(Reuters) - More than 2,000 firefighters aided by water-dropping helicopters battled a wind-driven wildfire raging out of control in Northern California on Monday, threatening homes and other structures as thick black smoke drifted across the San Francisco Bay Area.

The so-called County Fire, which broke out on Saturday afternoon in rural Yolo County, west of Sacramento, has already blackened more than 60,000 acres of grass, brush and dense scrub oak and was only 5 percent contained as of Monday evening afternoon.

Scott McLean of the California Department of Forestry and Fire protection said the job of hand crews and bulldozer operators trying to cut containment lines was made more difficult by high winds, which were blowing embers and starting new spot fires.

“Due to the erratic wind behavior it’s heading wherever it wants to head at the moment,” McLean said, adding that about 116 structures faced an imminent threat as of Monday morning.

“It’s our duty to save life and property,” he said.

Crews worked through the night to cut control lines around the blaze, which grew by one-third overnight Sunday, and officials ordered about 300 people to leave their homes near Lake Berryessa in neighboring Napa County as flames bore down.

The smoke reached some 75 miles (120 km) south to San Francisco, leaving films of ash on cars and windows. No casualties have been reported.

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Wildfires have burned through nearly 2.5 million acres (1 million hectares) in the United States as of June 29, well above the average of about 2 million acres for the same calendar period over the last 10 years, according to the National Interagency Fire Center.

The agency forecast significantly above-normal large fire for July across the U.S. West.

In Colorado, firefighters were hampered by adverse weather conditions to combat flames in six major wildfires that have charred more than 100,000 acres across the state, said Larry Helmerick, spokesman for the Rocky Mountain Coordination Center.

“We’ve got hot, dry and windy conditions,” Helmerick said. “Large parts of the Rocky Mountain region are in a prolonged drought, and Colorado is especially hard hit.”

The largest blaze, the Spring Fire in southern Colorado, has burned nearly 57,000 acres, destroyed an unknown number of structures, and has forced the evacuations of hundreds of residents, fire officials said.

The fire, which was human caused, is just 5 percent contained, according to InciWeb, a federal government wildfire website.