See Getting There on the main page to reach Lake Helene and beyond.
The half-mile long arc of Flattop Mountain's North face provides a variety of excellent snow climbs for all abilities. There are three prominent narrow couloirs that contain snow or ice year round. To the west of these is the permanent Ptarmigan snowfield, which also can be used as a descent or ascent route. Gerry Roach gives all of these routes his coveted * Classic * designation, and of course they warrant it.
The eastern (left) couloir is the longest of the three and the only one I have not climbed. Roach says it reaches 55 degrees near the top of its 600 feet of climbing. The middle and western (right) couloirs are a bit shorter but reach 60 at their crux necks..
These couloirs can be overhung with surprisingly large cornices in early season. I have seen significant amounts of avalanche debris at the base of them in the spring. These cornices have caused at least two fatalities, so they are best treated with respect. On one occasion all three of the couloirs were guarded by large cornices, so we ascended a small but prominent rib between the east and middle couloir. This was not as steep as the couloirs and provided for a very fun, ridge-like climb.
I have used these couloirs as a training ground for several trips to the Himalaya. I climbed easy stuff over there, so feeling competent on these couloirs meant that I would only have altitude issues to deal with. That strategy worked.
Once I was climbing the central couloir with my new Scarpa Invernos, Charlet Moser strap-on crampons, and one general purpose mountaineering ice axe. There was another solo climber who was going without crampons but with two ice tools. When we topped out he said he would have gladly traded a tool for my crampons!
Ptarmigan snowfield, farther to the right, gives an easy 35 degree climb on its eastern (left) side, and gradually steepens to 45 degrees or so as you move west (right). I have seen a very deep crevasse near the western edge, unusual for a Colorado snowfield. Ptarmigan provides for a fun descent glissade, so you can climb one couloir, walk a hundred yards to the top of Ptarmigan, slide down, and then climb a second or even third route!
Other options include the standard Flattop Mountain trail, or you can continue hiking south along the Continental Divide, and come down Tyndall or Andrews Glaciers.
Depending on snow conditions you will need at least one mountaineering ice axe. As the snow firms up during the summer season crampons are useful. By fall you may want to bring a short length of rope and some snow, ice, or rock pro to protect the steeper sections of the couloirs. There is no runout by then, so a fall will land you in a boulder-strewn talus slope.
Climbing the central couloir in June, avoiding the large cornice!