Authorities told people in the Bitterroot Valley to leave their homes Monday as a late-season wildfire fanned by gusty winds quickly spread.

In Ravalli County, the Sawtooth Fire grew overnight to 1,300 acres and was threatening about 350 homes spread out over a 10-mile area west of Hamilton, fire information officer Greg DeNitto said.

Residents west of Westside Road, south of Paradise Trail and Blodgett Camp Road, and north of North Gold Creek Loop were told to leave Monday afternoon. Law enforcement officers went door to door and set up roadblocks around the area, though the evacuations were not mandatory, DeNitto said.

“If they choose to stay they will not be removed, but they also have to recognize that they may not necessarily have any support,” he said.

The Red Cross was opening a shelter for evacuees on U.S. 93 on Hamilton’s south side. A public meeting updating residents of crews’ progress was to be held at 6 p.m. Monday.

In Eastern Montana, a fire that started Monday morning about 15 miles east of Roundup quickly spread to a square mile and at one point threatened a rural subdivision. 

Authorities say they  stopped the forward movement of the fire later Monday.

Musselshell County Commissioner Larry Leske says burned within a couple of miles of the Delphia Pines subdivision Monday before a containment line halted its advance.

The National Weather Service issued red-flag warnings for much of Western Montana through late Monday as a dry cold front pushed south from Canada, bringing wind gusts of up to 50 mph. At least 23 fires were burning across the state as of Monday, according to the Northern Rockies Coordination Center, as the wildfire season pushed toward fall with no end in sight.

Smoke from the Sawtooth Fire was thick, obscuring the mountain where it was burning and dropping ash into the fire camp, DeNitto said. 

Combined with smoke from fires burning in Idaho, the haze created air conditions in Hamilton that state officials described as very unhealthy. 

The air in Missoula, Butte and Great Falls was listed as unhealthy, and the quality was just slightly better in Helena and Seeley Lake.

Sarah Coefield, an air quality specialist for the Missoula City-County Health Department, said that with air quality as bad as it is in Hamilton, smokers, children, the elderly and people with lung disease should avoid outdoor activity, and all people in the area should avoid prolonged exertion outdoors.

About 20 miles south of Bozeman, fire crews prepared to keep the winds from pushing the 16-square-mile Millie Fire into the municipal watershed and threatening the water supply. A report prepared Sunday for the Northern Rockies center said modeling shows a “high probability” the blaze would move into the watershed. 

Fire information officer John Zapell said the threat of the fire spreading into the watershed has existed for a couple of weeks, and he did not know how much of Monday’s front was expected to increase that threat.

The fire is now on the watershed’s southeastern corner. Crews in helicopters were dropping water and retardant to keep it from spreading to the point that debris from the blaze overwhelms the city’s treatment plant.

“It if gets burning heavily, that will wash down to the treatment plant. They have equipment that can handle some, but if they get large debris flows, that’s the concern,” Zapell said.

Rick Moroney, the superintendent for Bozeman’s water treatment plant, said if the fire spreads to the area, the impacts to the plant won’t be seen until the first big rainfall or snow. 

The city has three sources for its water, and if one is temporarily lost, the other two would still fill the needs of the city outside of irrigation season, he said.

“If the fire were to do catastrophic damage to one of the other water sheds, we would still be able to rely on the other two,” Moroney said.