Thick smoke is beginning to clear from the forests of Northern California as firefighters battling the largest single wildfire in state history braced for a return of fire-friendly weather.

The winds are not expected to reach the ferocious speeds that helped the Dixie Fire grow to a vast size in the US state last week.

But the forecast is nonetheless concerning for firefighters working in unprecedented conditions to protect thousands of homes under threat.

Mark Brunton, of the California Department of Forestry and Fire Protection, said: “The live trees that are out there now have a lower fuel moisture than you would find when you go to a hardware store or a lumber yard and get that piece of lumber that’s kiln dried.

“It’s that dry, so it doesn’t take much for any sort of embers, sparks or small flaming front to get that going.”

Fuelled by strong winds and bone-dry vegetation, the fire incinerated much of Greenville on Wednesday and Thursday last week, destroying 370 homes and structures and threatening nearly 14,000 buildings in the northern Sierra Nevada.

The Dixie Fire, named for the road where it started nearly four weeks ago, grew overnight to an area of 725 square miles on Sunday morning and was just 21% contained, according to CalFire.

It has scorched an area more than twice the size of New York City.

With smoke clearing out on eastern portions of the fire, crews that had been directly attacking the front lines would be forced to retreat and build containment lines farther back, said Dan McKeague, a fire information officer from the US Forest Service.

On the plus side, better visibility should allow planes and helicopters to return to the firefight and make it safer for ground crews to manoeuvre.

“As soon as that air clears, we can fly again,” Mr McKeague said.

Crews have constructed 465 miles of line around the massive blaze, Deputy Incident Commander Chris Waters said. That’s about the distance from the central California city of Chico to Los Angeles. But officials are only confident that about 20% of the line is secure, he said.

“Every bit of that line needs to be constructed, staffed, mopped up and actually put to bed before we can call this fire fully contained,” Mr Waters said.

Erratic winds were predicted again Sunday afternoon. But the weather was expected to settle, starting on Monday.

Damage reports are preliminary because assessment teams cannot get into many areas, officials said.

The blaze became the largest single fire in California’s recorded history, surpassing last year’s Creek Fire in the Central Valley.

It is about half the size of the August Complex, a series of lightning-caused 2020 fires across seven counties that were fought together and that state officials consider California’s largest wildfire overall.

The fire’s cause is under investigation. The Pacific Gas & Electric utility has said it may have been sparked when a tree fell on one of its power lines. A federal judge ordered PG&E on Friday to give details by August 16 about the equipment and vegetation where the fire started.

Cooler temperatures and higher humidity slowed the spread of the fire, and temperatures topped 32C instead of the highs recorded earlier in the week.

But the blaze and its neighbouring fires, within several hundred miles of each other, posed an ongoing threat.

Governor Gavin Newsom surveyed the damage in Greenville Saturday, writing on Twitter that “our hearts ache for this town”.

“These are climate-induced wildfires and we have to acknowledge that we have the capacity in not just the state but in this country to solve this,” Mr Newsom said on CNN.

Heat waves and historic drought tied to climate change have made wildfires harder to fight in the American West.

Scientists have said climate change has made the region much warmer and drier in the past 30 years and will continue to make the weather more extreme and wildfires more frequent and destructive.