September in Key West is the rainiest month of the year. In September, the weather in Key West is typically hot. Average daily high temperature in September is about 28 degrees above zero. On average, Key West has 19 rainy, 6 sunny, 3 partly cloudy, and 2 cloudy days in September. The rains are usually heavy. September as well as June, July, and August is considered the most uncomfortable time to visit Key West.