If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Tumwater: 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 Tumwater are November, December, January, and March, with each month seeing an average of 8 inches of rainfall.