Monday, December 2, 2013

December Looking Downright Frigid

Forecasts for a warm start to winter (mine included) are quickly being beat down with the emergence of new data in the long range. (Note, this post will deal with how December will average out to be frigid in parts of the country, not necessarily continuous cold weather.)

WeatherBell Models
European ensemble projections of 500 millibar geopotential height anomalies for the 10-12 day forecast timeframe have a strong ridge of high pressure evolving over the Bering Sea and beginning to shift westward. If true, this would lead to a couple of consequences.

First of all, it appears that a Rex Block might try to set up shop across eastern Russia and into East Asia. This forecast graphic above somewhat depicts that, with negative geopotential height anomalies forming across East Asia as that big ridge tries to progress west. At the same time as the ridge retrogrades west towards Russia, the persistent negative East Pacific Oscillation that has been prevailing across the Gulf of Alaska may actually reverse into a positive EPO, which would cut off our chances of this cold weather. This is to be anticipated, as the Bering Sea correlation of ridging on November 30th should result in some warmer weather in the US for a brief period, almost right in the middle of the month. Long range ensembles are already catching on to this idea, expanding the Southeast Ridge north, as the +EPO and +NAO aid in warm weather prospects. Beyond that, though, cold weather should return. Very long range ensembles indicate the ridge will shift east once again, which would kick out the +EPO and return the nation to a cooler pattern once again. 

The Madden Julian looks to lock in this warm-up in the middle of December, as it pushes into Phases 3 and 4 around December 8th through the 12th, roughly, as model guidance shows below.

November-December-January Phase 3 & 4 temperature composites on left
ECMWF MJO projection on right
After this mid-month warm up, the ridge should retrograde as the Phase 3 & 4 MJO dissipates. Interestingly enough, mid-level height composites for Phase 4 of the MJO in December actually favor deep negative height anomalies in the Gulf of Alaska, which would help push the ridge west towards Russia and enhance that positive East Pacific Oscillation chance. See how everything's interconnected here?

When the ridge retrogrades, expect the warm weather to dissipate and at least average, if not below average temperature anomalies spread out over the nation. If you take a look at that ECMWF ensemble guidance image at the top of this post again, you'll see that very stormy weather over East Asia like we previously discussed. With GFS ensembles in agreement, it does seem that the middle and latter portions of December will be rather stormy and cold in the United States as a result of that East Asian storminess. If you recall, weather in East Asia correlates with weather in the United States 6-10 days later. It's still a ways away, but if these stormy East Asian projections verify, I don't see why a resurgence of colder weather isn't possible. 

WeatherBell Models
Adding to this whole mix is the fact that the majority of what is referred to as the polar vortex will be over North America during this timeframe. As the ECMWF ensemble image above, valid for December 10th shows, two major ridges of high pressure will be stationed at key points across the upper latitudes. The aforementioned Bering Sea ridge will be meandering around the north Pacific, and another ridge in western Europe will be lending its hand to contain the polar vortex in North America. There is ridging in the Southeast, a result of deep negative height anomalies over Greenland (positive North Atlantic Oscillation), and this will keep the coldest weather in the West, Plains, and parts of the Midwest and Great Lakes for the most part. Intrusions further east are probable, but for consistent cold, the aforementioned regions are at the highest risk. 

The stratosphere may also try to play a role here, as I discussed in yesterday's post.

The stratosphere is telling us that the Central and East US may be in line for some very cold weather in the second half of December, and verification from its last cold weather indication says that we should be monitoring this development.

Between October 30 and the first few days of November, the Central and East US experienced above normal temperatures in the upper stratosphere. This is displayed well in the Climate Prediction Center's reanalysis of the 30th in the image above. These few days between the end of October and the start of November also included slightly above normal stratospheric temperatures diving south from Canada, with below normal temperatures observed in the Pacific Northwest. Because the effects of stratospheric warmings are typically seen 2-4 weeks after the warming event, we took a look at surface temperatures from November 14-28 to see how well the surface temperatures matched up with the upper stratospheric temperature anomalies (note that this is a negative correlation relationship, meaning above-normal stratospheric temperatures will lead to cold surface temperatures, and vice versa).

Ground-level temperature verification from November 14th to November 28th, approximately 2-4 weeks after the height of this US upper stratospheric warming event, is nearly identical to the temperature anomalies experienced in the stratosphere. The regions that saw above normal stratospheric temperatures, including the Central and East US, as well as Canada, saw below normal temperatures during this timeframe. Similarly, the Pacific Northwest, which was bathed in slightly below normal upper stratospheric temperatures to kick off November, was included in slightly above normal surface temperatures from November 14th to November 28th. It would seem to me that the stratosphere had a direct involvement with these surface temperature anomalies, given how well the stratospheric and surface temperature anomalies line up.

In the last few days, the upper stratosphere has suddenly sprang to life, with a wide swath of much above-normal temperature anomalies extending from as far west as the Rockies to the Eastern Seaboard, through the Atlantic Ocean and off to far western Europe. This would seem to hint at colder than normal weather making a return to the nation somewhere in the December 14-28 timeframe, roughly 2-4 weeks after yesterday (November 30th). Normally, I wouldn't be putting so much faith in the stratosphere to dictate temperatures for the next few weeks. However, with its stellar verification in the prior event, I'm willing to give this stronger warming event a shot. That said, I'm going along with this colder than normal prognosis for December 14-28 with low confidence, as there are a few other factors that may try and intervene to bring about warmer temperatures for this timeframe than what the stratosphere is suggesting. But until those come to fruition, I'll go ahead with the stratosphere's call for cooler temperatures between December 14th and 28th.

Let's summarize.
The beginning of December will be cold, as we have been anticipating for some time now. Around the middle of the month, a warm-up is expected, as the MJO, Bering Sea Rule and +EPO come into play. Then, as the MJO dies down, a colder weather pattern is expected to regain control as the East Asian correlation comes into play, the -EPO re-emerges, and any effects from the warming stratosphere propagate down to the surface. Averaged out, the month of December should end up very cold/frigid for many parts of the country.