Napa Valley enjoys a long consistent growing season marked by sunny, warm days followed by cool evenings. The most moderate temperatures are found in the Carneros region in the south, with the cooling ocean influences from San Pablo Bay. Further north the Pacific Ocean has less influence and temperatures are warmer and the rainfall is greater. The eastern side of the valley tends to be more arid. Annual rainfall varies in the region from just over 20 inches in the south to 41 inches in the north, with mid-valley St. Helena averaging 34 inches. December is the wettest month and July is the warmest. Best months to visit Napa Valley wineries for good weather, and less crowds, are November, March, April & May.
