Other facts from our historical weather data: Which is the warmest month in Grand Manan Island? August has an average maximum temperature of 19°C (66°F) and is the warmest month of the year. Which is the coldest month in Grand Manan Island? The coldest month is January with an average maximum temperature of 2°C (36°F). Which is the wettest month in Grand Manan Island? December tops the wettest month list with 150mm (5.9 in) of rainfall. Which is the driest month in Grand Manan Island? July is the driest month with 85mm (3.3 in) of precipitation. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.