Not sure about conditions in your location but Colo. got the Northern Rockies snow. Usually 6 weeks would put you on water "browner than a chocolate shake" in the Yellowstone area. The air temperature during the spring is also a factor. Remember "Spring" in the Rockies is a lot later. There was 5 foot of snow at 8500 feet outside of Cooke City, Mont. a week ago. (With only 40% of average snowpack).
I "wasted" my first trip West in the early 70's, I got there at the very beginning of the runoff. I spent two weeks watching the water rise and muddy up. The only fishing I got were some very small trib. streams.
I'm heading to the Cooke City, Mont. area in two weeks to camp ground host for the Forest Service. I'm planning on fishing a lot of mtn. lakes at Ice Off until around mid-July.
Check with the fly shops!!