May in Lewistown is the rainiest month of the year. In May, the weather in Lewistown is typically warm. Average daily high temperature in May is about 20 degrees above zero. On average, Lewistown has 19 rainy, 7 sunny, 4 partly cloudy, and 1 cloudy day in May. The rains are usually heavy. May as well as February, March, and December is considered the most uncomfortable time to visit Lewistown.