Went through in early February a couple of years ago and hit a period with absolutely clear air and blue skies. Those were conditions for a long shot down the canyon that needed a long lens. A wide one, or very wide one like your 155, would have had to be tilted way down to avoid having half a frame of blank sky. For a major trip like you're undertaking I wouldn't want to be wishing I had also brought along a longer lens l6 to 18 inches - you can leave it in the room on any day when you don't need it. Also, as Sal noted, no matter what condition you are in, don't underestimate coming from sea level to 7000 feet. It will take you a couple of days to acclimate unless you are driving out. I live at 6000 feet and I can really feel it when I go up only 4000 feet more. If you are driving remember that Mother Nature has nasty ways of spoiling your plans. Enjoy your trip.