Anyone that has been watching the standard teleconnections such as AO, NAO, and PNA, would tell you that these indices are in an unfavorable state to bring cold weather to the US, and they would normally be correct. So what is the reason for the recent cold, and the deep cold that is forecast to reach our area around the 22nd?
Right now we're in a positive AO pattern which means there is an expansive area of low pressure (polar vortex) parked over the north pole southward to Greenland. Out west, we have a large area of positive height anomalies over the Bering Sea into Siberia. A +AO pattern would normally mean a nice toasty winter for most of us in the US as it keeps the cold air bottled up in Canada. But we have a little different setup this year compared to the last several years. The thought is that the large high pressure that's creating the -WPO is countering the the +AO and allowing cold air to penetrate the central part of the US.
According to a meteorologist:
"If you take the 20 most negative WPO Decembers, you get 13 with a -AO, 5 with a +AO and 2 with a neutral AO. So, overall from this small sample, the combination of a very anomalous -WPO with an overall +AO is the rarer combination. The 20 most positive WPO Decembers, on the other hand, have about 10 with +AO and 8 with -AO. That group, too, had about 2 with a neutral AO.
The -WPO/neutral AO years were 1956 and 1980. Both directed the pipeline of cold into Canada but the less negative AO either forced it well north of the CONUS (1956) or only into the eastern third of CONUS (1980). The PDO state likely played a role there. 1956 was, more or less, a blowtorch December. 1980 was exceptionally warm for the western 2/3s of the CONUS.
The +AO Decembers with an anomalous -WPO (in the 20 most negative only) looked more like a hybrid El Nino-like pattern. The cold anomalies were more focused in south, even if you take away the very cold 1983 year, and the northern-tier was above normal. This rare combination consists only of 5 years out of the 20 -WPO set: 1954, 1974, 1975, 1999 and 1983."
So as you can see, we're in a very unusual pattern.
In really simple terms, it opens up a hole for the polar air to come tumbling into. Again, all of this happening despite the AO and NAO in unfavorable states.
Here's what could happen according to WXRisk.com. "This combination of -WPO/-EPO have allowed the PNA values to rise toward positive values. With the +AO weakening, the +PNA goes "boom". Cross-polar flow would be firmly established. And IF... the -WPO moves north toward the Arctic Circle and the +AO goes negative... according to WXRisk.com, "the crap would really hit the fan".
:-) Lovely way to put it, but you get the jest.
All of these future "what if" scenarios will be determined over the next several weeks, but we may be moving into a pattern that will allow some VERY cold air to drop into the heart of the country. After this next week, the rest of the month appears to be well below normal for this time of year. I've seen model temperatures as cold as -40F invading the country with teens into Georgia, so LOTS of cold air will be available. Just to give you an idea of what may be on the horizon, here are the 2 meter temperatures from the GFS at 1pm on 12/28/13. AGAIN... we're 9-10 days away so much can change.
The thing that is so cool about this weather (no pun initially intended...!) is that it goes against what most people, including some meteorologists, might expect based on the setup we currently have now. And what is even more promising, is that the models are in such good agreement at this distance, which is pretty unusual. Tracking winter weather is like putting together a gigantic double-sided puzzle for us here in the southeast, and this puzzle appears to be coming together rather nicely despite the odds!
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