Planning for the weather is an important factor for making your trip to Rosemont a success, especially when you’re splurging. The hottest months are usually July and August with an average temp of 71°F, while the coldest months are January and February with an average of 30°F. The snowiest months in Rosemont are January, November, February, and December, with each month seeing an average of 5 inches of snowfall.