May in Sioux Falls is the rainiest month of the year. In May, the weather in Sioux Falls is typically warm. Average daily high temperature in May is about 21 degrees above zero. On average, Sioux Falls has 13 rainy, 9 sunny, 8 partly cloudy, and 1 cloudy day in May. The rains are usually heavy. May as well as February, March, April, and November is considered the most uncomfortable time to visit Sioux Falls.