Other facts from our historical weather data: Which months have the greatest rainfall in Eureka? Most rainfall (rainy season) is seen in March and December. Does Eureka have a dry season? Yes, the months of July and August are very dry. Which is the warmest month in Eureka? September has an average maximum temperature of 18°C (64°F) and is the warmest month of the year. Which is the coldest month in Eureka? The coldest month is December with an average maximum temperature of 12°C (54°F). Which is the wettest month in Eureka? December tops the wettest month list with 198mm (7.8 in) of rainfall. Which is the driest month in Eureka? August is the driest month with 4mm (0.2 in) of precipitation. No idea where to travel to this year? We have a tool that recommends destinations based on your ideal conditions. Find out where to go with our weather planner.