January in San Juan is the rainiest, the cloudiest, and the warmest month of the year. In January, the weather in San Juan is typically warm. Average daily high temperature in January is about 25 degrees above zero. On average, San Juan has 18 rainy, 10 sunny, and 3 partly cloudy days in January. The rains are usually heavy. January as well as February, March, and December is considered the most uncomfortable time to visit San Juan.