Firefighters in Lassen National Forest hold back Park Fire burning in Northern California

Firefighters overnight attacked flames near control lines in Lassen National Forest, holding the Park Fire within its perimeter on the northeast portion of the wildfire burning into its 20th day in Northern California, authorities said.

The Park Fire, as of Monday morning, had burned a total of 429,263 acres (670 square miles) with containment at 38%, according to Cal Fire. The fire grew by less than 100 acres overnight but containment remained the same.

The fire has burned more than 376,000 acres in Tehama County, where the active fire continued to burn. Flames burned nearly 53,000 acres in Butte County, where the suspected arson fire started more than two weeks ago. The wildfire has also affected areas in Plumas and Shasta counties.

The Park Fire, burning into its 20th day on Monday, started in Butte County just east of Chico in a suspected act of arson.

Cal Fire said the total acreage burned includes over 113,000 acres in Lassen National Forest, where firefighters overnight held the wildfire within its current perimeter by directly attacking any flames near control lines supported by night-capable aircraft. Firefighters on Monday were planning to aggressively respond to any new growth on the fire’s perimeter as “mop-up” efforts were underway to maintain and improve existing control lines by removing hazardous trees, according to a Cal Fire situational summary.

Conducting mop-up involves removing or extinguishing all burning or smoldering material along containment lines.

The burnout of heavy, dead vegetation and interior islands continued overnight, Cal Fire said, and incremental improvement in the weather will continue this week with slightly cooler temperatures and higher humidity.

A dry and cold weather front was moving into the area of the Park Fire, bringing along wind from the south and veering northwesterly Monday afternoon. Cal Fire expected gusts of 24 to 27 mph, especially over exposed ridges before calmer conditions return overnight with low temperatures from 50 to 60 degrees.

Jan Smoots, a fire behavior analyst for Cal Fire, hearing about atmospheric instability “makes the hairs stand up on the back of my neck.” He said those weather conditions could lead more active and erratic behavior from the Park Fire.

“We got some wind that kick up some spot fires for us possibly, wake up some of that old heat that’s been lying dormant and quite for us over the last few days,” Smoots told firefighters during a Monday morning operational briefing. “Stay engage, make smart decisions and have a productive day.”

Spot fires occur when floating embers drift in the air land on the unburned side of a fire containment line.

The Park Fire remained the fourth-largest wildfire in California recorded history. The third largest was the Mendocino Complex Fire that started in July 2018 and burned 459,123 acres. The August Complex Fire that burned 1.032 million acres in seven Northern California counties is the largest in state history; it killed one person and destroyed 935 structures.

Ronnie Dean Stout II, 42, of Chico is accused of pushing a burning car into a gully in Upper Bidwell on July 24, igniting what would become the Park Fire before emerging from heavy vegetation along the hiking path and joining evacuees fleeing the area, prosecutors have said.

Stout, who has been charged with felony arson, appeared in Butte Superior Court in late July and earlier this month for his arraignment but has not entered a plea. He is scheduled to return to court Aug. 22 for another arraignment hearing.

The Park Fire is California’s largest wildfire caused by arson in recorded history. The wildfire has destroyed 641 structures and damaged 52 others in Butte and Tehama counties, according to Cal Fire. The total includes infrastructure.

In Butte County, the fire destroyed 428 structures and damaged 47 others. Cal Fire said the fire destroyed 213 structures in Tehama County, damaging five others.

No fatalities to civilians or firefighters have been reported in the Park Fire, according to its latest incident update.