I've never checked the Allegheny for hatches, but I have seen a few on the Yough around the end of august.
However, it is very sparse, and doesn't happen until just about dark. I catch a few fish on them. It's nothing like the hatch on the Breeches, or when the Little Juniata used to have it good.
Whereever I've fished them, they never start until late, and you have maybe half an hour to 45 minutes on it. I don't really think it's worth hunting for, unless you're into night fishing.
As Tom mentioned, someone - Pennkiev? - has posted a few times on here about them being on Slippery Rock. I think that he's catching mostly bass there by then. It may be worth checking out though