We were there in July,great falls.
May is a good time to visit Shoshone Falls! You will love seeing the amount of water going over the Falls. If you stand on the Outlook you might even feel the spray :-) have fun on your trip in May
June is a great time as well. We went on a Girl Scout trip and got real wet from the spray. So be aware of that.
May should be just fine:)
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