The heat was so extreme that the Bay Area Rapid Transit system was running trains slower than normal to help engineers spot any warped tracks before hitting them.
It also created difficulties for crews fighting a fire that chewed through brush-covered mountains just north of downtown Los Angeles.
Several hundred firefighters worked to contain the blaze, which prompted mandatory or voluntary evacuations for 730 homes in Los Angeles, Burbank and Glendale.
No injuries have been reported, and one home has burned, officials said. At nearly 8 square miles (21 square kilometers), the fire had charred more land than any other in Los Angeles’ history, Mayor Eric Garcetti said.
It burned in the same area of a June wildfire that came perilously close to hillside homes.
Resident Tracy Goldman said flames were about 200 feet from her house — where the earlier fire burned across the street.
“It’s very unsettling,” she said as she watched, already having packed her car in case officials ordered her street to evacuate.
Officials said that if winds do not pick up, they were confident they could confine a blaze that was just 10 percent contained.
Triple-digit highs in Southern California were forecast to drop into the 90s over Labor Day weekend. The warmth extended up the West Coast and into mountain states.
Fire weather warnings were in effect for parts of Wyoming, South Dakota and Montana, where fires spanned more than 850 square miles (2,200 square kilometers).
In the Pacific Northwest, high temperatures and a lack of rain this summer have dried out vegetation that fed on winter snow and springtime rain. Officials warned of wildfire danger as hot, dry, smoky days were forecast across Oregon and Washington over the holiday weekend.
A fire about 80 miles (129 kilometers) southeast of Seattle has burned more than 23 square miles (60 square kilometers) and led to new evacuation notices Saturday. About 3,800 homes were threatened, authorities said.
Dozens of wildfires in Oregon were sending up large plumes of smoke, causing disruptions in holiday travel as roads close and shutting down camping areas.
The weeklong heat wave was generated by high pressure over the West, the National Weather Service said.
Forecasters said more heat could be expected when remnants of Tropical Storm Lidia move north from Mexico’s Baja California during the weekend.
In Northern California, the extreme heat sent Michelle Ogburn to a cooling center set up in Santa Clara’s North Branch Library, one of many that were opened throughout the state.
Ice-water stations were set up and dozens of people, many of them homeless, took shelter Friday.
“I work from home and I live in an old mobile home with no air conditioning and not very good insulation,” said Ogburn, who lives in Sunnyvale. “Today it was very hot and I just couldn’t work.”
Martha Bellisle in Seattle contributed to this report.