A fire burning deep in Montana’s Bob Marshall Wilderness ballooned to more than 4,000 acres Wednesday, making it the state’s largest active wildfire.

The Silvertip Fire was burning at about 7,000 feet along Silvertip Creek in rough terrain between Ibex Mountain and Minaret Peak. The lightning-caused blaze was reported at about 400 acres Tuesday before spreading that afternoon and overnight.

A Type 4 team was assigned to fight the fire. According to a statement from the Flathead National Forest, crews were challenged by the fire’s remote wilderness location.

The Flathead National Forest reported crews were working to protect the Pentagon Cabin, the only structure near the fire. Multiple trail closures were in place.

Southwest of Frenchtown, the Elder 1 Fire grew to 661 acres Wednesday. The North Fork Lost Horse Fire south of Hamilton remained at 105 acres.

Air quality across Montana improved Wednesday as the massive fires burning in Washington and Oregon were less active. Still, particulate levels were considered moderate in the Flathead Valley and Lincoln County.

Critical fire weather remained a concern for the weekend, with a cold front bringing the possibility of dry thunderstorms.

“With warm air, dry fuels and gusty winds, any of these storms have the potential to spark new wildfires,” the Montana DEQ stated in its smoke forecast.