The 1975 Ross' Gull was a thing of legend. It made the cover of the New York Times and got three minutes of coverage on the NBC Nightly News. However, since then there has been quiet a few records of this high arctic breeder reaching some southern clims.
In 1985, Anthony H. Bledsoe and David Sibley wrote "Patterns of vagrancy of Ross' Gull" for American Birds where they compiled subarctic records and (on pg 223) charted them in half-month intervals. Since then, there have been at least 34 additional records which hint at when these birds are likely to occur.
Kinda looks like there are two movements of these gulls that send them to the lower 48. So now that we are entering the first wave, keep an eye out for any gulls with gray underwings and wedge tails!