Thick smoke from the Park Fire raging northeast of Chico is expected to increase humidity and slightly cool active parts of the Northern California wildfire by late Wednesday afternoon.

As of Wednesday morning, the Park Fire had burned a total of 420,827 acres (657 square miles) of land, and the wildfire was still 34% contained, according to Cal Fire. The fire has burned more than 367,000 acres in Tehama County and nearly 53,000 acres in Butte County. The wildfire has also affected areas in Plumas and Shasta counties.

The wildfire remained active overnight due to poor moisture recovery, resulting in uphill elevations and intense fire behavior, according to a Cal Fire situation summary Wednesday morning. Cal Fire officials believe the fire remains actively burning and is spreading north and east, climbing slopes with critical dry vegetation.

Cal Fire Chief of Operations Billy See said at a briefing Wednesday morning that he was concerned about what would happen as smoke cleared and humidity dropped above the flames in areas.

“The lid is on the thing now, it’s heating up,” See told firefighters preparing for the day shift. “As soon as we get a breath of fresh air, the thing is going to want to get up and move.”

He urged firefighters to be aware of their surroundings and fire behavior, to retreat if necessary for their own safety and to re-fight the fire at the right time.

Cal Fire says ground crews continue to battle high heat and deep drainage and are fighting the fire aggressively where it is safe. Authorities expect a weather system to move into the area later this week, bringing higher humidity.

The Park Fire was the fourth-largest wildfire in California history. The Complex Fire in August, which burned 1.032 million acres in seven Northern California counties, is the largest in state history; it killed one person and destroyed 935 structures.

Cal Fire Chief of Operations Mark Brunton urged firefighters to monitor the situation closely and not become complacent, but said they must continue to “stay aggressive and participate in the fight.”

“This is unprecedented fire behavior,” Brunton said at the press conference on Wednesday morning. “The fact that this fire has become one of the five most devastating fires in California history says a lot. And you can see that out here.”

The Park Fire has destroyed 636 structures and damaged 49 others in Butte and Tehama counties, according to updated numbers from Cal Fire. The total includes infrastructure.

In Butte County, the fire destroyed 428 buildings and damaged 47 others. Cal Fire said the fire destroyed 212 buildings and damaged five others in Tehama County.

The total area burned is over 107,000 acres in the Lassen National Forest. Cal Fire officials said firefighters fought the fire overnight directly where the flames crossed Highway 172, and that ground crews and aircraft continued to aggressively attack the fire and keep it under control, with safety as their top priority.

On Wednesday, the Park Fire had been raging for 14 days after it was suspected to have been started by arson in Butte County east of Chico.

Ronnie Dean Stout II, 42, of Chico is accused of pushing a burning car into a ravine in Upper Bidwell on July 24, igniting what would later become the park fire, before emerging from thick vegetation along the trail and joining evacuees leaving the area, prosecutors said.