They're big gulls, heavy-chested, and even heavier billed. The heavy bill was even obvious to the naked eye as they would fly over the highways.
I saw a few juvenile birds at various places, the only juvenile gulls I encountered. The dark brown spangled-edged feathers of the mantle and reasonably neatly checkered wing coverts reminded me somewhat of lesser black-backed (though that bill does anything but).
The second year birds take on more of a herring gull look, with plainer dark plumage; basically big ugly brown things.
Based on the black-tipped yellow bill and the retained brown tertials with marbled white edges (also something that always stands out to me on lesser black-backed) I think this is a 3rd summer bird:
This is probably a 4th summer bird given the remnants of brown on the head and neck, mostly bi-colored yellow bill, but with adult primaries growing in.
Finally the adult birds. Sibley notes that eye color varies between dark in the north and pale clear yellow in the south with there being a perceptible change in central California. I didn't try to grade the ones I saw, but clearly there was some variation in the population based on these 2 photos:
Finally is a probably a member of the Glaucous-winged x Western Gull hybrid swarm sometimes known as "Olympic" Gull so-named for the overlap zone in that part of Washington where virtually all of the birds have mixed parentage. The bird has a paler mantle than a Western Gull and some marbled head and neck patterning far greater than what a Western Gull should have:
Coming soon to a blog near you, the other Western gulls I encountered, Heerman's and California...