I've been there in early November. This time of year you'd definitely be out on a limb. The avi paths are enormous, going down one side and back up the other in the valley. Plus the springs melt the snow from the slope above which creates pretty luke warm conditions. Our trip in early october this year bordered between survival and fun. Also know the springs are above timberline, just barely, but that gives you an idea of the temps you're in for.