Most, but not all, and there are large stretches at a fairly high elevation nonetheless. If you look at the historic weather data for towns along the Southern Tier route, there are places in AZ & especially NM with some pretty cold average low temperatures (in the 20s) this time of year. There's a pass in NM over 8,000 feet high. And although Ryan said someone at Adventure Cycling told him that people ride the ST in the winter, if you check crazymanonabike.com and search for "Southern Tier January", you'll find that few have attempted it in the winter. Fewer have succeeded, even over a much longer time period than Ryan has. Most of the hits you'll get are for journals that were updated or posted in January, but the cyclists actually did their trips in the spring or fall. Among the few who were actually riding in the winter, one guy encountered snow in NM in November and had to get a ride in a pickup truck across than stretch. Another guy went south into Mexico. Others talked about their frozen solid water bottles. Another stopped riding about halfway. Adventure Cycling organizes their own tours along the ST in the spring & fall, but not in the winter.

I have no idea if Ryan ever got lights. His website is asking for money for lights so I would presume not. That's going to be another major impediment if he has to stop by sundown each day.