Legit Cold Coming?
It's cold now, but this is nothing unusual for December. The cold air I'm going to discuss isn't anything unusual either, but it could be MUCH colder than what we have now. Long range guidance is hinting at a very healthy dose of cold air coming around mid month. It's too early to speculate about how strong and how long.
I like to look at 3 teleconnective indicies in the winter: Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and the Eastern Pacific Oscillation (EPO). Lining up all three as negative does give a high degree of confidence of a significant push of cold air. Guess what? They mostly go negative soon!
From the weatherbell.com glossary.
AO: Arctic Oscillation- The AO refers to opposing pressure patterns in the middle and high latitudes of the Northern Hemisphere. The negative phase is represented by high pressure over the polar region and lower pressure at the mid-latitudes. The opposite occurs during the positive phase. The AO plays a role in weather patterns as it affects the track of storms, and the location of cold air. When the AO index is negative, there tends to be high pressure at the pole and weaker zonal winds, so more cold air is able to drop down into the mid-latitudes.
EPO: Eastern Pacific Oscillation - A dipole pattern similar to the NAO in the Atlantic, but located in the eastern Pacific. There is a tendency for heights/pressures/temperatures to be higher to the north and lower to the south in the negative phase and lower to the north and higher to the south in the positive phase. The negative phase corresponds to widespread cooling over central and eastern North America and the positive phase to warming.
NAO: North Atlantic Oscillation- One of the most commonly looked at teleconnection patterns for weather forecasting. The value of the NAO is determined by the differences in pressures over the high latitudes and central latitudes of the North Atlantic. The positive phase of the NAO is characterized by a low difference in pressure between the two regions, a below average pressure over the high latitudes, and an above average pressure over the central latitudes. The opposite occurs during the negative phase. The NAO is significant in forecasting because its state affects the intensity and location of the jet stream and storm track in the North Atlantic. As a result, significant changes in temperature and precipitation occur in Europe and North America based on the NAO.
Model forecast of those 3 teleconnective indicies
NAO - See the horizontal zero line? This goes out to December 17th. Mostly negative!
AO - mostly negative
EPO - going negative soon
A birds eye view of the northern hemisphere from the Euro ensemble mid month
It seems as if the Euro Ensembles favor ridging in the part of the world needed to bring cold to us! Once that ridge builds up into western Canada and Alaska, it dislodges arctic air and sends it south into the trough over the central and east.
Euro temperature anomalies at 5000 feet show where the cold and warm may be located. Above average temperatures in the eastern Pacific and Alaska with a push of cold into the central U.S.
In summary, there is a reasonable degree of confidence in a bigger push of cold air around mid month. How strong and long long it sticks around is a big question. Considering the source region, the time of year, and the teleconnective indicies, it could be impressive. I will caution you, this is long term stuff and I have seen it turn out the complete 180 degree opposite. It will be fun to watch unfold.