Weather is probably a major factor when you plan your trip to St. Charles, but bear in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually July and August, with an average temperature of 17°C, while the coldest months are January and December, with an average of -6°C. The snowiest months in St. Charles are April, January, February and December, with each month seeing an average of 44 cm of snowfall.