Arctic Outbreaks Herald Start of December

A very cold pattern will herald the start of December, and unofficially the start of winter 2013-2014.

The ECMWF ensemble system and GFS ensemble system forecasts agree that we will see a pattern develop in the next 10+ days which will include two substantial bodies of high pressure in the northern Atlantic and Pacific. The Bering Sea will possess one half of the ridging system, while Greenland attempts to reel in the second component. These two ridges are then projected to split the polar vortex into two pieces, one of which will slide down into Europe, while the other pushes down into North America. 

It is expected that there will be a prolonged period of lower pressures across North America in the first couple of weeks of December, and this will lead to below normal temperatures for much of the nation. It is anticipated that this cold air will originate from Canada and will be transported from the Northern Plains into the Central Plains on east. Temperature anomalies from both ensemble systems have deep below normal anomalies stretched out across nearly all of the country. The likelihood of this prolonged cold weather is rather high, as the stratosphere will also be undergoing a Wave-2 response to two high pressure systems applying pressure in just about the same areas as the ensemble projection above shows.

Now, there are two main types of stratospheric events that disturb or split the polar vortex. There is a Wave-1 response, which involves the polar vortex becoming elongated and/or weakened, however a split does not occur. This can result in displacement of the vortex out of the Arctic, but that prospect is not as likely as it is in a Wave-2 scenario. A Wave-2 stratospheric response involves the polar vortex being split into two main vortices, as the image from NASA shows above. While the temperature images show the Wave-1 and Wave-2 (top and bottom rows, respectfully) responses occur over Greenland and Eurasia, especially with the two split vortices where one vortex goes over Greenland and one goes into Eurasia, the split can lead the vortices into any land mass, not just those two regions.

The ECMWF Ensemble set confirms the idea of a particularly harsh outbreak of cold weather to kick off December just over a week in...

Andrew