BWOs will be hatching as well. The reason I mention this is because the trout on spring were taking BWO duns instead of sulphur duns. When the sulphur spinners came, the fish switched to them. It was frustrating, because the BWOs weren't obvious.
I think I counted five simultaneous significant hatches on Penns in late may last year.
Early terrestrials will be present. So will golden stones on penns, bfc, and some others.
At least one day will probably see high, off color flows. It's a rainy time of year, and thunderstorms are common. Sculpins, slumpbusters, articulated streamers, etc are all effective. They cannot be too large.
Here's my favorite hatch chart:
http://thefeatheredhook.com/stream.html