When I arrived at the shelter, I asked everyone what temperature rating their sleeping bag was. These hikers out for a couple days were unprepared but after hearing ratings of twenty and thirty I figured they would survive at least. We were all about to camp at 3,500 feet in open air. It was in the twenties when we all rolled in to camp and I knew nothing was going to prevent that temperature from dropping into the teens at night.
They all pulled out their trash bags, space blanets, four pairs of socks, whatever they had to insulate them. A couple went so far as to even leave their unlaced boots on in their sleeping bag and put their feet in their packs.
And sure enough, they all were miserable and they all struggled to sleep. And sure enough, we all awoke to clashing of sleet hitting the tin roof around midnight. And the next morning we all had mixed feelings about the beautiful couple inches of snow on the ground.
I guess I should have expected this. Winter thru-hike plausible? Maybe. Winter thru-hike sensible? Probably not.