May in San Antonio is the rainiest and the cloudiest month of the year. In May, the weather in San Antonio is typically hot. Average daily high temperature in May is about 30 degrees above zero. On average, San Antonio has 13 rainy, 10 partly cloudy, 6 sunny, and 2 cloudy days in May. The rains are usually heavy. May as well as June, July, August, and September is considered the most uncomfortable time to visit San Antonio.