I had heard that sulphurs* had been coming off in the evening. As of last night I can confirm it firsthand. Despite water that’s higher than ideal, I don’t think I could have had too much more of an ideal evening last night (5/20).

I’d seen sporadic sulphurs and corresponding rises, but this was the first sustained hatch and rise I’d been a part of on the Gunpowder this year. I brought several very nice fish to hand (sorry, no photos–my waterproof camera is in the shop), and should have landed about twice as many.
The sulphurs I saw were in the 14 to 16 size range, mostly yellow with a bit of orange on the thorax. Rises were aggressive.
If we don’t get too much rain, it should (hopefully) be very good fishing the next few days.
*Is it sulphurs or sulfurs? Even the Interwebs don’t seem to know for sure.