I thought initially that this was a Thayer's too.
Its small bill puts it rapidly into the Kumliens/Thayers group. The dark eye is better for Thayer's as is the amount of head marbling. Many Kumlien's Iceland gulls show a lot less black in the primaries as well, but some can pretty much resemble a Herring gull at rest. Thayer's wings pretty much look like Herring gulls at rest, though usually have larger white terminal tips.
The photo only shows the tips of primaries 6-10. P5 can frequently be helpful since Kumlien's gull "rarely" shows black on this feather whereas Thayers' gulls only lacks it "sometimes," per Howell and Dunn. Olsen and Larson report a paper (by Howell) that the number of Thayer's lacking black on P5 is 25%. Either way it doesn't matter since we can't see it, though my assessment as it flew by was that the bird seemed to only have 5 feathers with black on them. However, what bothers me on this bird is that you can see the underside of P10 on the opposite wing which is almost entirely whitish with gray; I'm not seeing any solid black. Sibley shows a narrow strip of black in his painting of the adult non-breeding, which is less black than all the photos of Thayer's in either Howell and Dunn or Olsen and Larsson.
Here's another interesting gull (the one on the left). You'll have to take my word that the back was evenly mottled brown, it did not have any of the gray mantle that a 2nd cycle would be acquiring (or even some 1st cycle Herring gulls when they do more pronounced moult after juvenile plumage). This bird, however, has a bicolored pink and black bill which I've never seen a 1st cycle Herring gull exhibit.
There was also some atypical white feathering on the face over the forehead, and more even brown through the head and body than the darkening plumage that Herring tends to show.
Here's the bird as it flew off, I didn't quite time it/keep it framed to get an effective spreadwing, but did manage a spread tail!
The tail did parallel the flight feathers though, in that it was a lighter more medium brown than the dark brown-black that Herring gulls typically show. I suspect this to be a "Nelson's Gull," a hybrid between Herring and Glaucous.