September in Knoxville is the sunniest month of the year. In September, the weather in Knoxville is typically warm. Average daily high temperature in September is about 26 degrees above zero. On average, Knoxville has 19 sunny, 8 partly cloudy, and 3 rainy days in September. The rains are usually light. September as well as October is considered the most comfortable time to visit Knoxville.