Honestly, after I sent the post, I realized I should have said Penna., or even spelled out the whole name, but too late. Mea culpa.
As to the rarity of sterna terns around here, according to "Birds of the Lehigh Valley", both Forster's and Common are rare but regular in our area. There are only three records of Arctic from our area: one in '89, one in '03, and one in '12, but none in Lehigh County. Common Terns (and Forster's for that matter) are fairly regular in migration along the Susquehanna River about 70 miles/115 km west of here.
This bird was originally found by one of the best birders in the area, who called it a Common. He was actually hoping for a Forster's, I've been told, since's he's seen Common in the county but not Forster's. No idea whether he considered an Arctic, but if I know him, he may have.
I saw the bird a week after it was found, noticed the bill did not have a black tip, and wrote that off as a distinction between Common and Forster's and went no further than that. It was only after looking at the other tern threads in the ID Forum last night that I took a second look at the eBird photos of our bird and began to think maybe Arctic.
My thoughts. Pro Arctic: Short legs, deep red bill without black tip. Pro Common: Bright white breast and shorter tail streamers. I also spoke to another experienced birder this morning who saw the bird, but only in flight. He thought it was a common based on the breast color.
Full disclosure: Common is a new county bird for me. Arctic would be a new bird both for Pennsylvania as well as the Lower 48.