If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in Mountlake Terrace: The hottest months are usually August and July with an average temp of 62°F, while the coldest months are February and December with an average of 41°F. The rainiest months in Mountlake Terrace are November, January, December, and October, with each month seeing an average of 7 inches of rainfall.