The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
The 3 main globals (GEM, GFS [blah], and ECMWF) are showing one heck of a cold spell around Christmas (or perhaps a bit after). Actually, system after system looks to dig in to the main trough, acting to deepen it in the eastern 1/2 to 2/3 of the CONUS.
Before getting to the 00Z models, here is the 12Z Euro, showing a 516 decameter vortex digging rapidly into the Lakes and Ohio Valley at day 7.
This would produce an EXTREME Lake Effect event, although it would likely be relatively short-lived given the in and out nature of that upper low, according to the Euro.
Moving on to this evening's model runs -
The 00Z GEM continues with it's major shots of cold weather. At Day 10, there is a 504 dm vortex digging into the Northern Plains with a strong high close on it's heels acting to bring 1000-500 mb thicknesses well below 510 (meaning daytime highs near or below 0F) well into the central US. Cross-polar flow is well-developed, according to the Canadian, helping to support lows well below zero in any area laden with fresh snowcover. Just as a reminder, day 10 is the beginning of Christmas Eve, meaning that vortex would be in the heart of the Ohio Valley, Lakes and Northeast just in time for Christmas Day. That would be a Christmas to remember in much of the Northeastern 1/4 of the country.
Finally, the 00Z GFS, although crazy as usual that far out, brings the actual Polar Vortex into the United States by day 15.
It also has the cold Christmas shot, but the day 15 event is much stronger, with 850 mb temps pushing -30C (again below zero highs and much below zero low temperatures for much of the country north of the Mason-Dixon line...and yes sub-freezing in much of the south and southeast).
Also, for fun, the 12Z Japanese global brings -30C air at 850 into the Northeast on the 21st.
Now, we should all take these model runs with a grain of salt - with that said, the agreement (although not perfect) between models and each model's runs is impressive and something that doesn't happen often. In my educated opinion, the extreme nature of the projected cold is well supported by the various indices and teleconnections talked about by the professional mets here.
As exciting as all this is, it will mean very expensive heating bills for much of the nation - that is something none of us needs right now. I just hope there is a ton of fun snow to go along with the cold. There is nothing worse than cold, "brown" weather.
J/K!
