My initial impression this bird is a "Nelson's Gull", a hybrid between Glaucous Gull x Herring Gull (usually L. a. smithsonianus but perhaps possibly L. a. vegae).
The tail band rules out Glaucous Gull. The sharp-bicolored bill rules out Glaucous-winged, Slaty-back, Thayer's. Primaries are altogether too pale for Vega or Heuglin's. I think the primaries are also very pale for Mongolian Gull and the bill is extremely crisply bicolored which I don't think fits Mongolian too well.
Nelson's Gull are seen on both the east and west coast of North America. I can remember personally at least two adults during my North America birding days. Photos of 1st winter birds are very similar to this
