Hello Tweets, I'm not able to get out and check gull roosts this morning, but I thought I'd pass off some knowledge on where I know gulls to roost in case it helps anyone searching. All of these are in Snohomish county and we start from the south.
Edmonds Breakwater - Visible from the Edmonds pier with tons of parking. Presumably already checked this morning I'd assume. Usually has several hundred Heermann's and a mixture of California's, GWGU's, etc.
Shell Creek Exit - Visible from Sunset Avenue, just north of the Edmonds Ferry Terminal, by looking north along the beach. At low tides, you can walk along the beach to get closer, but you must start from the park next to the ferry terminal. Also visible from Ocean's avenue to the north, but there's literally one parking spot there.
Haines Wharf Park - When I checked this yesterday at low tide there were literally zero gulls on the roof of the old wharf building, but depending on tide and season there can be hundreds on gulls up there. Scan the point to the north which is in Meadowdale park. Sometimes there are gulls bathing there. Unfortunately this park can legally only be accessed via a long hike.
Picnic Point Park - I would expect a sunny Saturday during salmon season to make this a tough roost for gulls, but I have sometimes seen a few dozen bathing here. There are usually a few gulls on the posts south of the park (visible from the railroad overpass) and near the shipwreck to the north.
Everything north of here is in more protected waters and gets fewer birds that would seem to have diets similar to STGU, but it might be worth checking around Mukilteo, especially the docks west of the ferry terminal and perhaps Everett.
Outside of this area, I know there are large concentrations of gulls roosting at the south end of Whidey Island and along the beaches of Kitsap County north of Kingston. No idea what access is like at either place though.