Other facts from our historical weather data: Which is the hottest month in Río San Juan? September has an average maximum temperature of 31°C (88°F) and is the hottest month of the year. Which is the coolest month in Río San Juan? The coolest month is February with an average maximum temperature of 27°C (81°F). Which is the wettest month in Río San Juan? November tops the wettest month list with 149mm (5.9 in) of rainfall. Which is the driest month in Río San Juan? February is the driest month with 83mm (3.3 in) of precipitation. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.