The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large Brown Trout that only touch streamers for the rest of the year.
In Mayflies: An Angler's Study of Trout Water Ephemeroptera , Knopp and Cormier report "occasional" fishable hatches of this species from southern Alberta, and its known range extends down the east slope of the Rockies into the United States.