If you're hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Keystone: The hottest months are usually July and August, with an average temperature of 12°C, while the coldest months are December and January, with an average of -9°C. The snowiest months in Keystone are March, February, January and December, with each month seeing an average of 82 cm of snowfall.