I continue to remain confident on a wholesale change to the 500mb pattern over the next 10 days... this is mainly due to the strong MJO wave propagating towards the dateline, which will provide the impetus for -EPO ridging, as well as the anomalous Scandinavian trough which will retrograde into the NAO domain.
The MJO is currently in Phase 7, and looks to make a break for Phase 8 as well within the next 10 - 14 days, which is favorable for high-latitude blocking and cold air intrusions into the E/C US. Although there have been some murmurs about the -ENSO interfering with the propagation of the wave, having it die before reaching Phase 8 with high amplitude, the -QBO state often allows for stronger, slower waves. This should allow for the MJO to continue its propagation through favorable phases.
The Scandinavian ridge that's been talked about since the beginning of the month is now starting to form in the short range, and will reach +2.5 sigma:
This cutoff ridge will retrograde towards Greenland, providing stout -NAO blocking that will, in turn, lead to significant confluent flow over Nova Scotia. This should keep the SE ridge at bay even with the -PNA. There will also be a great cold feed into the US from the -EPO forced by the MJO, which is a complete 180 from what we've been seeing recently. Both of these drivers should lead to a coast-to-coast trough, which becomes more apparent towards each of these runs:
I've heard a lot of concerns about the -PNA, and I don't believe that they're totally founded. We will probably have to deal with the SE ridge before the blocking is fully established, but once the -NAO begins to retrograde, the compression of heights in SE Canada will ensue. This almost always crushes the SE ridge and allows for redevelopment of any systems that would try to cut. So it might be a problem towards the 20th, but the maturation of the blocking should allow for confluence to take over.
Anomalous -NAO blocks like this have the potential to completely overwhelm patterns, and this one looks to become more and more west-based over time, which is optimal. If anything, a +PNA here would likely lead to supression until the block eventually decays. The negative anomalies throughout the Atlantic also lead me to believe that this is a legitimate -NAO, as the flow in that region is completely backed up.
The high-latitude blocking will also lead to dramatic heat fluxes into the stratosphere, which look to constantly disrupt the SPV. I don't believe that the SPV will be able to consistently strengthen, and a weakening of the SPV may allow for prolonged blocking. A SSW isn't on the way as of now, but that would significantly disrupt -ENSO climo if it were to occur, and this would certainly be a step in the right direction.
This December will almost certainly finish with a -NAO/-PNA combo. The Januaries that historically followed Decembers with a -NAO/-PNA featured favorable patterns for the E US with a significant west-based -NAO:
Overall, I am quite excited for the end of the month, as things get very interesting any time we see blocking of this magnitude. I expect the SE ridge to continue to correct weaker as the -NAO trends stronger with time (as hemispherically forced -NAOs usually do), allowing for a coast-to-coast trough look. This would be the "just cold enough" type pattern where waves roll under the block with good climo. Hopefully we continue to see these corrections as we endure a dismal pattern through the 20th.