Planning for the weather is an important factor for making your trip to Grantsville a success, especially when you’re splurging. The hottest months are usually July and August with an average temp of 67°F, while the coldest months are January and February with an average of 32°F. The snowiest months in Grantsville are January, December, February, and May, with each month seeing an average of 31 inches of snowfall.