Other facts from our historical weather data: Which is the warmest month in Portstewart? August has an average maximum temperature of 17°C (63°F) and is the warmest month of the year. Which is the coldest month in Portstewart? The coldest month is February with an average maximum temperature of 9°C (48°F). Which is the wettest month in Portstewart? November tops the wettest month list with 145mm (5.7 in) of rainfall. Which is the driest month in Portstewart? May is the driest month with 79mm (3.1 in) of precipitation. Which is the sunniest month in Portstewart? May is the sunniest month with an average of 203 hours of sunshine. 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.