May in West Yellowstone is the rainiest month of the year. In May, the weather in West Yellowstone is typically cold. Average daily high temperature in May is about 8 degrees above zero. On average, West Yellowstone has 24 rainy, 3 sunny, 2 cloudy, and 2 partly cloudy days in May. The rains are usually heavy. May as well as April is considered the most uncomfortable time to visit West Yellowstone.