September in St. John's is the hottest month of the year. In September, the weather in St. John's is typically hot. Average daily high temperature in September is about 29 degrees above zero. On average, St. John's has 17 rainy, 11 sunny, and 2 partly cloudy days in September. The rains are usually heavy. September as well as August, October, and November is considered the most uncomfortable time to visit St. John's.