Firefighters in California were battling at least 18 wildfires on Tuesday, including a 41-square-mile (106-square-kilometer) blaze in the mountains of Santa Barbara County.
Extreme heat and a longstanding drought in the West has also dried out vegetation that fuels wildfires.
More than 161 million people around the U.S. were under heat alerts Tuesday.
The Lake Fire was only 12% contained, and forecasters warned of a “volatile combination” of high heat, low humidity and northwest winds developing late in the day.
North of Los Angeles, the 2-square-mile (5-square-kilometer) Vista Fire chewed through trees in the San Bernardino National Forest and sent up a huge plume of smoke visible across the region. There was no containment Tuesday.
The blaze prompted officials to issue an advisory of potentially poor air quality in the Victor Valley area of San Bernardino County.
The National Weather Service said Tuesday it was extending the excessive heat warnings across most of the Southwest U.S. through Saturday morning.
AP video shot by Corinne Chin