Officials said at least 8,000 firefighters and other personnel are battling the inferno, and resources are still pouring in from Oregon, Nevada, Washington and Arizona.
Five major wildland fires have burned more than 39,000 acres in Sonoma and Napa counties, including the Tubbs Fire that has destroyed homes and businesses in Santa Rosa.
Officials in Sonoma County said they've received more than 100 calls about missing people, NBC News reported. More than 1,500 structures have been destroyed. "It is very dynamic".
Officials say trucks are bringing fuel into inaccessible areas and helping fuel emergency vehicles directly from the trucks.
Wildfires raging across broad swaths of northern California have hospitalised hundreds, and charred about 170,000 acres of land, damaged or destroyed at least 3,500 structures and forced almost 20,000 residents to evacuate, local media reports said.
The equipment being used includes 550 fire trucks, 73 helicopters and over 30 planes. Because of the sharp, dry winds and what the fire chief called "smoke inversion", air tankers were forced to play a cautious role in dousing the blazes. By late Wednesday morning, 170,000 acres had been torched.
"I'm optimistic that we will get a lot of people connected", Sonoma county Sheriff Rob Giordano said on Wednesday night. "The devastation is enormous", he said.
"We are now assessing damage as first responders grant permission for our crews to safely access areas". Gusty winds and low humidity were possible again on Thursday.
About 25,000 people have been evacuated in Sonoma County and 5,000 have sought refuge in shelters, the department said.
NPR's Eric Westervelt, reporting from Calistoga, spoke with Colin Curtis, who has been eyeing with concern the rapidly moving fires. "That just shows that no one is safe anywhere around here".
"Understand these fires started after 10 p.m. around midnight on a Sunday night", Pimlott said when reporters asked about this issue Tuesday.
"The spread of the fire and predictions on the wind and seeing what we've seen had happen throughout our neighboring communities, obviously want to absolutely err on the side of caution here", he said.
This photo appeared several times on social media.
"I want to go up and see what's there", he said, "and I don't want to go, you know?"
Governor Jerry Brown declared a state of emergency for northern California's wine-making Napa, Sonoma, and Yuba counties as the blazes raged unchecked and engulfed the region in thick, billowing smoke that drifted into San Francisco and Oakland.
"We have had big fires in the past".
"One woman learned that her home was still standing, but her farm was gone", said de Grassi, a spokeswoman for the Mendocino County Tourism Commission.