May in San Juan is the cloudiest month of the year. In May, the weather in San Juan is typically warm. Average daily high temperature in May is about 27 degrees above zero. On average, San Juan has 15 rainy, 8 sunny, 6 partly cloudy, and 2 cloudy days in May. The rains are usually heavy. May as well as July, August, September, and October is considered the most uncomfortable time to visit San Juan.