Kinda hard to tell for sure... can't see the wings enough to see if there are any wingbars. Also can't see the legs. From what I can see , though, it looks like a Nashville Warbler. The gray head, white eye-ring, yellow breast, yellow undertail coverts, and green back and what I can see of the wings, also greenish, all point to Nashville. Well within it's range, too.
Edit: I think the eye-ring is too well-defined for a Mourning Warbler, and the yellow undertail coverts are too short for a Connecticut....... at least according to the descriptions and field marks listed in my Sibley's.
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