The weather in September in Napa Valley is fantastic. The average temperature in Calistoga is 69 degrees, with an average high of 88, and an average low of 50 degrees. The average rainfall in Calistoga for September is 0.3 inches. You can count on sunshine an average of 10 hours a day.
If you are coming from outside California, you will probably be surprised to learn that September and October can often have periods of temperatures in the 90’s. Indeed, in 2014, the temperature reached 90 degrees or higher eight days in the month. Regardless of how hot it is during the day, the temperature almost invariably cools down at night.
In the town of Napa, the average temperature in September is 67 degrees, while the average high is 81 degrees and the average low temperature is 52 degrees. The average rainfall for September is 0.4 inches. Temperatures and rainfall in Yountville and Oakville, several miles north of Napa, would be roughly the same as in Napa. Chances will be excellent that you will never see a drop of rain during your visit.