FOREST RANCH >> Firefighters made progress and were helped by improving weather over the weekend in the battle against wildfires covering massive areas in the western United States, but further evacuations have been necessary as thousands of personnel tackle the flames.

The Park Fire, the largest wildfire in California this year and the sixth largest in the state’s recorded history, was one of more than 100 large active wildfires burning in the U.S. on Monday. The man arrested on suspicion of starting the fire by pushing a burning car into a gully made his first appearance in court Monday and was charged with felony arson of an inhabited structure or property.

The Park Fire had scorched more than 575 square miles, an area greater than the city of Los Angeles, as of Monday, according to CAL Fire. It destroyed more than 100 structures and threatened 4,200 more.

Firefighters increased containment to 12% on Saturday, aided by cooler temperatures and more humidity, officials said. Containment remained at 12% on Monday.

Evacuation orders were in effect on 25 wildfires, according to the National Interagency Fire Center. More than 27,000 wildland firefighters and support personnel are assigned to wildfires that have burned more than 3,200 square miles nationwide, the center said.

Some were sparked by the weather, with climate change increasing the frequency of lightning strikes as the western U.S. endures blistering heat and bone-dry conditions.

In Cohasset, a small forest community north of where the Park Fire started, Ron Ward described on Monday the “nail-biter” experience of saving the buildings on his family’s ranch, which had lost insurance coverage about a month earlier. He and his son Ethan installed a fire protection system — a water line to a pond and sprinklers.

The fire approached with flames hundreds of feet high, Ward said.

“It hit our sprinklers and kind of died down and then went around our property and missed, missed all of our structures,” he said.

Although cooler-than-average temperatures are expected through the middle of this week, that doesn’t mean existing fires will disappear, said Marc Chenard, a meteorologist at the National Weather Service’s Weather Prediction Center in College Park, Maryland.

The service issued “red flag” warnings Monday for wide swaths of Idaho, Montana, Utah and Wyoming, in addition to parts of California, meaning dry fuels and stronger winds were increasing the fire danger, the weather service said.

Air quality alerts were also issued for Monday in the northwestern U.S. and western Canada.

Officials were assessing damage across eastern Oregon and eastern Idaho from a group of blazes called the Gwen Fire, which was estimated at 43 square miles, as of Sunday.

In Oregon, the Grant County Sheriff’s Office identified a single-engine air tanker pilot who died last week while fighting the Falls Fire as James Bailey Maxwell, 74.

Maxwell’s air tanker disappeared late Thursday while he was fighting the fire burning near the town of Seneca and the Malheur National Forest. Search and rescue crews found Maxwell and his plane the following morning.

The Falls Fire was roughly 222 square miles and nearly 70% contained Monday. Fire managers hope to have it fully contained by Tuesday.

In California, Paradise and several other Butte County communities were under an evacuation warning Sunday. Yet the fire’s southernmost front, which is closest to Paradise, was “looking really good,” Cal Fire operations section chief Jeremy Pierce said.

Officials did not expect it to move farther into Chico, a city of about 100,000 people just west of Paradise, and crews plan to extinguish hot spots and remove hazards over the next few days, Pierce said.