Thursday, November 14, 2013

Early December Long Range Outlook

December is still a ways away, but it is possible to see what the opening days of the last month in 2013 will be like. Let's take a look at how the atmosphere should be shaping up as November ends.

Ensemble guidance is in agreement that the Arctic Oscillation will quickly shift from a strong positive phase it is currently in, to a more negative stance by the days before Thanksgiving. This means that, rather than the stormy and cold activity that has been seen across the Arctic Circle recently, ridges of high pressure will start to push into the Arctic, resulting in a weakening polar vortex and thus a more neutral/negative Arctic Oscillation. The ECMWF ensemble guidance image above shows this trend well, and its ideas are matched by the GFS ensemble's projections. Long range ensemble guidance maintains this negative Arctic Oscillation, and as we enter into December, I am not ruling out a pattern shift in favor of some wintry weather in the East US.

European ensemble guidance for the North Atlantic Oscillation (NAO) follows the same pattern as the Arctic Oscillation, with a moderately positive NAO peaking in the next few days before dropping down into neutral and negative territory. The negative NAO signifies high pressure ridging prevailing over Greenland, and this would lead to additional chances for general wintry weather across the Central and East US. This is a possibility if recent ideas of a pattern change come to fruition, and this would mean good news for winter weather folk across much of the nation.

One item that is trying to throw a wrench into this early December pattern shift is some massive ridging across the Bering Sea. It is known that mid-level geopotential height anomalies correlate positively to height anomalies in the US approximately 2.5-3 weeks after the height anomalies in the Bering Sea. Thus, the mammoth high pressure system currently flooding the Bering Sea will come back to bite the US around the first couple days of December on through the first full week of the last full month in 2013. Beyond that time period, if the negative AO and negative NAO are maintained, I can see the possibility that the US may get off to a cold (and potentially snowy) winter.

All in all, it's looking like a big warm-up for much of the nation in the opening days of December, and depending on how the Northern Hemisphere works itself out in the days around Thanksgiving, we may see the emergence of some truly wintry weather across a good chunk of the US.