Thousands of people in California are under orders to evacuate in regions surrounding the San Francisco Bay Area as nearly 40 wildfires blazed across the US state amid a blistering heatwave now in its second week.
Smoke blanketed the city of San Francisco.
"Throughout the state of California right now, we are stretched thin for crews" because of the fires, said Will Powers, a state fire spokesman.
"Air resources have been stretched thin throughout the whole state."
Police and firefighters went door-to-door early on Wednesday in a frantic scramble to warn residents to evacuate as fire encroached on Vacaville, a city of about 100,000 that lies between San Francisco and Sacramento.
Fire officials said at least 50 structures were destroyed and 50 damaged, and four people were injured.
Television footage showed some homes in flames and thick ash dropping in a rural area near Interstate 80 as the fire appeared to head towards more densely populated areas.
Diane Bustos told KPIX-TV that she and her husband tried to drive out but their vehicle caught fire and they had to flee on foot.
"I got all these flames on me and I lost my shoe, but I made it. God saved me," she said.
The California Department of Forestry and Fire Protection said the blaze was exhibiting "extreme fire behaviour" and challenging firefighters.
To the south in San Mateo and Santa Cruz counties, about 22,000 people were ordered to evacuate because of a fire that was burning in dense wooded parkland but threatened communities, Cal Fire spokesman Jonathan Cox said.
"This is a very active timber fire burning in two counties with a serious threat to both public safety and for structures that are out in front of it," he said.
Thousands of homes and businesses were also threatened in the wine-growing counties of Napa and Sonoma in an area devastated by a series of deadly blazes in the last three years.
At least seven fires were grouped together as one of two major lightning fires in northern California, a nod to their origins just a day earlier.
Mr Powers said much of the fire was burning through rural areas with steep terrain, making it difficult to get crews in.
In Napa County, Gail Bickett, 80, loaded up her three dogs in a truck to evacuate as the fire burned behind houses across the road, the San Francisco Chronicle reported.
"It's scary," she said. "It's overwhelming."
Blazes engulfed rural and forest areas near the San Francisco Bay Area, near Salinas in Monterey County, around Oroville Dam north of Sacramento, forested areas west of Silicon Valley, in remote Mendocino County and near the Nevada state line north of Lake Tahoe.
Several were also burning in northern coastline areas and in southern California.
Governor Gavin Newsom ordered a statewide state of emergency on Tuesday.
The cluster of wine country fires threaten an area that only last year grappled with another massive blaze that forced 200,000 to flee – a task made more complicated this year because of the pandemic.
On Tuesday afternoon, the fire was moving towards an area called Atlas Peak that burned in 2017 in a blaze that killed six people and destroyed nearly 800 buildings.
Robin Sisemore hosed down vegetation in front of her mother's adjacent house.
Both homes were new, replacing ones that burned just three years ago.
To the south, evacuations were ordered for all of Boulder Creek to the west of Silicon Valley, a community of 5,000 high in the Santa Cruz mountains where windy, long, forested roads, some paved, some dirt, can easily become blocked during storms or fires.