The monster blazes have charred more than 100,000 acres (40,470 hectares), authorities said, destroying hundreds of homes and commercial structures and mobilizing thousands of firefighters.
Eight major fires are currently burning but two -- one about 100 miles (160 kilometers) southwest of the state capital of Sacramento and the other about 100 miles to the east -- have been particularly damaging over the last week, with little letup in sight.
Driven by dry conditions, high winds and soaring temperatures, the Butte Fire has burned 65,300 acres and was just 25 percent contained while the Valley Fire had grown to 50,000 acres and was zero percent contained, state fire agency CAL FIRE said late Sunday.
Governor Brown issued "an emergency proclamation for Lake and Napa counties due to the effects of the Valley Fire, which has burned thousands of acres of land and caused the evacuation of residents and damage to highways and other infrastructure," his office said.
The town of Middletown, population 1,300, was particularly devastated by the Valley Fire, according to local daily Santa Rosa Press-Democrat, which said the fire grew from 50 acres to 10,000 over just five hours Saturday -- before quadrupling in size overnight.
"I'm looking in all directions, and all I see is fire," local fire chief Steve Baxman told the paper.
More than a thousand firefighters have been called in to combat the fast-moving blaze, which was tearing through oak-and-grass hillsides dried out from a fourth year of drought.
CAL FIRE said four of those firefighters suffered second-degree burns while battling the blaze.
Throughout Saturday night and Sunday morning, new evacuation orders were issued for nearby communities, including all the way south to the town of Calistoga, at the northern edge of the Napa Valley wine region.
"Thousands of residents have been evacuated from this fire," CAL FIRE spokesman Daniel Berlant told AFP. "The Red Cross has opened emergency shelters to provide evacuated residents a place to go."
In a tweet, Berlant said crews have not had a chance to do a full damage assessment on the Valley Fire, but "we know 100s of structures have been destroyed."
On the other side of California's Central Valley, the Butte Fire has drawn more than 4,500 firefighters to the Sierra Nevada foothills since Wednesday.
More than 80 homes have been destroyed there and some 6,400 other structures are threatened, Berlant told AFP.
Costs soar, air quality drops
Berlant said that CAL FIRE has so far spent more than $212 million, just since July 1, to fight fires under its jurisdiction and that approximately 5,000 firefighters were currently assigned to major blazes.
The costs of containing other fires, such as those ablaze in Sierra Nevada forests, are largely covered by federal agencies and not included in those figures.
The US Forest Service has spent an additional $1.31 billion battling fires and says it is approaching its record, from 2002, of $1.65 billion.
Brown's state of emergency will release additional funds to cover the costs of the firefighting efforts.
National Guard troops have also been called in to help.
After a briefing on the Butte blaze, the governor's emergency services chief said this summer's fires were the most volatile he had ever seen.
"The fires are spreading faster than I have seen in my 30 years," Mark Ghilarducci was quoted as saying by the Sacramento Bee newspaper.
Meanwhile, the state's air quality has also taken a hit from the fires.
"As a result of the Valley Fire, combined with the drought conditions and other wildfires burning in the state, California's air quality has significantly deteriorated," the governor said in his emergency declaration.
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