May in Boulder is the rainiest and the cloudiest month of the year. In May, the weather in Boulder is typically warm. Average daily high temperature in May is about 13 degrees above zero. On average, Boulder has 16 rainy, 7 sunny, 5 partly cloudy, and 3 cloudy days in May. The rains are usually heavy. May as well as February, March, and April is considered the most uncomfortable time to visit Boulder.