If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Missoula: The hottest months are usually July and August, with an average temperature of 18°C, while the coldest months are December and January, with an average of -4°C. The snowiest months in Missoula are January, December, February and October, with each month seeing an average of 14 cm of snowfall.