The Napa County has a Mediterranean weather pattern characterized by warm, dry summers and cold, moist winters. The majority of annual precipitation occurs as rain that falls during the winter and early spring.
The highest rainfall occurs on the western side of the watershed. Rainfall gages throughout the County show north-south and east-west trends of precipitation patterns.
The average annual precipitation is 35–40 inches in the western side of the County, and 20–25 inches to the east (1961-1990, Western Regional Climate Center 2002). Average annual precipitation is 38 inches at Calistoga and 25 inches at the Napa State Hospital (1961-1990, Western Regional Climate Center 2002).
The WICC offers a rainfall map that represents lines of equal rainfall (isohyets) based on long-term mean annual precipitation data, collected over a sixty year period (1900-1960). Use the "Water Resources" link in the map legend to view the data.
The Napa Valley Regional Rainfall and Stream Monitoring System (Napa Onerain) provides current and historical rainfall, river and stream level monitoring data from a network of automated gage sites throughout Napa County.
The average daily maximum temperature is lower in the south, because coastal fog keeps the lower valley cooler.
The National Weather Service is a good resource for local weather data and forcasts.