August in Roscommon is the rainiest and the cloudiest month of the year. In August, the weather in Roscommon is typically warm. Average daily high temperature in August is about 16 degrees above zero. On average, Roscommon has 30 rainy and 1 sunny day in August. The rains are usually heavy. August as well as January, February, March, and April is considered the most uncomfortable time to visit Roscommon.