September in Cancun is the rainiest month of the year. In September, the weather in Cancun is typically hot. Average daily high temperature in September is about 30 degrees above zero. On average, Cancun has 25 rainy, 2 sunny, 2 partly cloudy, and 1 cloudy day in September. The rains are usually heavy. September as well as June, July, August, and October is considered the most uncomfortable time to visit Cancun.