Evacuation orders have been given to 13,000 people in California as firefighters struggle to contain some 20 wildfires.
Some 9,000 firefighters worked throughout Monday in steep terrain and rugged conditions, officials said.
The biggest blaze – the so-called Rocky fire north of San Francisco – has already consumed more than 90 square miles (145sq km) of land.
On Monday afternoon the fire jumped a highway that had been containing it.
The BBC’s David Willis reports from Los Angeles that the Rocky fire tripled in size over the weekend, with officials describing its rate of growth as unprecedented.
At least 24 homes were destroyed as the blaze was whipped up by winds that fuelled the flames, our correspondent says.
“I’ve never seen a fire act like this one,” evacuee Vicki Estrella said.
“It’s amazing the way that thing spread. There was smoke 300ft (91m) in the air.”
More than 6,000 properties currently lie in the path of the flames.
Four years of drought in the western US have left the landscape tinder dry. Coupled with unseasonably humid conditions, lightning strikes and gusting winds, this has posed a severe challenge for fire fighters and water-dropping aircraft.
While some progress has been reported in tackling the blazes, partially due to a slight fall in temperatures, there is no rain forecast.
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