If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Eureka: The hottest months are usually July and August with an average temp of 62°F, while the coldest months are January and February with an average of 23°F. The snowiest months in Eureka are December, February, January, and March, with each month seeing an average of 13 inches of snowfall.