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 Eureka: The hottest months are usually July and August, with an average temperature of 17°C, while the coldest months are January and February, with an average of -5°C. The snowiest months in Eureka are December, February, January and March, with each month seeing an average of 33 cm of snowfall.