The "Birdking" writes:

Hello Andy. I had an adult alternate Thayer's Gull at the mouth of the
Okanagon River near the town of Brewster last week. The bird flew by rather
quickly, but I saw all the field marks. I was wondering how regular they are
east of the cascades... Thanks.

I respond:

Charlie,

This species is rare in the interior in most parts of North America, so any
record is of interest. As far as I know, there is but one prior record from
Okanogan County, WA ; I had one adult in April. They are rare, but regular
on the large lakes in the Okanagan Valley of southern BC.

Andy Stepniewski

As a postscript, I believe field identification of a fly-by Thayer's Gull is
a challenging call, even by those with extensive prior experience with this
species.
Wapato WA
steppie at nwinfo.net