If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in San Isidro: The hottest months are usually March and April with an average temp of 69°F, while the coldest months are January and February with an average of 68°F. The rainiest months in San Isidro are October, June, September, and August, with each month seeing an average of 22 inches of rainfall.