August in Thunder Bay is the sunniest and the warmest month of the year. In August, the weather in Thunder Bay is typically warm. Average daily high temperature in August is about 20 degrees above zero. On average, Thunder Bay has 13 sunny, 13 partly cloudy, 3 rainy, and 2 cloudy days in August. The rains are usually light. August as well as {{month_nom: February}}, and {{month_nom: February}} is considered the most comfortable time to visit Thunder Bay.