One of the heaviest hatches and a special favorite of mine is the Sulphur Mayfly. The duns start coming off about noon and continue until dark. The spinner start coming back two hours before dark and are on the water until dark. When both the duns and the spinners are on the water it is not unusual to see 6 or 8 trout rising to the naturals in the same pool. I use the Murray's Sulphur Dry size 16 & 18 to match this. I have switched to a 6X leader since the streams are getting low. Be sure to watch your approach and sneak, sneak, sneak so that you do not spook the fish.
For more information on these streams, see my book Trout Fishing in the Shenandoah National Park or stop by the fly shop. Please note the fly shop will be closed Sunday May 30 through June 1 for the Memorial Holiday.