Me and my wife are planning a trip to the states, spring 2011. We want to climb/hike some of the best U.S. peaks in a row: Mount Whitney, Maroon Bells, Mount of the Holy Cross, Grand Teton, Mount Rainier, Mount Shasta and visit Yosemite, RMNP and Yellowstone in between. We want to go in spring in order to avoid the summer crowds and because we think the spring high up is even more beautiful than the summer.
Do I miss some essentials here?

I'm wondering what the conditions of south Maroon south ridge and/or bells couloir and Mount of Holy Cross couloir are in early june:
How many snow is there left on the mountain? Are snowshoes necessary? What about avalanche danger, especially in the couloirs?
What about the temperature at night?
Is there less risk of thunderstorms than in summer time?
Are there some other things we have to bear in mind this time of the year?

June is typically a very good time of year to climb couloir routes in Colorado. By then the snow has settled into a nice "freeze/thaw" cycle with good hard freezes at night to stabilize the snow for morning climbing. Often times its usually better to descend another route as the afternoon warming increases the risk for "wet" avalanches.

Overnight temps at high elevations will be below freezing, but usually a little warmer around the trailheads where you are more likely to be camping, maybe low 40s even.

There is usually not enough snow on the approaches to warrant the extra weight of snowshoes, although gaitors and waterproof boots will be necessary. Expect to post hole for short sections, particularly in soft snow in the afternoon, in areas of dense shaded tree cover. Most of the ridges will be dry as will most of the south facing slopes. The snow melts quickly, so from one week to the next the snowline can move significantly.

Afternoon thunderstorms are usually not much of an issue in June, typically not until early July do they really start their cycle.

Please note that Mount of the Holy Cross you would not be able to access the standard route or any of the couloir routes until June 21st, as the road is closed annually for elk calving in the area. So you should plan your schedule accordingly.

Hope all of that is helpful, June is a wonderful time to climb in Colorado, have fun!

Thanx for your information, it sounds all really nice! I'm very looking forward to it. I've done Longs Peak a few years ago in late autumn, that was a very good experience. So hopefully Colorado will be nice for me again.