A bit of weather:
aer.com
"This could be the most important weather event of the upcoming winter for the NH. As the energy transfer increases from the troposphere to the stratosphere, I expect an overall milder pattern across the NH with two possible regional exceptions – East Asia and western North America. It does seem to me that this scenario is consistent with the model forecasts heading into mid-December. Close to the peak of the PV disruption I would expect some immediate and short-lived tropospheric response with some regional severe winter weather. Then about two weeks later I would expect a more long-lasting and larger scale increase in severe winter weather across the NH but there will be places that will turn milder and it is difficult to know ahead of time. Based on past events the most likely place to see colder temperatures is Siberia (I know what you are thinking - who cares, its cold no matter what). But I do think that warm sea surface temperatures in the Gulf of Alaska favor downstream troughing and cold and possibly snowy weather across the interior of North America.If the PV disruption is significant and the coupling with the troposphere robust, this could potentially setup a cold January possibly persisting into February across large regions of the NH. On the other hand, if the PV disruption and/or the coupling with the troposphere are weaker than anticipated then we could be off to the races as far as NH temperatuers.
Ironically, I was thinking, though I argue that PV disruptions lead to more severe winter weather across the NH mid-latitudes, the only guarantee with PV disruption seems to be a multi week period of milder weather for Europe and the Eastern US leading up to and including the PV disruption. Much more ephemeral is a multi-week and even multi-month period of severe winter weather for those regions. It is almost damned if you do and damned if you don’t. If the PV is stable and strong that overwhelmingly favors mild weather and if the PV is significantly disrupted only milder weather is nearly for certain with much more uncertainty with the ensuing severe winter weather. The one possible exception seems to me are the reflected PV events where there seems to be a much stronger likelihood of North American cold weather east of the Rockies.
Near Term Conditions
1-5 day
The AO is currently neutral (Figure 1) with mixed geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). And with positive geopotential height anomalies across Greenland (Figure 2), the NAO is negative.
Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 25 November 2019 GFS ensemble. (b)
The predicted daily-mean near-surface AO from the 00Z 25 November 2019 GFS ensemble. Gray lines
indicate the AO index from each individual ensemble member, with the ensemble-mean AO index
given by the red line with squares.
This week troughing/negative geopotential height anomalies across Northern Europe and ridging/positive geopotential height anomalies across Southern Europe (Figure 2) will result in normal to below temperatures across most of Europe including the UK under westerly flow with the possible exception of Scandinavia where more northerly flow favors normal to below normal temperatures (Figure 3). This week troughing/negative geopotential height anomalies across much of Asia (Figure 2) will favor normal to below normal temperatures across much of Asia (Figure 3). The one exception is Southeastern Asia where ridging/positive geopotential height anomalies (Figure 2) will result in normal to above normal temperatures (Figure 3).
Figure 2. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 26 – 30 November 2019. The forecasts are from the 25 November 00z GFS ensemble.
This week ridging/positive geopotential height anomalies centered over Alaska and Greenland are predicted to force troughing/negative geopotential height anomalies in Western Canada, the Western US, New England and the Canadian Maritimes with more ridging in between the two troughs (Figures 2). This is predicted to result in widespread normal to above normal temperatures in Alaska, Northern and Eastern Canada and the Eastern US with normal to below normal temperatures across the Western Canada, the Western US and possibly in the Saint Lawrence River Valley (Figures 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 26 – 30 November 2019. The forecast is from the 00Z 25 November 2019 GFS ensemble.
Troughing and/or cold temperatures are predicted to bring new snowfall across Siberia, Northwestern Russia, Central Asia, and Scandinavia (Figure 4). However, intrusion of warm air on southerly winds will melt snow in Iran and Afghanistan (Figure 4). Troughing and cold temperatures are predicted to bring new snowfall to Alaska, Southwestern Canada, the Northwestern US, the Upper Midwest and the Canadian Maritimes (Figure 4).
Figure 4. Forecasted snowdepth anomalies (mm/day; shading) from 26 – 30 November 2019. The forecast is from the 00Z 25 November 2019 GFS ensemble.
Mid-Term
6-10 day
The AO is predicted to trend positive into positive territory this period (Figure 1) as geopotential height anomalies turn more negative especially across the Eurasian Arctic with mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 5). And with weakening positive geopotential height anomalies predicted across Greenland (Figure 2), the NAO is predicted to trend positive as well.
Figure 5. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 1 – 5 December 2019. The forecasts are from the 25 November 00z GFS ensemble.
Predicted ridging/positive geopotential height anomalies in the central North Atlantic will force downstream troughing/negative geopotential height anomalies and normal to below normal temperatures for much of Europe Including the UK with the exception of the Balkan states where southwesterly flow will transport milder temperatures to the region (Figures 5 and 6). Ridging/positive geopotential height anomalies will amplify in Western Asia downstream of the European trough with more troughing/negative geopotential height anomalies across East Asia (Figure 5). This is predicted to yield normal to above normal temperatures for Western Asia including the Middle East with normal to below temperatures for East Asia (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 1 – 5 December 2019. The forecasts are from the 00Z 25 November 2019 GFS ensemble.
The pattern is predicted to continue from the previous period across North America with ridging/positive geopotential height anomalies across the north with troughing/negative geopotential height anomalies to the south (Figure 5). This pattern is predicted to bring normal to above normal temperatures across Alaska and much of Northern Canada with normal to below normal temperatures in much of Southern Canada and the US(Figure 6).
Figure 7. Forecasted snowdepth changes (mm/day; shading) from 1 – 5 December 2019. The forecasts are from the 00Z 25 November 2019 GFS ensemble.
Troughing and/or cold temperatures will support the potential for new snowfall across much of Siberia, Scandinavia, Eastern and Central Europe, Northwest Russia, the Tibetan Plateau, Northeast Asia, Alaska, Canada and the Northern and Eastern US (Figure 7). Some snowmelt is predicted in the Western US (Figure 7).
11-15 day
With mostly negative geopotential height anomalies predicted for the Arctic (Figure 8), the AO is predicted to remain positive this period (Figure 1). With predicted weak negative pressure/geopotential height anomalies across Greenland (Figure 8), the NAO is likely to turn positive this period as well.
Figure 8. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 6 – 10 December 2019. The forecasts are from the 25 November 00z GFS ensemble.
Troughing/negative geopotential height anomalies across Europe are predicted to still dominate Europe (Figures 8) persisting a northerly flow with relatively cool temperatures across much of Europe including the UK this period with the possible exception of far Southeastern Europe due to a mild southwesterly flow (Figures 9). Ridging/positive geopotential height anomalies are previously over Western Asia are predicted to spread eastward with troughing/negative geopotential height anomalies confined to Far East Asia (Figure 8). This pattern favors normal to above normal temperatures across Western and Central Asia including the Middle East and the Indian subcontinent with normal to below normal temperatures in Eastern Siberia, Northeastern and Southeastern Asia (Figure 9).
Figure 9. Forecasted surface temperature anomalies (°C; shading) from 6 – 10 December 2019. The forecasts are from the 25 November 00z GFS ensemble.
Troughing/negative geopotential height anomalies are predicted to become more widespread across Alaska Northern Canada with ridging/positive geopotential height anomalies now across the Central US (Figure 8). This is predicted to favor a cooling trend across Alaska and Northern Canada with a milder trend across Southern Canada and the US (Figure 9). However residual cold temperatures are still likely across Southern Canada and the Northern US (Figure 9).
Figure 10. Forecasted snow depth changes (mm/day; shading) from 6 – 10 December 2019. The forecasts are from the 00z 25 November GFS ensemble.
Troughing and/or cold temperatures will support new snowfall across much of Siberia, Northwestern Russia, Central Asia, Scandinavia and possibly parts of Eastern Europe, western Alaska, much of Canada and the Northwestern US (Figure 10). Snowmelt is possible in China and the US Plains (Figure 10).
Longer Term
30–day
The latest plot of the polar cap geopotential height anomalies (PCHs) currently shows near normal PCHs in both the stratosphere and the troposphere (Figure 11). However, the trends in both are in opposite directions with stratospheric PCHs predicted to turn above normal next week, a sign of a sudden stratospheric warming (SSW) while tropospheric PCHS are predicted to turn below normal (Figure 11).
Figure 11. Observed and predicted daily polar cap height (i.e., area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecasts are from the 00Z 25 November 2019 GFS ensemble.
The plot of Wave Activity Flux (WAFz) or poleward heat transport shows a relatively active upcoming two-week period (Figure 12). One major WAFz pulse is for this week and another one for the second week of December (Figure 12). In the mid-stratosphere some negative/cold PCHS are predicted possibly a sign of a reflective event but it is confined to the mid-stratosphere which seems unusual to me.
Figure 12. Observed and predicted daily vertical component of the wave activity Wux (WAFz)
standardized anomalies, averaged poleward of 40-80°N. The forecast is from the 00Z 25 November 2019
GFS ensemble.
The stratospheric AO is currently slightly positive (Figure 1) reflective of a near normal PV. However, in response to the positive WAFz predicted over the next two weeks, the stratospheric AO is predicted to continuously trend negative and could be in strong negative territory by the second week of December (Figure 1).
Figure 13. (a) Analyzed 10 mb geopotential heights (dam; contours) and temperature anomalies (°C;
shading) across the Northern Hemisphere for 25 November 2019. (b) Same as (a) except forecasted
averaged from 6 – 10 December 2019. The forecasts are from the 00Z 25 November 2019 GFS operational
model.
Despite the strong circulation around the PV center and relatively low heights, the PV is not circular in shape but rather elongated and displaced towards Eurasia, signs of some disruption already (Figure 13). The counterclockwise flow around the PV center is bringing northerly flow to North America rather than westerly flow more common with a strong PV with a more circular configuration (Figure 13). The northerly flow is supportive of the predicted cold temperatures in eastern North America in Southern Canada and the US.
Currently there is warming and a ridge centered near Alaska in the stratosphere (Figure 13). But over time the new WAFz pulses are predicted to cause warming over much of Northern Eurasia and reinforce the ridging centered over Alaska with relatively warm temperatures widespread across the Arctic (Figure 13). Also, the PV center is predicted to remain displaced towards northwest Eurasia over the next wo weeks. The displacement of the PV center towards Scandinavia is likely contributing to a tropospheric reflection helping to deepen the troughing across Europe next week (e.g., Figure 5). The displacement of the stratospheric PV towards Eurasia is usually the first sign of a more significant PV disruption that is now being predicted by most models.
Figure 14. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for December 2019. The forecasts are from the 25 November 2019 CFS.
I include in this week’s blog the monthly 500 hPa geopotential heights (Figure 14) and the surface temperatures (Figure 15) forecast for December from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). The forecast for the troposphere is ridging across Northern Europe, East Asia, Alaska and Western Canada with troughs over Greenland and Iceland, Southern Europe, Central Asia, Eastern Siberia, the Dateline, and Eastern North America (Figure 14). This pattern favors relatively mild temperatures for much of Europe, Western Asia, Southeast Asia and Western North America with seasonable to relatively cold temperatures for Siberia, Northeast Asia, Eastern Canada and the Eastern US (Figure 15). The CFS has also shown little consistency from run to run.
Figure 15. Forecasted average surface temperature anomalies (°C; shading) across the Northern Hemisphere for December 2019. The forecasts are from the 25 November 2019 CFS.
Surface Boundary Conditions
Arctic sea ice extent
In this week's plot I included a plot of Arctic Seas sent ot me by David Birch; seems like a good idea. Arctic sea ice growth rate has accelerated but remains well below normal. Large negative sea ice anomalies exist in three regions: the Chukchi-Beaufort, west of Greenland-Candian Archipleagos and Barents-Kara Seas. The anomalies in the North Pacific sector have emerged as the most well below normal (Figure 16), however, based on model forecasts sea ice in the Chukchi-Beaufort Seas may grow more quickly in the next two weeks. Below normal sea ice in and around Greenland and the Canadian Archipelagos may favor a negative winter NAO. Based on recent research low sea ice anomalies in the Chukchi and Bering seas favors cold temperatures in central and eastern North America while low sea ice in the Barents-Kara seas favor cold temperatures in Central and East Asia, however this topic remains controversial. Recent research has shown that regional anomalies that are most highly correlated with the strength of the stratospheric PV are across the Barents-Kara seas region where low Arctic sea ice favors a weaker winter PV.
Figure 16. a) Observed Arctic sea ice extent on 24 November 2019 (white). Orange line shows
climatological extent of sea ice based on the years 1981-2010. b) Map of Arctic Seas. Images courtesy of National Snow and Ice Data Center (NSIDC). Snow and Ice Data Center (NSIDC).
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) anomalies have cooled and Neutral El Niño/Southern Oscillation (ENSO) conditions seem most likely (Figure 17). Observed SSTs across the NH remain well above normal especially near Alaska and along the north slope of Asia though below normal SSTs exist regionally especially west of South America. Warm SSTs around Alaska may favor mid-tropospheric ridging in the region this upcoming winter.
This is really outside of my expertise but the relatively warm SSts east of Africa and realtive cold SSTs west of Indonesai in the Indian Ocean are known as the positive phase of the Indian Ocean Dipole (+IOD). This has been shown to suppress convection over the Maritime continent. These corrrespond to some of the Madden Julian Oscillation (MJO) phases associated with warmer weather patterns in the Eastern US during the winter months ( Benedict et al. 2015).
Figure 17. The latest weekly-mean global SST anomalies (ending 17 November 2019). Data from NOAA OI High-Resolution dataset.
Currently no phase of the Madden Julian Oscillation (MJO) is favired (Figure 18). The forecasts are for the MJO to emerge into phase 1 and then to phase 2 over the next two weeks. Some MJO influence is possible across North American weather in the forecast period as theses phases favor high latitude blocking and troughing in the US transitioning to ridging in the Eastern US and troughing in the Western US.
Figure 18. Past and forecast values of the MJO index. Forecast values from the 00Z 18 November 2019 ECMWF model. Yellow lines indicate individual ensemble-member forecasts, with the green line showing the ensemble-mean. A measure of the model “spread” is denoted by the gray shading. Sector numbers indicate the phase of the MJO, with geographical labels indicating where anomalous convection occurs during that phase. Image source: atmos.albany.edu
Northern Hemisphere Snow Cover
Snow cover advance continues its climb across Eurasia and is currently near decadal averages. Snow cover will likely continue to advance especially across East Asia this week and Europe the following week as troughing and cold temperatures spread across the region. Above normal snow cover extent in October, favors a strengthened Siberian high, cold temperatures across northern Eurasia and a weakened polar vortex/negative AO this upcoming winter followed by cold temperatures across the continents of the NH.
Figure 19. Observed Eurasian (top) and North American (bottom) snow cover extent through 24
November 2019. Image source: star.nesdis.noaa.gov
North American snow cover melted back from decadal highs and is below last year at this time closer to decadal means. However snow cover advance should pick up with predicted colder temperatures.The early advance of snow cover across Canada this fall, has likely contributed to an early start of cold temperatures across the Western US and now Eastern US.
Video: Judah Cohen discusses the forecast methodology used in the blog to the NWS Bismark office
 (click image to play)
Video: How Arctic Amplification Affects the Polar Vortex |
