The largest blaze has forced hundreds of people to evacuate and destroyed two dozen homes. Experts have predicted a long and volatile summer wildfire season in California in its fourth year of crippling drought.
Firefighter David Ruhl, 38, a married father of two from Rapid City, South Dakota, died on Thursday while on assignment at the Frog Fire in the Modoc National Forest near California's border with Oregon.
No one else was with Ruhl when he was killed, and he was the incident commander on the fire at the time, said Modoc National Forest spokesman Ken Sandusky. It is common for a leader on a fire to travel alone, Sandusky said, but he declined to release more details on the death.
The Frog Fire, which broke out on Thursday and is about 5 percent contained, has grown to 3,000 acres (1,214 hectares) and erratic winds have pushed it in all directions, according to the U.S. Forest Service's InciWeb online fire information center.
A red-flag warning, designating the threat of gusty winds that risk fanning flames, was expected to remain in effect until late Sunday in the area of the Frog Fire. InciWeb said the flames are not expected to be fully under control until Aug. 12.
The Rocky Fire, in the foothills and canyons on the inland flanks of California's northern coastal range in Lake County north of San Francisco, has also ballooned in size and now covers 25,750 acres (10,420 hectares).
Nearly 2,000 firefighters are working on the fire, which broke out on Wednesday and is only 5 percent contained, according to a state fire information website.
"It's making some good runs right now,” said California Department of Forestry and Fire Protection (Cal Fire) spokesman Captain Ron Oatman.
In particular, fingers of the blaze were advancing on Highway 20 where firefighters built containment lines to hold back flames, he said.
The Rocky Fire has destroyed 24 homes and 26 more outbuildings, and it threatens an additional 6,156 structures, according to Cal Fire.
More than 12,000 residents have either been ordered to evacuate or are living under evacuation advisories, according to Cal Fire.