If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Eureka: The hottest months are usually August and September, with an average temperature of 15°C, while the coldest months are February and March, with an average of 9°C. The rainiest months in Eureka are December, January, March and February, with each month seeing an average of 176 mm of rainfall.