September in Roseau is the hottest month of the year. In September, the weather in Roseau is typically hot. Average daily high temperature in September is about 29 degrees above zero. On average, Roseau has 14 rainy, 11 sunny, and 5 partly cloudy days in September. The rains are usually heavy. September as well as May, June, August, and October is considered the most uncomfortable time to visit Roseau.