Years ago when I biked from Denver to Jasper, it was hot everywhere in the Rockies that summer. The highest temperature of the entire trip was in a valley in eastern BC south of Radium Hot Springs, where it reached 104F - 40C. Cycling in the summer months in 6 provinces in Canada in various years, I've never had weather that I would even describe as "cool". I'm sure it can happen, though, and I guess you need to be prepared for it.