North America Winter Outlook 2020-21

Welcome to my North American outlook, discussing the models and drivers to come to conclusions about the winter ahead. Discussion is reasonably technical, so if one wishes just to see the results scroll to the bottom.

Seasonal models

This outlook is to be driven by the climate drivers, rather than modelling, but we will give it a look for a point of comparison.

The ECMWF model shows for the Jan-Feb-Mar period, troughing over Alaska and over the Northwestern US, as well as ridging over the East Coast and the SW. This would bring the best snowfall for the PNW and the Northern Rockies.

Sunspots

SILSO data/image, Royal Observatory of Belgium, Brussels

Same as the last few outlooks, basically we are still in a solar minimum, which in theory helps a negative Arctic Oscillation. This would therefore assist snowfall in Eastern North America, but I don’t weigh sunspots highly as a tool in winter seasonal prediction.

QBO

We are looking at a descending positive/westerly QBO, that tends to have an effect at avoiding stratospheric warmings. And as we can see a certain effect on the North American climate:

The general effects shown by the analog on the right:

  • Greenland ridge and Atlantic ridge wedge Canadian trough from the east.
  • QBO driven Aleutian ridge concurs with the ENSO prognosis.
  • Warm and ridge-driven in the south, particularly in the SW.
  • More active and cold in the north, the most in New England and the Midwest.

Our specific ENSO and QBO analog shows:

  • A positive Arctic Oscillation, shown by the trough.
  • Weak troughing for the Eastern US and parts of Western Canada
  • Same Aleutian ridge, with a mild ridge in the SW.

So not great for snow in the Southwest, but potentially hopeful for the Northern US especially on the eastern side of the continent.

ENSO

We are heading into a Moderate Nina for this winter, with us expected to be at the depths of this Nina in the next month or so.

We are in the midst of an east-based La Niña that is stretching throughout the ENSO basin, but still focused on the Eastern end of the Pacific.

Our humble moderate Nina (east-based to basinwide) analog shows:

  • Aleutian ridge pushing more towards the Gulf of Alaska and to the West Coast.
  • Canadian trough, driving cold and snow for Canada and the Northern US.
  • A mild SE CONUS ridge.
  • Less precipitation/snowfall for California and parts of the Southern US.

Stratosphere

I see an average to slightly above average chance for a SSW, most likely in January:

  1. A descending westerly or positive QBO will weaken chances for the development of Sudden Stratospheric Warming events.
  2. Brewer Dobson Circulation is stronger than normal in both of the NH and SH subtropics as well as over the tropics this month. I expect a stronger than normal BDC over the winter. This increases ozone in the SPV during the winter, which increases the chance of a SSW.
  3. Average Siberian Snow Cover does not really affect the stratosphere, but the low Barents-Kara Sea Ice does improve chances for a SSW.
  4. Solar minimum is still taking place at this point, favouring a -AO and potential stratospheric disruption.
  5. December’s +EAMTs and amplified pattern could help to amplify a stratospheric disruption down the road.

Taking a look at the modelling for the stratosphere this season, January looks like the most interesting month for potential stratospheric disruptions on both EC and UKMO, with EC the more enthusiastic option.

These processes if a SSW is successful and is able to propagate from the stratosphere to the troposphere, would be able to assist roughing and snowfall on the Eastern side of the North American continent.

Sea Ice

The NASA GMAO forecast for around Christmas is for lower than average sea ice in the Barents-Kara Sea Ice region, which is more likely to induce a -AO/-NAO and to help to disturb the stratospheric vortex. This would help snowfall/troughing in the Eastern US.

Snow Cover

October in terms of the expansion of the Siberian snow cover was pretty average with a pretty neutral impact. Late October and Early November brought a slump of the expansion, which may have had a negative impact down the line on the potential extratropical cooling effect and setting up of SSWs.

November saw a large increase of snowfall extent across Eurasia, and the resulting impact of that has been the strengthening of the Siberian High. This helps to increase pressure on the stratospheric polar vortex.

Although October wasn’t very exciting in Siberia, November shows some hope for a colder winter for the Eastern US, with potentially a more -AO/-NAO outlook focused on the potential for a SSW.

MJO


Here we have the expected tropical wave pattern for the winter ahead, that focuses on a Phase 2-3 (Dec) to phase 4/5/6 (Feb) approach to the winter, with a slow shift of focus from the Eastern Indian Ocean as the focus of tropical activity, to the Maritime Continent region (North of Australia).

This may assist Western US troughing in the Phases 2-4 of the MJO. We want to see MJO Phases 7-8-1 for snowfall and cold correlation in the Eastern US.

Late October/November have featured a far more positive AAM/amplified pattern, which makes things more interesting with the potential for SSWs and disruption to the polar vortex.

It has also delivered good results in terms of cold for the Eastern/Southern US this month, which could prove more fruitful if it continues.

North Pacific

This is the January North Pacific SSTA forecast, and is possibly quite decent for the Eastern US (and less so for the West Coast). We see that strong warm anomaly off the coast of the PNW, which could help to anchor a ridge closer to the CONUS than the Aleutians, which would have positive impacts for the Central and Eastern US for cold and snowfall.

We also have a warm SSTA setup from 10N and north, and tight anomaly bars/warm waters coming off Japan, which as part of a +PMM helps to drive the North Pacific jetstream. This may provide some help to California, in what is likely to be a lacking winter, and have positive downstream effects for cold/snowfall for the East.

Conclusions

  1. We see an average season for the PNW/BC in terms of snowfall, including into the Northern Rockies (MT, ID, WY). It could possibly be better inland over the Rockies than over the coast, if a Gulf of Alaska ridge pushes in like above.
  2. We see an average winter for Utah.
  3. We see an average to slightly above average snow season for Colorado.
  4. We see a below average season for the Sierra Nevada (CA).
  5. A strong snowfall season for the Midwest.
  6. An above average winter season for New England.
  7. An average to slightly above average snowfall season for the Mid Atlantic to NYC.
  8. An average snowfall season for the Southern half of the US, better the further north one gets.

Thanks so much for reading. Long term forecasts on a sub-seasonal scale for snowfall coming very soon!

Seasonal outlooks tend to have bias and errors, due to the fact that these forecasts are so far out. So don’t use these outlooks to make important decisions. These outlooks is meant to be interesting information, that can help to see what the season would be like. 

I appreciate all for reading, please follow me on Twitter @longrangesnow and subscribe to my email list by clicking on the tab on the main header above

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