I would not be surprised to see a strong storm system traversing the nation over the December 19-23 period. We'll start off this analysis with a portion of the post from December 11.
The teleconnection forecast for the next little while from the Earth System Research Laboratory (ESRL) above shows projections for the Pacific-North American index (PNA), the North Atlantic Oscillation (NAO), the West Pacific Oscillation (WPO), and the East Pacific Oscillation (EPO). For this discussion, we will focus on the PNA and the NAO.
The forecast for the Pacific North American index is negative over the time period for this storm system, even bottoming out to its minimum during when I anticipate this storm system to strike. So what does the atmosphere look like during a negative PNA?
The next item to discuss is the projected positive phase of the North Atlantic Oscillation. During the positive NAO, we see stormy weather over Greenland, which shifts the jet stream so the weather pattern is not conducive to those big high pressure systems you might sometimes see over the Bering Sea or even the Western US. The positive NAO pattern is what is called 'progressive', meaning no atmospheric feature really sticks around in any single area, because the atmosphere wants to keep this moving, so to speak. That said, previous forecasts for this storm had the system going north-northeast from the southern Plains into the Dakotas and Upper Midwest. Because of that progressive pattern, I'm really doubting we see such a spike northward. Will we see northward movement? Yes, because the -PNA will supply that ridging in the Southeast. However, the progressive nature of the positive NAO should limit any NNE or even due-north movement by this storm. I would watch out for the storm to move northeast, or even ENE.
The first item we'll discuss in this image is the ridging over the southern portion of the Gulf of Alaska. This ridging is encased in solid oranges right over the heart of the big arcing in the waters west of California and the coast, signaling high confidence that this ridge will be present. If that ridge is present, then is essentially guarantees the set-up I described above. Why? Well, that ridge in the waters off the West Coast then creates West US troughing, which provokes high pressure in the Southeast, and suddenly we have a classic negative PNA pattern.
Second, we see a swath of oranges extending from southern Canada into the Rockies. The troughing in the West from that negative PNA pattern tells us that the ensembles are sensing the trough to be our storm system in the December 19-23 timeframe. Like the Gulf of Alaska ridge, the oranges signify over a 60% confidence among the ensemble system with this storm system. Rising confidence among the ensembles for not only the storm itself, but for the NE Pacific ridge tells me that this storm is becoming even more likely.
Third, there is a wide body of oranges in the Southeast US. Going back to that negative PNA pattern, high pressure/ridging in the Southeast is expected, so it really isn't surprising that the ensemble set is going with a >60% chance of that ridge occurring. However, with the aforementioned progressive pattern thanks to the +NAO, it is unlikely that we see this ridge take over the East entirely.
Model forecasts aren't as consistent as I'd like them to be, but here's what the GEM and ECMWF models show for this timeframe. I picked them, because they seem to be following the progressive pattern, and do respect the Southeast ridge that is likely to be present in this timeframe.
The timing is a little off, though that could be attributed to the ECMWF tendency to keep energy in the Southwest a little too long, and the fact that we're still some time out from this event actually occurring. Nonetheless, other forecasts have the storm pushing more to the Southeast, and I'm really not so sure we'll see that happen. It would appear that the storm currently ongoing from the Midwest to the Northeast might be similar to this upcoming storm, as teleconnections will be nearly the same what they are now. Additionally, the negative PNA will be even stronger, and this would mean a more persistent Southeast ridge that would keep away any southern suppression of this upcoming storm system.
It's to be determined exactly where the storm will go, but you can kind of get an idea of my early thoughts from this post. I do expect this storm to be strong, and it will most likely have a wintry side with accumulating snowfall. The intensity is TBD, but confidence in the storm itself and the set-up surrounding the storm is rather high.