The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
That was my impression as well, also a female spinner. I'm thinking Siphlonurus alternatus. Don't see any Siphlonurus species listed for PA in my most trusted (national) source. However it's most likely incomplete in that regard.
I can confirm that we do have Siphlonurus in PA. I've seen and even fished them on a few streams. Greg Hoover's 2002 survey of PA mayflies lists barbaroides, mirus, quebecensis, rapidus, and typicus as PA species.
Well, we seem to be in violent agreement on genus. And after all, identification to species based on a photograph of a female imago is problematic, at best. However, as Konchu has observed, it is fun.