May in Hillsborough is the rainiest month of the year. In May, the weather in Hillsborough is typically warm. Average daily high temperature in May is about 17 degrees above zero. On average, Hillsborough has 14 rainy, 7 partly cloudy, 6 sunny, and 4 cloudy days in May. The rains are usually heavy. May as well as March, April, November, and December is considered the most uncomfortable time to visit Hillsborough.