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 Mission: The hottest months are usually August and July, with an average temperature of 17°C, while the coldest months are February and December, with an average of 3°C. The snowiest months in Mission are January, March, February and December, with each month seeing an average of 15 cm of snowfall.