If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Roseau: The hottest months are usually July and August, with an average temperature of 18°C, while the coldest months are January and February, with an average of -10°C. The snowiest months in Roseau are April, January, March and October, with each month seeing an average of 26 cm of snowfall.