I just hope we can score within the next couple weeks, the retrogression of the Alaskan/NE Pacific ridge due to planetary vorticity advection to northeastern Eurasia will shut off the heightened upward tropospheric wave propagation and momentum deposition onto the polar vortex and thus will lead to a strengthening polar vortex (increasingly +AO/NAO) as we get into mid-late month, barring that this NE Eurasia vortex is a lot stronger than forecast and manages to hold firm... The retrograding ridge will also probably leave lower heights and a trough in its wake over Alaska (+EPO) which could shut-off the cold air transport in about 3 weeks or so from now. It's uncertain whether or not we'll return to a cooler/stormier pattern thereafter near the end of January but its climatologically not favored in a NINA. We honestly should consider ourselves lucky that we've held onto a good pattern this long into January, and managed to squeeze out a huge storm in early December, a majority of NINA years tend to turn ugly before we flip into January (if they haven't done so already)... There are obviously rare exceptions but I wouldn't bank on this year being one of those for now at least...
From Nishii, Nakamura, & Orsolini (2011) "Geographical Dependence Observed in Blocking High Influence on the Stratospheric Variability through Enhancement and Suppression of Upward Planetary-Wave Propagation".
This is a fantastic paper, highly recommend it if you ever get the chance, obviously there's a lot of material that may go over your head but the basic concepts are very interesting and can help deepen your understanding of long range weather forecasting... I bolded the main conclusions of the paper in the abstract below.
http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-10-05021.1
"Previous studies have suggested the importance of blocking high (BH) development for the occurrence of stratospheric sudden warming (SSW), while there is a recent study that failed to identify their statistical linkage. Through composite analysis applied to high-amplitude anticyclonic anomaly events observed around every grid point over the extratropical Northern Hemisphere, the present study reveals a distinct geographical dependence of BH influence on the upward propagation of planetary waves (PWs) into the stratosphere. Tropospheric BHs that develop over the Euro-Atlantic sector tend to enhance upward PW propagation, leading to the warming in the polar stratosphere and, in some cases, to major SSW events. In contrast, the upward PW propagation tends to be suppressed by BHs developing over the western Pacific and the Far East, resulting in the polar stratospheric cooling. This dependence is found to arise mainly from the sensitivity of the interference between the climatological PWs and upward-propagating Rossby wave packets emanating from BHs to their geographical locations. This study also reveals that whether a BH over the eastern Pacific and Alaska can enhance or reduce the upward PW propagation is case dependent. It is suggested that BHs that induce the stratospheric cooling can weaken the statistical relationship between BHs and SSWs "
Shown below is a portion of figure 4 from this paper showing the composite evolution of 30 blocking high events in the North Pacific at 250mb (near the tropopause). Notice, over the course of several weeks, the blocking high which initiates in the Northeastern Pacific & Alaska retrogrades westward towards NE Eurasia (-EPO ridge evolves into a -WPO ridge) (again, this happens due to planetary vorticity advection. What this really means is that the ridge is so large that the coriolis parameter varies a lot across the breadth of the blocking high, ((df/dy) f- coriolis parameter y-latitude) that this causes the wave to advect itself westward against the mean flow so strongly that it's able to overwhelm the mean easterly flow imposed by the mid-latitude jet stream.) Over time this retrograding blocking high strengthens the polar vortex and leads to an increasingly positive Arctic Oscillation because this -WPO ridge becomes superimposed onto the climatological mean elongated trough that encompasses most of northeastern Eurasia. The juxtaposition of this ridge over the mean trough in NE Eurasia reduces upward planetary wave propagation into the stratosphere which is crucial to decelerating the polar night jet that encapsulates the polar vortex and generating high-latitude blocking in the troposphere.
Therefore we can make a few conclusions given these very crucial pieces of information:
1) -EPO ridges tend to evolve into -WPO blocking highs over the course of a few weeks (thus the negative phase of the WPO tends to lag the negative EPO)
2) -WPO blocking highs in Northeastern Eurasia often serve as a precursor to a stronger polar vortex in the weeks that follow. Thus, when you see a very stout -WPO ridge, it's often a big red flag that the AO may become increasingly positive after it develops.
Here's the evolution of the EPS weeklies for the next few weeks, notice how similar this is to the composite of blocking high events in Nishii, Nakamura, & Orsolini (2011). First, the big -EPO ridge goes up this week, it takes a few weeks to retrograde towards Northeastern Eurasia, results in a -WPO, then the tropospheric polar vortex intensifies after the -WPO develops.
