It's a bad idea. Too many long, cold nights and lots and lots of opportunities to get caught in the backwoods in lots of snow. Mother Nature is indifferent about outcomes. Bad stuff can happen in cold weather and there are not a lot of folks on the AT in many sections in the winter to help. I started NOBO on Feb 20 and don't think I would want to start much earlier. I experienced about 35 warm days on the entire hike.