Thousands of firefighters have been battling raging wildfires throughout drought-parched California, where officials have evacuated entire neighbourhoods and closed off miles of highway in the path of the inferno.
At least one firefighter has been killed so far in the Rocky Fire.
Some 6000 homes were evacuated as 21 large fires began burning throughout the western US state, which is currently in the throes of a historic drought.
Governor Jerry Brown's office said that some 9600 people were battling the fire.
Dry lightning strikes are believed to have sparked most of the fires, but the causes of others remained unknown and are still being investigated.
But authorities said they did not suspect foul play.
Wildfires are a fact of life in much of California, but were far worse than usual this year because of the bone-dry conditions, with the state gripped by a fourth year of record drought.
Witnesses described dramatic scenes as thick smoke from the giant fire turned day into night and flames swallowed up large tracts of woodland and forest.
Vehicles had been left to the flames, abandoned and burnt-out.
On Friday, Mr Brown declared a state of emergency and the California National Guard was called in, underlining the scale of the threat facing the state.
Firefighter Dave Ruhl, 38, from South Dakota, was killed while fighting the Frog Fire in the Modoc National Forest outside Alturas.
"California's severe drought and extreme weather have turned much of the state into a tinderbox," Mr Brown said in a statement.
"Our courageous firefighters are on the front lines and we'll do everything we can to help them."
Mr Brown added in a separate statement: "Firefighter Ruhl will be remembered for his service and bravery and we extend our deepest condolences to his family, friends and colleagues with the US Forest Service."
California Department of Forestry and Fire Protection (CAL FIRE) spokeswoman Lynnette Round said weather had not been kind to firefighters who had been "working extensively throughout the last two weeks".
"The dry, warm windy conditions then the dry lightning hitting, it just fuels those fires," she said.
Mr Brown's office said neighbouring Nevada and Colorado had also contributed resources to battle the blazes.
A total of 24 homes and 26 outbuildings, which included barns and sheds, were destroyed in the community of Lower Lake, home to the picturesque Mendocino National Forest.
With very hot, dry weather and erratic winds expected only to make matters worse, evacuations in the face of the Rocky Fire were expected to impact 12,190 people, CAL FIRE said.
"Mountain thunderstorms will continue through today with little rain which could spark fire starts from dry lightning strikes," the agency said in a statement.
"Evening thunderstorms will shift northward late in the night, putting northern California at risk for new lightning fire starts."