By Laila Kearney
CLAYTON, Calif., Sept 9 (Reuters) - A wildfire sweeping over a mountainside park east of San Francisco doubled in size for second straight day on Monday, and officials said they were concerned about increasing winds and the steep terrain hampering their ability to fight the flames.
The blaze, burning in dense, dry scrub, grass and timber in and around Mount Diablo State Park, had scorched some 3,700 acres (1,500 hectares) by Monday afternoon, forcing the evacuation of about 100 homes at the edge of the town of Clayton.
A force of 500 firefighters, backed by about a dozen bulldozers, helicopters dropping water and tanker planes dumping chemical flame retardant, had carved containment lines around 20 percent of the blaze by mid-afternoon on Monday.
Authorities went door-to-door Sunday afternoon ordering residents of about 75 homes on the outskirts of Clayton to flee, and later another 25 dwellings were ordered evacuated.
Fire officials said there were no immediate plans for further evacuations, but they could revisit that decision based on the behavior of the flames.
Winds began to pick up late on Monday afternoon, Contra Costa County Fire Department spokesman Robert Marshall said, triggering concerns that the fire could again spread rapidly toward inhabited areas.
"We expect the fire to grow some more today, but I don't know how much," Marshall said.
Fire officials said the blaze also was threatening three communications towers and a historic stone building that serves as a visitors' center on the summit of Mount Diablo, which rises 3,800 feet (1,158 meters) above sea level.
The state park, about 20 miles (32 km) inland from the eastern edge of San Francisco Bay, has been closed to the public due to the fire.
Flames burned beneath some electrical transmission lines on mountain overnight, fire spokesman Dennis Rein said.
But the utility company Pacific Gas and Electric said those lines were undamaged. However, about 30 customers on the mountain briefly lost power, possibly due to damage to smaller distribution lines, a PG&E spokeswoman said.
The blaze pales in comparison with the huge Rim fire still burning in and around Yosemite National Park, roughly 200 miles (322 km) to the east.
That fire, believed to have been sparked by a hunter's campfire that grew out of control, has blackened more than 253,000 acres, or 395 square miles (102,000 hectares), of timber and dry brush since it erupted on Aug. 17 in the Stanislaus National Forest west of Yosemite.
It ranks as the third-largest California wildfire on record and the biggest of dozens of blazes that have raged across several states in the drought-parched west this year (Reporting by Laila Kearney; Additional reporting and writing by Steve Gorman; Editing by Dan Whitcomb, Cynthia Osterman and Jackie Frank)