San Francisco Weather
San Francisco's weather can be unpredictable because it's surrounded on three sides by the relatively cool waters of the Pacific Ocean and San Francisco Bay. If you are coming to the Napa Valley from outside California, chances are excellent that you will spend some of your time visiting San Francisco. One question that you are sure to ask is: “What will the San Francisco weather be like?” The answer is that the weather and climate in San Francisco will be unlike anything you have ever experienced, and most of the time you will find the weather in San Francisco delightful. But, when you come to San Francisco, you will need to plan accordingly, because you are sure to be surprised by the weather in San Francisco.
If you are coming during the summer months of June, July & August, you need to know that San Francisco has the coldest mean maximum and coldest mean minimum temperatures of any major US city. What this means is that you need to bring at least a warm sweater, or you may be quite uncomfortable during the nights in San Francisco. For example, the average high in July is just 68 degrees, and the average low is 54. Moreover, the summer months can be quite foggy, especially in the morning and in the evening, which makes the temperatures seem even cooler.
But while it may be quite cool in San Francisco during the summer, you can virtually be guaranteed that you will not have any rain. The average rainfall in July for example, is less than 1/10 of an inch. No need to bring an umbrella, and the same advice applies throughout your visit to California.
On the other hand, if you will be coming during the winter, be prepared for the rainy season. The vast majority of the rainfall in San Francisco falls between November and March, with an average of 1/3 of the days having some measurable rain. Do not expect, however, cold temperatures, as the average high in January is 58 degrees, and the average low is 46. You will be in heaven if you are coming from the east coast or the Midwest.
If you are looking for the best weather in San Francisco, in terms of lack of rainfall, warmer temperatures, and decent hours of sunshine, September and October may be the perfect weather for you.
One thing that you should remember is that the San Francisco climate in the summer is almost completely restricted to the geographical area of San Francisco. If you go over the Golden Gate Bridge in August, for example, you may leave San Francisco in the fog with a temperature of 57 degrees, drive a few miles to Marin, and notice that the fog has lifted and climbed into the 80’s. By the time you arrive in Napa Valley, the temperature may be in the high 90’s.