Check out this MODIS image of the central Idaho Mountains from yesterday.
The valleys are choked with smoke. There may be a bit of fog in there as well, but for the most part, its smoke.
Stable cold pools that form in these valleys at night prevent the vertical transport and mixing of the smoke until late in the day this time of year. This is very similar to what happens in Salt Lake during an inversion. As a result, the smoke remains trapped in the valleys.
One can really see just how strong these nocturnal cold pools are by looking at the weather the past few days at Stanley, Idaho. Yesterday they observed a high of 75ºF, but that was preceded by a morning minimum temperature of 26ºF. It got down that low again last night.
A few years ago I was on a camping trip in the Sawtooths in September and it was in the 20s at night and near 80 during the day. We started hiking with every layer we had on, but finished the day in shorts and t-shirts. Such is the fall climate of the central Idaho mountains.