I'm certainly no shorebird expert, but one thing I've noticed in my field guides is that Mountain Plovers typically have darker lores, even in non-breeding plumage, and I also see the tan color coming all the way around the front of the breast, and this bird seems to have a more pale breast. That all said, I don't know what's within the range of expected plumage for Mountain Plovers.
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The Black-bellied and Golden-Plovers were the first ones I thought of and eliminated. Those options all seemed a little too large and the spotting patterns of winter plumage. There are no neckbands which eliminated all other plover options
Internet, Stokes Field Guide and Petersen's Guide to Western Birds