Thousands of firefighters on Sunday battled raging wildfires across swathes of parched California that forced residents to evacuate and shut miles of highway after claiming the life of a fireman.
Some 6,000 homes were evacuated as 21 large fires burned in the bone-dry western US state, which is in the throes of a historic drought.
A total of 121,000 acres (49,000 hectares of land have burned so far, according to figures provided by the state.
Dry lightning strikes sparked most of the fires, while the causes of others remained unknown. But authorities said they did not suspect foul play.
Thousands of lightning bolts since Thursday caused hundreds of smaller fires throughout the state, but most of the blazes were concentrated in the north, California fire chiefs said.
About 9,000 firefighters backed by helicopters and air tankers were battling the blazes, CAL FIRE spokeswoman Lynnette Round told AFP, with particular concern about the raging Rocky Fire in Lake County, northwest of the capital Sacramento.
"Our firefighters have been working extensively throughout the last two weeks, working hard to contain these fires. The weather has not been so kind," Round said.
"The dry, warm windy conditions then the dry lightning hitting, it just fuels those fires."
The California governor's office said neighboring Nevada and Colorado had also committed 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.
CAL FIRE said the Rocky Fire had grown to 46,000 acres and was only five percent contained. Its cause was still under investigation, the agency said.
Stretches of Highway 20 were closed in both directions, spelling weekend travel misery for motorists.
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."
The National Weather Service issued a red flag warning, cautioning residents about extreme fire conditions caused by dry thunderstorms and gusty winds.
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, California Governor Edmund 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 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 state gripped by a fourth year of record drought.