The Tamarack Fire crossed into Nevada, growing about 10,000 acres and prompting new evacuations.
The fire south of Lake Tahoe has burned 50,129 acres of timber and head-high chaparral in national forest land, and is 4% contained, according to Humboldt-Toiyabe National Forest.
The Tamarack Fire erupted July 4 and was one of nearly two dozen blazes sparked by lightning strikes.
More than 1,200 firefighters were battling the Alpine County blaze, which has destroyed at least 10 buildings, forced evacuations in several communities and had closed parts of U.S. 395 in Nevada and California.
Fire officials expected active or extreme fire behavior on Thursday, which could see 14 mph winds and temperatures approaching 90 degrees Fahrenheit.