Weather:
With September comes the crush. The Napa Valley grape harvest is possibly one of the most popular event destinations in California. The term Wine Country automatically invokes a vision of the Napa Valley in all its glory. The wineries are in full swing as the crush reaches its crescendo, their doors are open and all are encouraged to witness this massive undertaking first hand. In September, the average temperature in Calistoga is 68 degrees, the average high is 85, and the average low is 50 degrees. The average rainfall in Calistoga for September is 0.1 inches. If you are coming from outside California, you will probably be surprised to learn that September and October can often have periods where temperatures reach into the 90’s during the day. Indeed, these months are the warmest of the year in San Francisco, as the fog is usually brief giving a chance for the temperature to warm during the day.

http://www.wunderground.com/US/CA/Calistoga.html