Yes the monsoons are here, but that doesn't mean you'll be ducking for cover all the time. Just keep the rain gear handy and be ready to slip it on with only short notice. Up at the high lakes, lightning will become an issue. Although bear paranoia seems to be rampant on these forums, lightning is the true danger for anglers in the Rockies
If you are up at an exposed high lake like Lake of Glass or Sky Pond, descend into cover at the first hint of a lightning storm. Do not fret about running into cover during times of good fishing. Electrical storms come and go so quickly, you'll be back on the water in no time.
Standard procedure for this time of year is.......mornings are calm and sunny. After lunch time, the winds pick up and then by 3-5pm the electical storms blow through with terrific force. At the blink of an eye, the thunderstorms disappear and it's blissful evening fishing until sundown. Enjoy the fishing and the Park!!!
Oh and as far as flies go for the high lakes. Midges, griffith's gnats, and ant/beetles. And elk hair caddis and parachute adams go without saying. During non-hatch times, go with wooly worms, scuds, and buggers. When there is no activity on top, I always go with prospecting orange and olive colored scuds.