May in Hillsboro is the rainiest month of the year. In May, the weather in Hillsboro is typically hot. Average daily high temperature in May is about 29 degrees above zero. On average, Hillsboro has 11 rainy, 9 sunny, 9 partly cloudy, and 2 cloudy days in May. The rains are usually heavy. May as well as January, June, July, and August is considered the most uncomfortable time to visit Hillsboro.