The best times to visit Thompson Falls are late spring (mid-May to mid-June) and early autumn (September to mid-October). In late spring, the river is running high and cold from snowmelt, the cottonwoods are leafing out, and the Cabinet Mountains still have snow on the high ridges — the valley is green and the days are long. Autumn brings clear skies, cooler temperatures, good fishing conditions, and elk activity in the forests north of town.
Midsummer (July and August) is perfectly fine — warm and dry, with long days that suit outdoor recreation — but fire smoke from wildfires in the region is increasingly common and can settle into the valleys for days at a time. The Clark Fork corridor is particularly susceptible to smoke trapping due to the narrow canyon geography. Check air quality forecasts if you're planning extended time outdoors.
Winter is cold and quiet. US-2 stays open year-round, but the Cabinet Mountains roads can be snowy and the wilderness trails are buried. Thompson Falls does see some ice fishing on the reservoir in deep winter, but services in town are minimal and some businesses close or operate reduced hours from November through March. The town's small population means this isn't a ski destination — the closest downhill skiing is at Turner Mountain near Libby, about 90 minutes northwest.