SANTA ROSA, Calif.— The deadly wildfires devastating Northern California continued to spread across dry hills and vineyards Wednesday, prompting more evacuations from a menacing arc of flames that has killed at least 21 people, destroyed more than 3,500 buildings, and battered the region’s wine-growing industry.
Officials expect the death toll to rise as crews begin to reach heavily burned areas. Hundreds in flame-ravaged Sonoma County remained missing Wednesday, and higher winds coupled with low humidity and parched lands could hamper efforts to contain the fires or create new ones.
‘‘We’re not out of the woods, and we’re not going to be out of the woods for a number of days to come,’’ Cal Fire Chief Ken Pimlott said Wednesday. ‘‘We’re literally looking at explosive vegetation. These fires are burning actively during the day and at night.’’
What makes the fast-moving fires particularly dangerous, Pimlott said, is that they ‘‘aren’t just in the backwoods. . . . These fires are burning in and around developed communities.’’
Nearly two dozen large fires have been raging in the northern part of the state, sending thousands of residents to evacuation centers and burning roughly 170,000 acres — a collective area larger than Chicago. That size is likely to grow.
Pimlott worried that ‘‘several of these fires will merge.’’
‘‘This is a serious, critical, catastrophic event,’’ he said.
The cause of the fires was unknown and is likely to remain so for some time, officials said. ‘‘Trying to speculate on any cause is premature. At this point, it’s way too early to talk about it,’’ Pimlott said. ‘‘Primary efforts are stopping the fire and protecting lives.’’
In Sonoma County, where 11 people have died, officials had ordered a round of evacuations — some announced by deputies ‘‘running toward the fire, banging on doors, getting people out of their houses,’’ said Misti Harris, a Sonoma County Sheriff’s Office spokeswoman.
‘‘It’s rapidly changing, it’s moving quickly, it’s a very fluid situation,’’ she said.
Sonoma Sheriff Rober Giordano said crews had not been able to reach most of the ‘‘hot zones’’ that were immolated in the firestorm. When they begin searching those areas, ‘‘I expect that [death toll] to go up.’’
Late Wednesday morning, 560 people in the county remained unaccounted for, Giordano said. It was unclear if those who were missing had been harmed or were simply unable to reach friends and families, as fires have disabled much of the communication system in the region.
Evacuation zones in Sonoma County will remain off limits, partly to limit the possibility of looting, which has resulted in several arrests. Giordano doubted residents would be allowed to return to their homes this week. ‘‘If you have a place to go, go; you don’t need to be here,’’ he said, adding later: ‘‘I can’t stress this enough. If you’re in an evacuation zone, you cannot come home.’’
Losses are equally grim in Mendocino County, where two fires had merged into one, and the death toll climbed from two to six in 24 hours.
‘‘What’s irking people around here is the national news is only talking about Napa and Sonoma, and we’ve lost just as much here,’’ Alison de Grassi, spokeswoman for the Mendocino County Tourism Commission, told the San Jose Mercury News. ‘‘People have built their lives around these wineries and these ranches, and now they’re gone.’’
High winds that whipped up 22 large wildfires had faded Tuesday, and the humidity increased, assisting an operation that has drawn resources from throughout the state and neighboring Nevada. But the sharp northern wind, known as a diablo, soon returned, allowing only a brief window for firefighters to carve clearings in place to stop the fires from spreading to vulnerable populated areas.
The National Weather Service expected ‘‘red-flag’’ conditions — dry air and wind gusts up to 40 miles per hour — to remain until Thursday in the North Bay Area, which includes Sonoma and Napa counties.
More than 25,000 people have fled from seven counties north of San Francisco, filling dozens of shelters that state officials had hoped to consolidate in coming days to provide more-efficient services. Many left houses carrying nothing, and officials acknowledged it could be weeks before some are able to return to what is left. In Sonoma County, 5,000 people had taken refuge in 36 shelters as of Wednesday morning.
The scope of the damage prompted President Trump on Tuesday to approve federal emergency aid, agreeing to a request by Governor Jerry Brown. The declaration, announced by Vice President Mike Pence during a visit to the state’s Office of Emergency Services, provides immediate funds for debris clearing and supplies for evacuation centers, among other aid.
Brown cautioned that the recovery would be very costly but seemed optimistic when asked Wednesday about the fires’ impact on California’s economy. The wine industry generates more than $55 billion in economic activity in California — and twice as much nationally — each year.
‘‘Overall, California’s economy is very large, about $2.5 trillion. . . . The machinery of the markets grinds on,’’ he said. ‘‘I don’t think you’re going to see a slowdown because of the fires.’’