September in South Plainfield is the sunniest month of the year. In September, the weather in South Plainfield is typically warm. Average daily high temperature in September is about 23 degrees above zero. On average, South Plainfield has 15 sunny, 8 rainy, and 7 partly cloudy days in September. The rains are usually heavy. September as well as August is considered the most comfortable time to visit South Plainfield.