If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in St. Charles: The hottest months are usually July and August with an average temp of 70°F, while the coldest months are January and February with an average of 29°F. The snowiest months in St. Charles are January, November, February, and October, with each month seeing an average of 7 inches of snowfall.