September in Mazatlan is the rainiest month of the year. In September, the weather in Mazatlan is typically hot. Average daily high temperature in September is about 29 degrees above zero. On average, Mazatlan has 21 rainy, 5 sunny, and 4 partly cloudy days in September. The rains are usually heavy. September as well as July and August is considered the most uncomfortable time to visit Mazatlan.