There had been “thousands” of lightning strikes since Thursday night, California fire chiefs said, sparking several hundred small wildfires.
At least 23 large fires were burning menacingly across California but many were clustered in the north of the bone-dry US state, which is in the crippling throes of a historic drought.
About 9,000 firefighters backed by helicopters and air tankers were battling the blazes, CAL FIRE spokesman Daniel Berlant said, with particular concern about the Rocky Fire in Lake County, northwest of Sacramento.
Witnesses described apocalyptic 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.
Berlant, writing on Twitter, warned that the Rocky Fire had grown to 25,750 acres (10,000 hectares) “& continues to burn north towards Hwy 20. Still 5% contained.”
Long stretches of Highway 20 were closed in both direction, spelling weekend travel misery for motorists.
Dozens of homes and outbuildings in Lake County, home to the picturesque Mendocino National Forest, had been destroyed and an estimated 6,000 more structures were under grave threat, fire chiefs said.
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 12,100 people, CAL FIRE said.
On Friday, California Governor Edmund Brown declared a state of emergency and the California National Guard were called in, underlining the scale of the threat facing the state.
Firefighter Dave Ruhl, 38, from South Dakota, was killed Thursday 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,” Brown said in a statement.
“Our courageous firefighters are on the front lines and we’ll do everything we can to help them.”
“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,” Brown said in a separate statement.
Wildfires are a fact of life in much of California, but were far worse than usual this year because of bone-dry conditions, with the western US state gripped by a fourth year of record drought.