Three major collections of fires – dozens in all – carved their way through forests, canyons and rural areas in the San Francisco Bay Area and central California.
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 this week 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, 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, was not affected but mayor Justin Cummings urged residents to be prepared to evacuate by filling 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,” Mr Cummings said.
Although temperatures were predicted to ease slightly on Friday, they were also expected to be hot enough that firefighters will not be able to count on cool evening weather aiding them.
Erratic winds could also drive the blazes unpredictably in multiple directions, state fire officials said.
“There’s so much heat in these fires that they create their own wind … and they may blow in any direction, and very erratically,” said Daniel Berlant, an assistant deputy director with the state Department of Forestry and Fire Protection, also known as Cal Fire.
Winds gusting to 20mph over ridge tops could challenge the overnight firefighting efforts in Santa Cruz and San Mateo counties, said the incident commander, Cal Fire assistant chief Billy See.
“This country likes to burn at night, more so than during the day, and that’s because of the wind patterns,” he said.
Nearly 50,000 people were ordered evacuated in those counties.
The ferocity of the fires was astonishing so early in the fire season, which historically has seen the largest and deadliest blazes when gusty Santa Ana winds start blowing over the next two months.
But the death toll has already reached at least five since the majority of blazes started less than a week ago.
Mr Berlant said three civilians have died in Napa County and one in neighbouring Solano County since the fires began.
It was not immediately clear whether the fatalities included a Pacific Gas & Electric utility worker who was found dead on Wednesday in a vehicle in the Vacaville area.
Also, in central California, a pilot on a water-dropping mission in western Fresno County died on 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, including homes, and threatened tens of thousands more.
More than 10,000 firefighters were on the front lines.
Some 3,000 firefighters had arrived in the past 24 hours, along with hundreds of fire engines from neighbouring states, and National Guard troops that were staffing hand crews and flying helicopters, Mr Berlant said.
Some C-130 military aircraft had also been outfitted as air tankers, he said.
More firefighters were sent to battle the complex of fires in Santa Cruz and San Mateo counties but “it’s still not enough”, Mr See said.
“We’re still drastically short for a fire of this size,” he said.
Cal Fire spokesman Dan Olson said there are concerns that some people are trying to organise through social media to create volunteer brigades and fight the fire themselves.
“The dangers out there to their own lives outweigh anything they can accomplish,” he said.
“They’re putting their lives in jeopardy.”
In a last-minute video recorded for the Democratic National Convention, governor Gavin Newsom said “if you are in denial about climate change, come to California”.
