If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in St. Helens: The hottest months are usually August and July with an average temp of 63°F, while the coldest months are February and December with an average of 40°F. The rainiest months in St. Helens are December, January, November, and March, with each month seeing an average of 7 inches of rainfall.