Sonoma Coast AVA
You may observe that the area along the coast in the north where the temps are lower, and also that the elevation is generally below 1000' MSL (Mean Sea Level) and demarcates the fog zone. As one looks southward, the Russian River channel allows fog to move in, and the land flattens out along the 'Petaluma gap' - which is the main area that allows the fog & maritime breezes to travel inland unhindered (elevations are 100~400'). The fog then fills the Santa Rosa plain, and the areas south of Sonoma in the Carneros region (elevation from MSL~120'). This 'fog bowl' as it could be viewed as, collects the cooling maritime layers and holds it through much of the morning, and sometimes well into the afternoon. A secondary route for the fog is northwards from San Pablo bay (which is how Napa County generally receives its fog, though some may flow through Calistoga from Knight's Valley in the north)...
Another piece of evidence supporting the idea that the AVA is pertinent is provided by satellite imagery. The following is from the visible camera of the GOES-10 satellite on July 14th, about 10 AM, and shows quite clearly the area in question under a contiguous layer of fog:
(Please note also how the fog has spread into Napa & Sonoma from the San Pablo bay, as well as heading into Sonoma County from the coast...)
To further this argument, I'd like to point out the temps throughout the area which are very similar (my rough outline of the AVA is lightly overlaid) :
So it would appear that the Sonoma Coast AVA shares more characteristics than one would assume just by looking at, say, a road map of the Bay Area. Sure, there are soil differences across the area, but there are also soil differences across something as small as a vineyard, and generally that shouldn't be considered a primary factor in AVA designation. And the fact that the Carneros region is further inland from the Pacific really doesn't change things too much as there is still ample fog & similar temps which unify the area.