If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in North San Juan: The hottest months are usually July and August with an average temp of 71°F, while the coldest months are December and February with an average of 43°F. The rainiest months in North San Juan are December, March, February, and January, with each month seeing an average of 11 inches of rainfall.