The Horse Creek fire south of Hysham exploded from 3 to more than 30 square miles overnight. It was the largest of more than a dozen new fires sparked by weekend lightning strikes.
Residents along the Horse Creek and Sarpy Creek basins in Treasure County were evacuated, said Paula Short with the state Department of Natural Resources and Conservation. A natural gas pipeline and a transmission line that pass through the remote, hilly area also were threatened by the blaze, officials said.
Short said the area where the fire is advancing is sparsely populated.
A brief period of cooler weather Monday was expected give way by Tuesday to a return of hotter and drier conditions.
With the Fourth of July approaching, a special federal fire prevention team arrived in Montana as part of a stepped-up effort to keep man-made fires from adding to the danger.
"It's so volatile out there. If there are winds, the fires can quickly get out of hand," said Melodie Lloyd with the Bureau of Land Management. "We always this time of year beat people over the head with `Be careful. Be careful.' We really mean it this year."
To the southeast, the 290-square mile Ash Creek fire jumped Highway 212 west of Broadus early Monday. The Powder River County Sheriff's Office was evacuating Sonnette and Ten Mile Creek roads.
Because the fire was burning in thick timber, firefighters were staying away from the forward edge of the blaze and instead concentrating on trying to contain it along the edges, said Ash Creek fire spokeswoman Kathy Bushnell. The fire was reported at 40 percent contained Monday morning, with 16 residences and 22 outbuildings burned so far.
In Musselshell County, authorities say the perimeter lines around the 22,000-acre Dahl fire held overnight despite heavy wind and thunderstorms. That fire has burned 73 structures and 150 outbuildings, and is 75 percent contained.
A voluntary evacuation order for the Dahl fire was lifted Monday morning.