September in Plymouth is the cloudiest and the hottest month of the year. In September, the weather in Plymouth is typically hot. Average daily high temperature in September is about 28 degrees above zero. On average, Plymouth has 21 rainy, 4 sunny, 4 partly cloudy, and 1 cloudy day in September. The rains are usually heavy. September as well as July, August, October, and November is considered the most uncomfortable time to visit Plymouth.