If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Renton: The hottest months are usually August and July, with an average temperature of 18°C, while the coldest months are February and December, with an average of 5°C. The rainiest months in Renton are November, January, December and March, with each month seeing an average of 176 mm of rainfall.