The Bear Creek Fire added over 8,000 acres over the past couple days as smoke-filled skies and temperature inversions allowed it to grow steadily under the radar, to an estimated 28,700 acres in the Flathead National Forest.

“What we’ve had is those thermal belts where the fire has been able to burn in the evening with a slow progression south and north in those areas,” said fire spokesman Al Koss, likening the effect to overcast skies holding the heat in during a normally cool night.

Much of the movement was south and east into the Bob Marshall Wilderness, but despite the sizable gains, the fire isn’t threatening any new structures.

The Wilderness Lodge is three to four miles from the flame front, Koss said, with another four to five miles to the Black Bear Cabin.

The Trail Creek Fire, burning to the northeast through similar terrain and vegetation, also added significant acreage, growing to about 11,700 acres from the previous estimate of 9,500 acres.