CASTAIC, Calif. >> California firefighters worked in extreme conditions today as they battled wildfires in rural areas north of Los Angeles and east of San Diego amid a blistering heat wave that is predicted to last through Labor Day.

Progress was made in containing both blazes but authorities warned that the explosive fire behavior that occurred after they erupted Wednesday showed the potential for what could happen during the prolonged torrid conditions.

“The days ahead are going to be challenging,” said Angeles National Forest Fire Chief Robert Garcia, one of the commanders of the battle against the Route Fire near the Interstate 5 community of Castaic in northwestern Los Angeles County.

The Route Fire was 12% contained after scorching more than 8 square miles (21 square kilometers) and destroying one house. Traffic on the major north-south interstate, a key route for big rigs, was jammed due to lane closures.

Temperatures in the area hit 107 degrees (42 Celsius) on Wednesday.

Seven firefighters had to be taken to hospitals for treatment of heat injuries, said Los Angeles County Fire Department Deputy Chief Thomas Ewald.

All were released, Ewald said, adding that he expected more heat emergencies to occur.

Temperatures in much of California were so high that Gov. Gavin Newsom declared a state of emergency and the state power grid operator asked residents to voluntarily reduce use of electricity during critical afternoon and evening hours.

After strong work by ground crews and helicopters and airplanes dropping water and fire retardant on the Route Fire, authorities planned to lift evacuation orders for a mobile home park and other homes, Ewald said.

“The big thing today is all about boxing the fire in,” he said.

In eastern San Diego County, the Border 32 Fire was 5% contained after swiftly growing to more than 6 square miles (15.5 square kilometers), the California Department of Forestry and Fire Protection said.

The fire burned at least four buildings, including a house, and prompted evacuations for some 400 homes in the Dulzura area near the U.S.-Mexico border. State routes in the area were closed and a local school district called off Thursday’s classes.

There were “multiple close calls” as residents rushed to flee, said Cal Fire Capt. Thomas Shoots.

“We had multiple 911 calls from folks unable to evacuate” because their homes were surrounded by the fire, Shoots told the San Diego Union-Tribune.

U.S. Customs and Border Protection closed the Tecate port of entry with Mexico hours early Wednesday night because of the fire and said it wouldn’t reopen until conditions improved. Travelers could still use the 24-hour Otay Mesa crossing.

Wildfires have sprung up this summer throughout the Western states. The largest and deadliest blaze in California so far this year erupted in July in Siskyou County. It killed four people and destroyed much of the small community of Klamath River.

Scientists say climate change has made the West warmer and drier over the last three decades and will continue to make weather more extreme and wildfires more frequent and destructive.