An 87,000-acre wildfire burned along a milelong front Sunday, threatening about 60 homes, and the number of firefighters assigned to the blaze nearly doubled.

A voluntary evacuation order remained in effect for the threatened homes near Summer Lake, and several homes in the Silver Lake area also were in danger, said David Widmark, a spokesman at the Northwest Interagency Coordination Center.

Thunderstorms were forecast in the area of south-central Oregon with a likelihood of lightning but no rain, said meteorologist Paul Werth at the interagency center.

Any new fires started by lightning could draw crews off the state's major blazes, Widmark said.

"What happens with the weather will tell us what the rest of the week will look like for fire behavior," Widmark said.

Officials dispatched an additional 800 firefighters to the blaze Sunday, bringing the total force to about 2,000 people, and National Guard troops are expected to arrive Monday.

The blaze was about 15 percent contained Sunday, Widmark said.

In addition to stiff wind and high temperatures, firefighters have faced a threat of electrical arcs as dense smoke creates short circuits between the ground and Bonneville Power Administration power lines in the area.

Blue bolts of electricity from the power lines already have killed several cows and firefighters won't go near the lines.

Two fires joined on Saturday to form the huge blaze on rolling hills between Summer Lake and Silver Lake, producing one leading edge about a mile long.

Altogether, about 25 major fires active Sunday in Oregon had burned a total of 216,000 acres, officials said.

Elsewhere, a 350-acre wildfire damaged or destroyed 10 homes Sunday in Lake Isabella, Calif., and forced the evacuation of an undetermined number of residents, officials said.

About 75 firefighters and three helicopters are working to save homes in the area. Lake Isabella is about 150 miles northeast of Los Angeles.

In north-central Washington, a wildfire near the north shore of Lake Chelan had burned about 17,000 acres, little changed from Saturday, officials said. It was about 10 percent contained.

A voluntary evacuation order remained in effect for nearly 300 homes near Lake Chelan. Several outbuildings had been destroyed but no homes had burned.

In Colorado, a fire near Rocky Mountain National Park had burned 5,000 acres and residents had been evacuated from about 225 homes in a subdivision near Lyons, 45 miles northwest of Denver. Occupants of an additional 400 to 500 houses were told to be ready to leave.

The Colorado blaze was 5 percent contained. It was 4 to 4.5 miles from Estes Park, one of the gateways to the national park.

A heavy air tanker was used to help firefighters Sunday. On Thursday, a PB4Y-2, a converted World War II-era bomber, crashed near Lyons while fighting the blaze and both crew members were killed.