Bracing for winds, authorities order thousands to flee California wine country fire

California firefighters raced against time on Saturday as they cut defensive lines against a wildfire in Sonoma County's famed wine country and authorities ordered 50,000 people to evacuate, ahead of winds that are forecast to pick up at night and spread the flames.

The 25,000-acre Kincade Fire in rugged and steep terrain north of San Francisco was one of two major wildfires burning in California, with the other, the Tick Fire, raging in suburban Los Angeles where it has charred 4,600 acres.

The more than 2,000 firefighters battling the Kincade Fire, which officials said was 10 per cent contained and had destroyed nearly 50 structures, faced a more immediate threat from weather than crews battling the Tick Fire.

The National Weather Service has issued a "red flag" warning for areas around the Kincade Fire, with low-humidity drying out plants and trees and leaving the flames with plenty of combustible fuel. And conditions were only expected to worsen.

The cause of the Kincade Fire is still under investigation, but officials have said it erupted on Wednesday near the base of a damaged high-voltage transmission tower owned by the troubled utility Pacific Gas and Electric Corporation.

Earlier in the week, authorities had ordered 2,000 people to evacuate their homes, with Sonoma County Sheriff's Office spokeswoman Karen Hancock describing the flames as having come closest to the community of Geyserville.

On Saturday, authorities expanded evacuation orders in the nearby communities of Windsor and Healdsburg, with more than 50,000 people told to leave home by 4 pm local time when power was expected to be shut off in those areas.