Firefighters, military planes and National Guard troops are arriving in California from nearby states to help fight two dozen wildfires.
Crews have arrived from Arizona, Montana, Nevada, Texas, Utah, and more are on the way.
Some 200 members of the National Guard have been activated and military planes have been sent.
The fires, ignited by lightning from dry storms in northern and central California, have killed at least six people and destroyed around 700 buildings.
Nearly one million acres have been blackened and smoke has been so thick it could be seen from other states.
The worst of the fires, including the second and third-largest in California since records began, are burning around the San Francisco Bay Area.
Some 200,000 people have been told to leave their homes.
Officials warned that more dry lightning storms are approaching, bringing a risk of further fires.
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California Department of Forestry and Fire Protection said: “Extreme fire behaviour with short and long range spotting are continuing to challenge firefighting efforts.
“Fires continue to make runs in multiple directions and impacting multiple communities.”
One of the largest fires is the LNU Lightning Complex fire, which began as several smaller fires and has burned through 340,000 acres in Napa, Sonoma, Lake, Yolo and Solano counties.
A high-pressure ridge has been taking moisture from a now-dissipated tropical storm off Mexico, creating ideal conditions for thunderstorms in California.
Most of the precipitation evaporates before hitting the ground, leaving dry lightning that has made the fire season more volatile.