SAN FRANCISCO (AP) — Firefighters already dealing with extreme heat in Northern California braced Wednesday for the possibility of thunderstorms and strong winds as they tried to conquer several raging wildfires.

Crews fought to re-establish containment lines as the Chips Fire in Plumas National Forest threatened more than 900 homes and prompted voluntary evacuations.

Firefighters struggled to regain ground lost Tuesday as blowing embers helped spread the blaze along its southern edge. Thunderstorms could aid their work or make their jobs even more difficult, fire spokeswoman Alissa Tanner said.

"That's the biggest question," she said. "If the thunderstorms will just be rain and not gusts of winds that will be a real blessing. If not, then it could spread the fire in many different directions."

The blaze has burned 66 square miles and was about 20 percent contained.

It's among the largest of nearly a dozen major wildfires burning across California that some 8,000 firefighters are battling, state fire spokesman Daniel Berlant said.

Fire officials also issued a statewide burning ban that will stay in effect until there's a significant change in the weather or until the end of fire season.

Elsewhere in Northern California, firefighters made significant progress against a wildfire in Lake County, despite dry weather and triple-digit temperatures.

The fire was 70 percent contained and hundreds of evacuees were allowed to return after the blaze burned more than 12 square miles and threatened nearly 500 homes in the Spring Valley community.

The turning point came Tuesday when the more than 1,200 firefighters on the ground and in the air were able to slow its progress just two days after the fire started, Berlant said.

"They did a great job of getting in and building some containment lines even though the fire is still very active," Berlant said. "Their actions helped save hundreds of homes."

Officials estimated the fire could be out as early as Monday, Berlant said.

In Southern California, wildfires threatened dozens of homes after burning through more than 19 square miles of brush in the midst of a brutal heat wave.

In rural northeastern San Diego County, a complex of five wildfires caused by lightning had burned more than 14½ square miles of wilderness and was only 5 percent contained, state fire Capt. Mike Mohler said. He noted an earlier estimate of 15 percent was incorrect.

Evacuation orders were issued for the communities of Ranchita and Santa Fe, covering about 180 homes and 400 residents.

The two largest and most active fires were above the desert floor in an area subject to erratic winds. Forecasts called for a return of monsoonal moisture that could create thunderstorms with even more erratic winds Thursday, Mohler said.

More than 1,400 firefighters and numerous aircraft were on the scene, trying to carve dozens of miles of fireline and douse flames while the dry, relatively calm weather prevailed, Mohler said.

They were hampered by rugged terrain and the distance from roads, he said.

Meanwhile, a 4½-square-mile blaze in the foothills of Riverside County's San Jacinto Mountains threatened 47 homes and was 15 percent contained. It didn't move much overnight, authorities said.

The fire near the community of Aguanga, east of Temecula, had burned four structures, including at least one home.

A resident living in a trailer was seriously burned and a second resident received lesser injuries after the fire broke out Tuesday, authorities said. Two firefighters received minor injuries.

Elsewhere in Southern California, a 273-acre wildfire caused by lightning in Joshua Tree National Park was 60 percent contained.

Military helicopters made water drops on the Jawbone Complex, two fires burning on more than 12,000 acres in rugged Kern County mountains above the Mojave Desert about 80 miles north of Los Angeles.

The Jawbone fires also forced the Bureau of Land Management to temporarily close about 20 miles of the popular Pacific Crest Trail. The trail through three western states between Canada and Mexico draws thousands of hikers every year.

Berlant said fire officials were concerned that wildfire season began earlier than usual in the state.

"We have definitely seen an increase in fires this season in comparison to previous years," Berlant said. "Most of the damaging fires happen in September and October, not during the summer months."

He said other western states have been experiencing early fires as well.

"We're starting to see the same level of activity that's been occurring in Arizona, Colorado and New Mexico," Berlant said. "We're just like the rest of the West; We continue to be hot and dry, just like them."