This would have been a video, and it may still be one later, but it’s hard to do videos when I’m at home, and they may have to wait until school resumes.
It is now 2012, and the heart of the winter is now beginning in earnest. December 2011 defied many forecasts that had significant cold invading the eastern half of the nation, and the ideas presented here that it should be warmer than normal instead turned out to verify nicely, as seen in the CPC analysis below:
January and February now lie before us, and some of the model ensembles, namely the GFS and CFSv2, are showing a gradual breakdown of the positive AO that we have had, allowing blocking to become established over the north pole during the next few weeks. Below are today’s 12z GFS ensemble 500mb mean valid January 19th (left) and the CFSv2 500mb mean Week 4 forecast for the last week of January (right).
A lot of this blocking may be due to a stratospheric warming event that is currently taking place, which may work itself down into the troposphere and produce blocking over the pole. What’s interesting about these model forecasts is that despite this, the arctic vortex doesn’t vanish, but rather simply moves a little bit, namely farther south deeper into Canada. The mean location of the arctic vortex in the winter is right over northern Baffin Bay, and all it has done here is moved southward over Hudson Bay instead. Something is keeping this strong low alive this winter, and so far it has prevented any blocking from developing over Canada or Greenland, and even as the models show the AO going negative, Canada remains a massive cold anomaly at 500mb.
If this indeed is to be the pattern that we see develop during the heart of the winter, I decided to take a look at the past and try to find some analogs that might give us a similar look. I searched for strong negative PDOs (November 2011 had the 3rd-strongest negative PDO for that month on record) with a fading multi-year La Nina and an AO that started positive in early winter but crashed later. Two winters fit the bill nicely: 1956-57 and 1971-72. Below are the PDO and AO values for these winters, with 2011 through November thrown in (December values aren’t in yet, but we know the PDO is still strongly negative and the AO was strongly positive).
Pacific Decadal Oscillation:
YEAR JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 1956 -2.48 -2.74 -2.56 -2.17 -1.41 -1.70 -1.03 -1.16 -0.71 -2.30 -2.11 -1.28 1957 -1.82 -0.68 0.03 -0.58 0.57 1.76 0.72 0.51 1.59 1.50 -0.32 -0.55 1971 -1.90 -1.74 -1.68 -1.59 -1.55 -1.55 -2.20 -0.15 0.21 -0.22 -1.25 -1.87 1972 -1.99 -1.83 -2.09 -1.65 -1.57 -1.87 -0.83 0.25 0.17 0.11 0.57 -0.33 2011 -0.92 -0.83 -0.69 -0.42 -0.37 -0.69 -1.86 -1.74 -1.79 -1.34 -2.33
YEAR JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 1956 -1.204 -2.029 0.470 -0.868 1.391 0.280 -0.215 -0.652 -0.202 1.139 -0.066 0.001 1957 2.062 -1.513 -2.013 0.238 -0.966 -0.760 -0.646 0.097 -0.956 0.903 -1.380 0.828 1971 -0.163 -0.922 -1.091 -0.583 0.679 -0.668 -0.578 0.818 0.153 1.185 0.419 0.824 1972 0.166 -0.195 -0.141 1.007 0.140 -0.049 -0.553 -0.082 -0.920 0.392 -0.380 1.238 2011 -1.683 1.575 -1.424 2.275 -0.035 -0.858 -0.472 -1.062 0.665 0.800 1.459
And here’s a montage showing 500mb anomalies for the Decembers of 2011, 1956, and 1971:
Notice the similarities to this December, with a strong polar vortex over northeast Canada and ridging over much of the United States bringing warmth to the east and central part of the country. Now if we progress into the heart of the winter and look at the 500mb anomalies during January and February of 1957 and 1972, we see a strong similarity to the CFSv2 and GFS ensemble forecasts for late January:
As blocking develops southwest of Alaska and up into the Arctic Ocean, the polar vortex is shoved southward into Canada, imposing itself upon the northern United States. The fact that it remains strong is important, because it means that the the U.S. won’t necessarily get entirely overrun with cold. Some forecasts now are setting their hopes on this new polar blocking to get the cold into the southern and eastern United States during late January. However, although the arctic vortex would shift southward if this pattern comes to pass, the limitations on how deep the U.S. troughs can get are still there. In December I talked about the angular momentum conservation which makes it hard for monster troughs to develop over the eastern U.S. when there is no blocking to the north. Here the same principle applies, and the flow is likely to want to remain fairly flat most of the time over the CONUS. We still don’t have any blocking showing up over Canada or Greenland on the models, and without that, it’s hard to rock the the eastern U.S. with frigid, snowy weather.
However, this pattern overall would be much colder than what we had in December, and a greater portion of the U.S., mostly the Pacific Northwest and the midwest, would see bitter temperatures at times due to the main jetstream flow that would now be out of Alaska and the north pole instead of off the Pacific Ocean. A fierce battleground would set up between cold to the north and warmth still hanging on in the southeast, causing snow and ice problems for the country’s midsection. We saw this in our two analog years being discussed here: 1957 and 1972. Here are the U.S. temperatures during January and February of both:
Notice that despite the arctic blocking, the south remains warm because Canada is still dominated by low heights aloft. There is far more to winter forecasting than just looking at the AO/NAO. They are indices that describe the pattern, but they do not give the full picture. They are but a shadow of the real thing, and it is the actual real thing that we should be looking at. A negative AO/NAO does not guarantee a cold, snowy eastern winter, as we have learned here from 1957 and 1972. One must look at the entire picture to see what’s going on.
The moral here is that the pattern is changing, likely in response to the recent changes in the stratosphere, and a shift to a generally colder pattern for North America may be on the way. If it does come, the models and analogs are in agreement that the arctic vortex will not be replaced by the blocking, but only shifted southward by it, imposing itself on the warmth in the southern U.S. and generating a battle between the different air masses right over the United States, causing nasty winter weather in many places and probably above-average snowfall in parts of the midwest and northern New England. However, such a pattern would still support normal to slightly above normal temperatures and below-normal snowfall for most of the eastern seaboard and southeast United States, in accordance with my winter forecast. You may recall that it also called for cold in the west and across the northern part of the country, which hasn’t happened in earnest so far except in the southwest, but may start to if the models are correct on this coming pattern change.
We shall see what happens!