I grew up in Wyoming and spent summers in the Colorado Rockies when growing up. Much more recently, I've taken several trips back to Colorado and to Wyoming's Wind Rivers.

A 20* Western Mountaineering Ultralight works fine for me. There were a few times that it was a bit marginal even with a vapor barrier suit and all my insulating clothing, but most of the time I was snuggly warm. IMHO, the most wonderful thing about this bag is the draft collar!

As mentioned, there will be a lot of frosty nights at higher elevation, and it can snow any time during the summer. Be prepared!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey