September in San Luis Potosí is the cloudiest month of the year. In September, the weather in San Luis Potosí is typically warm. Average daily high temperature in September is about 19 degrees above zero. On average, San Luis Potosí has 17 cloudy, 9 rainy, and 4 sunny days in September. The rains are usually moderate. September as well as July and August is considered the most uncomfortable time to visit San Luis Potosí.