidence including “all failed poles, conductors and associated equipment from each fire event” in Napa, Sonoma and Solano counties, according to a letter sent to PG&E official Meredith Allen.

The electric company must also tell employees and contractors to preserve all emails and documents related to potential causes of fire, maintenance and tree-trimming.

Sonoma County accounted for 17 of the North Bay fatalities, all from the Tubbs fire, which now ranks as California’s deadliest single wildfire since 2003.

Some people killed were asleep when flames engulfed their homes, fire officials said. Others had only minutes to escape as winds fanned fast-moving blazes.

Mark Ghilarducci, state director of emergency services, said the loss of cell towers likely contributed to difficulties in warning residents.

As many as 900 missing-person reports have been filed in Sonoma County and 437 people have since turned up safe. It remains unclear how many of the 463 still unaccounted for are fire victims rather than evacuees who failed to alert authorities, Ghilarducci said.

The year’s wildfire season is one of the worst in the history of the United States, with nearly 8.6 million acres (3.5 million hectares) burned so far, just behind 2012, according to the National Interagency Fire Centre. In the worst year, 2015, about 9.3 million acres burned.

The fires struck the heart of California’s world-renowned wine-producing region, wreaking havoc on its tourism industry and damaging or destroying at least 13 Napa Valley wineries.

The state’s newly legalised marijuana industry was also hit hard, with at least 20 pot farms in Sonoma, Mendocino and Napa counties ravaged, a growers’ association said.