January 20, 2025
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) embarked on
an experimental process of regular research, review, and analysis of the Arctic
Oscillation (AO) and Polar Vortex (PV). This analysis is intended to provide researchers
and practitioners real-time insights on one of North America’s and Europe’s leading
drivers for extreme and persistent temperature patterns.
During the winter schedule the blog is updated once every week. Snow accumulation
forecasts replace precipitation forecasts. Also, there is renewed emphasis on ice and
snow boundary conditions and their influence on hemispheric weather. In late Spring,
we transition to a spring/summer schedule, which is once every two weeks. Snow
accumulation forecasts will be replaced by precipitation forecasts. Also, there will be
less emphasis on ice and snow boundary conditions and their influence on hemispheric weather.
Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.
The AO/PV blog is partially supported by NSF grant AGS: 1657748
Summary
- The Arctic Oscillation (AO) is currently neutral and is predicted to remain positive the next two weeks as pressure/geopotential height anomalies across the Arctic are currently mostly mixed and are predicted to remain mostly mixed to negative over the next two weeks. The North Atlantic Oscillation (NAO) is currently negative with mostly positive pressure/geopotential height anomalies across Greenland and the NAO is predicted to be mostly positive the next two weeks as pressure/geopotential height anomalies are predicted to reverse to negative across Greenland.
- The next two weeks troughing/negative geopotential height anomalies across Greenland will support ridging/positive geopotential height anomalies across much of Europe with the exception of troughing/negative geopotential height anomalies across Northern Europe this week. This pattern will support mostly normal to above normal temperatures across much of Europe including the United Kingdom (UK) with the exception of normal to below normal temperatures across Scandinavia and the UK this week under low heights.
- The next two weeks ridging/positive geopotential height anomalies are predicted to dominate Asia with the exception troughing/negative geopotential height anomalies across the Urals and far northern Siberia this week and then Western Siberia pushing into East Asia next week. This pattern favors widespread normal to above normal temperatures across much of Asia, with normal to below normal temperatures across the Urals and far northern Siberia this week and then Western Siberia pushing into East Asia next week.
- The general pattern across North America this week is ridging/positive geopotential height anomalies centered in Alaska, and the Gulf of Alaska supporting troughing/negative geopotential height anomalies across Central Canada and the Eastern United States (US). The next week troughing will return to Alaska and spread across Canada with ridging across the US. This pattern favors normal to above normal temperatures across Alaska and Northern and Western Canada with normal to below normal temperatures across Southern and Eastern Canada and much of the US this week. However, next week above normal temperatures will become more widespread across Canada and the US with normal to below normal temperatures limited to Alaska, Northwestern Canada and the Northeastern US.
- This winter has been all about the polar vortex (PV) alternating among three different states: strong, a Canadian warming but most of all a stretched PV. This pattern looks to continue through January but could it all change in February? Also still watching the battle between high latitude blocking and a strong PV, which has been at a stalemate all winter long. Models are predicting near record strong PV will it finally gain the upper hand?
Plain Language Summary
What you see is what you get seems to be the theme of the weather this winter. Overall looks mild for Europe while I see more opportunities for cold in Canada and the US for the end of January and heading into February. I also think that a realignment of the atmospheric circulation favors an Arctic outbreak into East Asia the end of January and into early February. But overall, I don’t see why the parade of stretched polar vortex events this winter that has focused the relative cold in the US this past January (see Figure) won’t continue into February. Northern Europe has also been cold this month so far, but it does seem that the catalyst for the cold, Greenland blocking, shows no signs of returning for the foreseeable future.
Figure. Estimate of the observed surface temperatures (°C; shading) from 1 January 2025 – 19 January 2025 based on GFS initializations and the GFS forecast from the 20 January 2025 forecast.
Impacts
It is really amazing how it feels like I can just change the date on the blog (update by seven days) and then hit publish. This really is the winter of the stretched polar vortex (PV) and in quick even accelerating succession and seemingly in an infinite loop of lather, rinse repeat.” The stretched PV events or periods are punctuated by either a more circular or strong PV or a Canadian warming. Two more stretched PVs are predicted by the end of January bringing the monthly total to four (about once a week) and the seasonal total since the end of November to eight. The rapid transitions of the PV to different modes or phase can be seen in the latest PV animation in Figure i. There is the seventh stretched PV of the season on the 20-24 January and then the eighth stretched PV after 27 January through the end of the animation on 4 February. Though I will admit in the animation it just looks like one long stretched PV. And though at the end of last week it looked to me like two separate events, now I am not sure.
Figure i. Initialized 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 13 January 2025 and forecasted from 21 January to 04 February 2025. The forecasts are from the 00Z 20 January 2025 GFS model ensemble.
In the blog over the past several weeks I have been focusing on the wave reflection diagnostics and show it again in today’s blog. Wave reflection is the physical underpinning of stretched PV events, and it has seemed it has occurred continuously throughout the month of January. During wave reflection, wave energy goes up over Asia, bouncing off the stratospheric PV and then downward over North America. The downward wave energy amplifies the ridge-trough wave over North America and determines the strength or amplitude of the North American wave and the axis or position (see Figure ii). Once again, the wave reflection seems to be uninterrupted for the next two weeks. Wave reflection is supportive of two more or maybe one continuous stretched PV right through the end of January. One reason that I am considering them as two separate events is because it is cold this week in the Eastern US (see Figure 3), it turns milder for part of next week (see Figure 6) and then the cold returns clearly focused in the Northeastern US for the last two days of January. The cold looks more impressive in the European ensembles than the American models and I do believe that the European model has the best handle on the 7-14 day time period.
Figure ii. Longitude-height cross section of geopotential eddy height anomalies (shading) and wave activity flux (vectors) forecasted for a) 21 January through 25 January 2025 and b) 26 January through 30 January 2025 . The forecasts are from the 00Z 13 January 2025 GFS ensemble.
Stretched PVs deliver severe winter weather not only to North America but also East Asia but clearly nothing as impressive as what is transpired across North America this month (see Figure in Plain Language summary). But as can be seen in the animation of 500hPa geopotential heights shown in Figure iii, the high latitude blocking is predicted to become spread across the Eurasian Arctic and disappear across the North American Arctic. This should favor East Asia for an Arctic outbreak for the end of January and even into early February. However, with low heights draped across the North American Arctic cold air should build up across Alaska and Northwestern Canada, potentially setting up a future Arctic outbreak into the US.
The weather models are predicting the absence of Greenland blocking and with Greenland troughing instead for the foreseeable future (see Figure iii). This should allow high pressure ridging over Europe and an overall milder pattern for at least the next two weeks (see Figures 3, 6 and 9). For now, I don’t see much evidence for a return to colder weather, unless models push the European ridging further north into the Arctic.
Figure iii. Initialized 500 mb geopotential heights (dam; contours) and decameter anomalies (dam; shading) across the Northern Hemisphere for 20 January 2025 and forecasted from 21 January to 04 February 2025. The forecasts are from the 00Z 14 January 2025 GFS model ensemble.
In last week’s blog I discussed the possibility of cold air making it into the Eastern US as January comes to a close. The European has committed to this solution much more so than the American models with the European predicting a decent shot of cold air into the Northeastern US the last couple of days of January but limited in space. Certainly, the cold is much more limited than this week’s historic Arctic outbreak. I think this makes sense given that the northwesterly flow in the stratosphere is really directed at the Northeastern US and Southeastern Canada and not the Plains. The American models are much more tepid in their forecasts of cold air (see Figure 9), but from what I have seen, the European model has been performing the best (I guess no surprise there).
Looking ahead into February, I believe that the forecast remains complicated and challenging. There are two major factors that are making the forecast of US temperatures heading into February especially challenging. Both the GFS and European ensembles are predicting a near record strong PV (see Figure 1). It is very difficult to get cold in the Eastern US with a PV that strong instead, if there is cold it is much likely to be in the Western US. However, when I look certainly at some but not all the operational models, they are suggestive of yet another stretched PV the first week of February (see Figure iv). But admittedly there is no consistency in any of the operational forecasts and no support from the ensembles.
Figure iv. Forecasted 10 mb geopotential heights (dam; contours) and anomalies (meters; shading) across the Northern Hemisphere for 4 February 2025. The forecasts are from the 0Z 20 January 2025 ECMWF operational model. Plot taken from https://weathermodels.com.
I think the pattern most supportive of stretched PVs is ridging across northwestern Eurasia (Scandi-Ural blocking) and Alaskan and Gulf of Alaska ridging. Instead, the models are predicting ridging to stretch from Europe to Eastern Siberia or across the entire Eurasian Arctic (see Figures 5 and 8). I don’t know for sure, but this pattern seems less supportive of stretched PVs. A pattern that is less supportive of disrupting the PV and the never-ending wave reflection seems to be the key factors for the predictions of a near record a strong PV. With a PV that strong and especially if it becomes circular in shape a mild pattern across not just the US but the entire mid-latitudes has to be strongly considered.
The second complicating factor is the tropical convection. Tropical convection is currently supportive of cold in the Western US and mild in the Eastern US at least into early February (see Figure 18). The models keep predicting the cold to shift into the Western US just to eventually lose it just like Lucy and her yanking the football away every time Charlie Brown goes to kick it. Just last Friday, the 0Z European ensembles had all this cold air in western North America at the end of January that evaporated in the 12Z run and instead shifted some of that cold into the Northeastern US. For now, my focus remains on the variability in the PV.
At least for the foreseeable future, I still think that the PV will be supportive of cold in the Eastern US, at least when it stretches. Into February, I still favor a continuation of the pattern we have seen all winter, stretched PVs that most likely favor cold in the Eastern US and some kind of relaxation. The ensembles are predicting the most circular PV of the season so far for early February and there is every reason to expect a fairly mild pattern for the US for the end of January and into early February with the possible exception of the Northeastern US. However, I am expecting another stretched PV either at the end of the first week or second week of February based on the wave reflection, the return of ridging to the Gulf of Alaska (right now purely inferred by me from the model forecasts) and the predicted buildup of Arctic air in Alaska and Western Canada. If I am wrong and the PV does not stretch, then a much milder solution for the US should be expected for at least the first half of February and maybe all of February.
I was happy to receive my 5 and bit inches of snow last night in Boston. But that is fairly routine and only seems less so because of our two and a half year snow drought. But similar amounts of snow could fall along the Gulf of Mexico from Houston to New Orleans and Tallahassee. Will be interesting to see if it materializes where it would be an historic snowstorm. Will cause havoc down there but meteorologically incredibly interesting.
Near-Term
This week
The AO is predicted to be mostly positive this week (Figure 1) with mixed to mostly negative geopotential height anomalies across the Arctic and with mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). With predicted mostly negative geopotential height anomalies across Greenland (Figure 2), the NAO is predicted to be neutral to positive this week.
