May in Granby is the rainiest month of the year. In May, the weather in Granby is typically cold. Average daily high temperature in May is about 7 degrees above zero. On average, Granby has 19 rainy, 6 sunny, 3 cloudy, and 3 partly cloudy days in May. The rains are usually heavy. May as well as April is considered the most uncomfortable time to visit Granby.