Winter delayed but not denied
January’s pattern has already begun a dramatic flip from one of the warmest Decembers on record. A strong cold shot struck the east over the last few days and will moderate as we head into the weekend. However, a more sustained and lasting pattern change will take hold.
The latest QBO stat for December at -16.25 was a pretty sharp drop over December. This is a good sign that the easterly QBO is in full blast and in theory should help to weaken the westerly winds and consequently the Polar Vortex as we head through winter.
The latest MEI reading for Nov-Dec is in at -.979 which is a drop from the La-Nina peak in Oct-Nov when the bimonthly reading reached a low of -.980. This to me indicates - as the statistical and dynamical models indicated earlier in the year - that La-Nina would be weak to perhaps borderline moderate. The overly aggressive CFS model was obviously going to be wrong from the start. One of the reasons January will not average warmer than December which the CFS was forecasting pretty much all fall, is because it overdid the La-Nina from the start, a known CFS bias. I would expect the MEI to have a steady rise through the winter. This can also be seen on the reliable statistical and dynamical model blend on the link below,
The PDO did take a hard fall over November to -2.33, however, the SST distribution and layout would perhaps argue for a higher number over December, which has yet to be released. Regardless, the PDO will remain negative as expected throughout the winter into the spring. This will continue to make a true cross polar/+PNA pattern difficult to attain, especially in persistent manner.
The AO averaged extremely positive in November and December leading to both being very mild months across the entire CONUS. Cold air remained bottled up in the northern latitudes with a very expansive and intense PV sitting over southern Greenland pretty much the entire time as can be seen in the 500mb anomaly image attached.
However, recent EURO and GFS ensembles have shown this PV beginning to break apart and split up into smaller pieces, much like we saw in the previous two winters. The AO has fallen substantially thus far in the first week of January and the EURO weeklies continue this pattern and preview continued warming aloft at the poles. This should allow the pattern to consistently become more favorable for not only cold shots, but also our first realistic snow chances as we push through January.
As we see pattern shifts in the northern latitudes, the main thing to watch for will be the potential for blocking over Greenland and the Davis Strait. The analogs I looked at favor this as we head through winter, especially over February.
Overall I expect this winter to be back-ended, which was the general gist of my winter forecast and the trend of the analogs factored in. No one can truly predict the AO and its impacts on December being as warm as it was, however there were hints. That being said, the idea this winter was going to be identical to any other winter is being obtuse to the nature of meteorology, however, in my view there is no doubt the winter is just starting from a cold lovers perspective.