September in Rapid City is the sunniest month of the year. In September, the weather in Rapid City is typically warm. Average daily high temperature in September is about 21 degrees above zero. On average, Rapid City has 21 sunny, 5 rainy, and 4 partly cloudy days in September. The rains are usually moderate. September as well as {{month_nom: February}}, and {{month_nom: February}} is considered the most comfortable time to visit Rapid City.