May in Boulder is the cloudiest month of the year. In May, the weather in Boulder is typically warm. Average daily high temperature in May is about 12 degrees above zero. On average, Boulder has 15 rainy, 8 sunny, 7 partly cloudy, and 1 snowy day in May. The rains are usually heavy. May as well as February, March, June, and July is considered the most uncomfortable time to visit Boulder.