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 19 degrees above zero. On average, Sioux Falls has 14 rainy, 9 sunny, and 8 partly cloudy days in May. The rains are usually heavy. May as well as March, April, and November is considered the most uncomfortable time to visit Sioux Falls.