FOREST RANCH, CA — Firefighters made progress and were aided by improving weather Sunday in the battle against wildfires covering massive areas and leaving a trail of damage in the western United States, but further evacuations and resources have been necessary as thousands of personnel tackle the flames.

The so-called Park Fire, the largest wildfire in California this year, was one of more than 100 blazes burning in the U.S. on Sunday, according to the National Interagency Fire Center. 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.

The Park Fire had scorched an area greater than the city of Los Angeles as of Sunday, darkening the sky with smoke and engaging thousands of firefighters. The blaze spanned more than 562 square miles (1,455 square kilometers) of inland Northern California.

Millions of people were under air quality alerts Sunday in the northwestern U.S. and western Canada.

The Park Fire started Wednesday, when authorities say a man pushed a burning car into a gully in Chico and then fled. A man accused of setting the fire was arrested Thursday and is due in court Monday.

Firefighters increased containment to 12% on Saturday, aided by cooler temperatures and more humidity, officials 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.

A “red flag” warning was in effect for the region on Sunday, meaning dry fuels and stronger winds were increasing the fire danger, the weather service said.

The National Weather Service also issued a “red flag” warnings Sunday for wide swaths of Idaho, Montana, Utah, Colorado and Wyoming, in addition to parts of California.

Fires burned across eastern Oregon and eastern Idaho, where officials were assessing damage from a group of blazes called the Gwen Fire, which was estimated at 43 square miles (111 square kilometers) as of Sunday.

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 around midday.

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

The focus on saving lives and endangered property has shifted to confronting the blaze head-on, Jay Tracy, a Park Fire headquarters spokesperson, told The Associated Press by phone Sunday.

Nearly 4,000 firefighters are battling the fire, aided by numerous helicopters and air tankers. Reinforcements are expected to give much-needed rest to local firefighters, some of whom have been working nonstop since Wednesday, Tracy said.

“This fire is surprising a lot of people with its explosive growth,” he said. “It is kind of unparalleled.”

The Park Fire has drawn comparisons to the 2018 Camp Fire that tore through Paradise, killing 85 people and torching 11,000 homes.

Cohasset exhibited remnants of the devastation Sunday. Mailboxes and vehicles were covered with pink fire retardant dropped by aircraft. The husks of a washer and dryer set were surrounded by burned debris and a charred motorcycle was propped upright, balancing on rims after its tires apparently melted away.

Another part of Cohasset was relatively unscathed, said Garrett Sjolund, the Butte County fire chief.

“We have an unburned island in that community that we are continuing to patrol and ensure that there are no hot spots in it,” Sjolund said.

Managing evacuation orders in the area has been complex. Authorities were about to downgrade an order to an evacuation warning for Forest Ranch when they learned a number of hot spots were reported nearby, Butte County Sheriff Kory Honea said.

“That illustrates how rapidly things can change,” Honea said. “We were all set to be able to reduce that order to get people back in there.”

In Southern California, about 2,000 people were ordered to evacuate because of a fire sweeping through the Sequoia National Forest. The wind-driven blaze was fed by dried, dead plants and moving fast, eating up more than 60 square miles (155 square kilometers) in four days, Andrew Freeborn of the Kern County Fire Department said.

No fatalities have been reported in the Park and Borel fires, but some people were increasing the danger for everyone by disregarding evacuation orders, Freeborn said.

“When people are trying to ignore the orders and later call for rescue, that takes firefighters away from the task of fighting the fires,” he said. “This fire is moving at a pace and with such intensity that individuals should not be thinking they can wait until the last minute. They need to get out of the way.”

The historic mining town of Havilah and several other communities were “heavily impacted” by the fires, but it was too soon to count the burned homes, Freeborn said.

The Park Fire has destroyed at least 66 structures and damaged five others, Tracy said. Authorities initially believed 134 structures were lost, based on drone footage, but lowered the number after assessing the damage in-person, while acknowledging the figure could increase.

“Each day that number has potential to grow. Our teams obviously don’t do damage inspections when there is active fire in an area,” Tracy said.