Video report by ITV News Washington Correspondent Robert Moore
One of the victims was found in a car apparently attempting to flee the fires, California Highway Patrol Officer Ben Draper told reporters.
Hundreds, if not thousands, of homes and other buildings are believed to have been damaged or destroyed by the blaze northeast of San Francisco, fire officials said.
The fire has also threatened Paradise, a town devastated just two years ago by the deadliest blaze in state history that prompted a traffic jam as panicked residents tried to escape.
Other wildfires charred huge swathes of the west of the US amid gusty, dry conditions, with Washington, Oregon and Idaho also impacted.
A blast of polar air helped slow wildfires in Colorado and Montana.
Since the middle of August, fires in California have killed 11 people, destroyed more than 3,600 structures, burned old growth redwoods, charred shrubland and forced evacuations in communities near the coast, in wine country and along the Sierra Nevada.
Thick smoke choked much of the state and cast an eerie orange hue across the sky on Wednesday as thousands of people in communities near Oroville were ordered to evacuate.
Daniel Swain, a climate scientist at the University of California, Los Angeles, conservatively estimated the fire had burned about 400 square miles in 24 hours.
“The unbelievable rates of spread now being observed on these fires — it is historically unprecedented,” he tweeted.
The US Forest Service, which had taken the unprecedented measure of closing eight national forests in Southern California earlier in the week, ordered all 18 of its forests in the state closed Wednesday for public safety.
In Paradise, where 85 people lost their lives and nearly 19,000 buildings were destroyed, the sky turned from black to cherry red and ash carried on strong winds rained down in a scene reminiscent of the fateful morning of November 8 2018, former Mayor Steve “Woody” Culleton said.
“It was extremely frightening and ugly,” Mr Culleton said.
“Everybody has PTSD and what not, so it triggered everybody and caused terror and panic.”
In Southern California, fires burned in Los Angeles, San Bernardino and San Diego counties.
People in foothill communities east of Los Angeles were warned to be ready to flee, but the region’s notorious Santa Ana winds were weaker than predicted.
“We’re encouraged that the wind activity appears to be dying down,” Governor Gavin Newsom said. “The rest of the week looks a little more favorable.”
California has set a record with nearly 2.5 million acres burned already this year, and historically the worst of the wildfire season does not begin until the autumn.