Sunday, November 24, 2013

December 4-8 Potential Winter Storm

It looks rather possible that the US will experience a winter storm around the December 4-8 time period.

As the image above shows, there will be a rather deep area of negative height anomalies over East Asia for November 28th, as exhibited by the dark blues in the top left of the left-hand panel. There is a knwon correlation that indicates storm systems or ridges of high pressure over East Asia can be reciprocated in the United States 6-10 days after the event's occurrence in East Asia. As we see a stormy period begin in East Asia over November 28th, I would expect December 4-8 to be the timeframe for a winter storm in the US as a result of this East Asian correlation. There are already a couple of guesses put out by the ECMWF control run. The most recent forecast has a storm system targeting the Midwest and Great Lakes for snow, as shown below (note the yellow arrow denoting the storm track)...

ECMWF Ensemble Control MSLP anomaly projection for December 5th
The second-newest ECMWF control run actually develops a stronger storm towards the end of this December 4-8 timeframe, unlike the December 5th projection from the ECMWF control...

ECMWF Ensemble Control MSLP anomaly projection for December 8th
As you can see, there is still a lot of time for this to be figured out, and the level of uncertainty is very high as far as who may be affected. What I am decently certain of, though, is that there is a chance for a winter storm in this timeframe.


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...