Weather is probably a major factor when you plan your trip to St. Charles, and keep 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 temp of 63°F, while the coldest months are January and December with an average of 22°F. The snowiest months in St. Charles are April, January, February, and December, with each month seeing an average of 17 inches of snowfall.