Fire crews have shifted their focus to the Southwest Idaho side of a wildfire that has expanded rapidly the past seven days.

The Big Grass fire, which was sparked by lightning on July 23, continued its rapid expansion across Owyhee County, Idaho, and Malheur County, Oregon, growing from 358,911 acres to 579,366 acres. High winds earlier this week caused the fire to spread by more than 22,000 acres from Monday to Tuesday, most notably around the Battle Creek area, according to a news release from the Northwest Incident Management Team 10.

Firefighting aircraft responded by “hammer(ing) this area with water and retardant,” the management team said.

The intensive suppression efforts led to a successful day, with the fire growing by just 5 acres on Wednesday.

The ongoing fire response includes 989 personnel, 33 dozers, 69 engines, 33 water tenders and six helicopters, according to the management team.

Officials said no homes had been lost as of Thursday, but 159 people were evacuated, and 315 residential and 420 nonresidential structures were under threat.

“There’s a lot of people working really, really hard,” Stephanie Spencer, Northwest Incident Management Team 10 public information officer, told the Idaho Statesman in a phone call. “They’re out there in the heat and the dust, and they’re trying to get as much of this fire contained as quickly as possible.”

The safety of Duck Valley Indian Reservation became a particular concern this week after a finger of the fire stretched onto reservation land and began threatening structures there, according to the management team. Spencer said crews succeeded in protecting those structures and were continuing to secure the area.

“Really heavy winds caused the fire to push out a little bit across a dozer line that was in place and move out into the corner of the reservation,” Spencer said. “We got right on it with a lot of aircraft. There were a lot of very large air tankers, scoopers, helicopters. They dumped a lot of loads of retardant and water, and just tried to keep the fire in check.”

The National Weather Service issued a weather warning that incoming thunderstorms and lightning could pose further fire risk to the area southwest of Boise on Thursday.

With the exception of South Mountain, fire efforts have been successful at combatting the northern section of the Big Grass Fire, near where it began in Oregon. The northern end of the fire is “really solid” and “hasn’t had any activity in a couple of days,” Spencer said.

But the wildfire has continued to push further into Idaho, prompting fire management to shift resources from Jordan Valley to an established fire base at the Murphy Airport, according to Spencer. This shift will eliminate long helicopter flight times and allow for faster aircraft response.

“The bulk of the support was here in Jordan Valley, so we’re moving a lot of those support systems down to Owyhee (County) to better support those folks on the southern end,” Spencer said.

To “ensure the safety of the public and firefighters,” the Bureau of Land Management announced a temporary closure on Monday of the entire area south of Trout Creek Road and Silver City Road from the Oregon state line east to Highway 78 and Highway 51, to the Idaho-Nevada line in Owyhee County.

The agency said the closure was expected to last until 11:59 p.m. Aug. 20.

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