The Rim Fire burns through trees near Yosemite National Park, Calif., on Tuesday, Aug. 27, 2013. Firefighters gained some ground Tuesday against the huge wildfire burning forest lands in the western Sierra Nevada, including parts of Yosemite National Park. (AP Photo/Jae C. Hong)
Unnaturally long intervals between wildfires and years of drought primed the Sierra Nevada for the explosive conflagration burning the rugged landscape on the edge of Yosemite National Park, forestry experts say.
The fire had ravaged 288 square miles (745 sq. kilometers) by Tuesday night, making it the biggest fire in the Sierra's recorded history and one of the largest on record in California.
Containment held steady at 20 percent, but the number of destroyed structures rose to 111, and some 4,500 structures remained threatened. At least 31 residences were among those lost. Firefighters were making stands at Tuolumne City and other mountain communities.
Officials said they expected the blaze to continue to grow overnight.
"We're going to get a handle on it, but we're going to have to give some ground to get some," said Lee Bentley, fire spokesman for the U.S. Forest Service.
The blaze was just 40 acres (16 hectares) when it was discovered near a road in Stanislaus National Forest on Aug. 17, but firefighters had no chance of stopping it in the early days.
Fueled by thick forest floor vegetation in steep river canyons, it exploded to 105,620 acres (42,743 hectares) within the next two days. On its 11th day, it had surpassed 179,400 acres (72,602 hectares), becoming the seventh-largest California wildfire in records dating to 1932.
Federal forest ecologists say that historic policies of fire suppression to protect Sierra timber interests left a century's worth of fuel in the fire's path.
"That's called making the woodpile bigger," said Hugh Safford, an ecologist with the U.S. Forest Service in California.
Two years of drought and a constant slow warming across the Sierra Nevada also worked to turn the Rim Fire into an inferno. For years forest ecologists have warned that Western wildfires will only get worse.