October Preliminary 2020-21 Winter Outlook

Welcome to another winter outlook update for the Northern Hemisphere, including Europe, North America and Japan. For those who prefer a simple reading/conclusion and prefer not to read the long technical analysis, scroll to the Conclusions section at the bottom.

Model Outlook

December actually shows a more -NAO-like pattern on the ECMWF model, which is shown by significant blocking around Greenland, which benefits the UK and Southern Europe in terms of more cold. We also see a strong Aleutian ridge develop, which aligns with the Nina pattern and benefits the Rockies and the PNW. With a weaker Siberian High and Aleutian Low, the Japanese snow seasons suffers.

In January, we start to see a more +NAO outlook take hold in the Atlantic, which brings European ridging and less snowfall for the continent. Troughing over British Columbia benefits snowfall over the PNW and of course BC. Ridging weakens snowfall across the rest of the North American continent, leading to a dry month. Again persistent ridging over Japan weakens snowfall there too.

February brings a clear +NAO with a Greenland low and Azores High, leading to mild and wet conditions for Western Europe, and dry conditions for Eastern Europe. Ridging shifts slightly north, but persists over Japan. Again we see a favouring of snowfall for the PNW/BC at the detriment to the rest of North America.

We see similar scenes on the wider C3S multi-model ensemble for the whole of winter, with a strong Canadian vortex, bringing cold and snowfall over Canada and the Northern US. A strong Aleutian ridge dominates the image, with ridging stretching across America to Europe. We see a +NAO pushing the jet stream into the UK/Northern Europe. And a poor ridged outlook for Japan unfortunately.

ENSO

The current ENSO state is of a Eastern Pacific and perhaps could also be considered pushing into basin wide La Niña, as recently declared by the Australian Bureau of Meteorology.

We know it could be a reasonably strong one and will continue to do so, with the forecast colder than average surface and sub-surface sea temperatures. CFS shows a deep subsurface cold underneath the ocean surface that provides a strong base for this Nina can continue across the ENSO basin.

Following on from that, last discussion I had about this in Early September, I was looking at a Central Pacific Nina. Now I have shifted my personal thoughts have shifted towards a basin wide Nina over winter. Most forecasts show that the emphasis currently upon the Eastern Pacific in the most recent observations will shift towards a more general basin wide look in the coming months, as the Nina progresses (ECMWF as an example below).

So based on the NMME multi-model forecast ensemble, we are looking at a moderate-strong La Niña in terms of looking at the strength of the event:

CFS demonstrates a forecast for a strong Nina, some of the other American models show a weaker Nina and faster recovery.

Putting together an analog for a moderate-strong Nina winters since the 1980s, based in the Eastern Pacific extending into the entire basin:

  • Troughing/low pressure in the Eastern Indian Ocean and over Maritime Continent, certainly goes with the idea of a strong tropical standing wave over Maritime Continent.
  • Strong Aleutian Ridge, supports snowfall and cold in Northern America and Canada. Less North Pacific Jet extensions, less atmospheric rivers.
  • A very weak -NAO and more obvious Scandinavian blocking, which may help support snowfall and cold in Southern Europe/UK.
  • Decent troughing of east of Japan versus strengthened Siberian blocking, which supports Japanese snowfall.

Arctic Sea Ice

Sea-ice in the Arctic is continuing the lower than average trend of the past few decades.

Some analysis of this:

However low sea ice in certain regions is of more importance to us.

NASA GMAO

The sea-ice (NASA GMAO) forecast for the Arctic is pointing to low sea ice in the Barents and Kara Seas, which leads to more blocking in the Arctic region and the proliferation of a more negative AO state that benefits for snowfall, Eastern US, Japan and UK/Southern Europe.

Slightly lower than normal ice coverage out into the Chukchi Sea in that forecast, but mostly a neutral impact from that particular region. It is worthy mentioning that ice in that region is currently very low, so a quick turnaround would be needed to get to a more neutral impact stage. The below map from LDEO shows stronger than normal sea ice in the Bering Sea for DJF (and lower sea ice in the Barents-Kara Sea), and this isn’t an outlook I am really buying, it would be a very strong turnaround of sea ice to get to that stage.

LDEO

Overall I certainly believe we will see a lower than average sea ice year through winter, which in the case of the Barents-Kara Sea, will be a positive influence for higher snowfall and more cold for Europe/UK, the Eastern US and Japan.

Snow cover

We are currently seeing lower than average Siberian snow cover as of midway through the month of October.

Judah Cohen

It is however expected that snowfall will ramp up to some extent in Siberia over the next 10 days, as seen on ECMWF below.

So upon that analysis, I am envisioning a pretty average snow cover by the end of the month, middle of the road, but I may change this prognosis in the formal regional seasonal outlooks once the month has concluded. So it’s impacts are therefore pretty neutral on the climate.

MJO

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

To say the least, these forecasts are not overly great for most seeking out snow.

  • The US West Coast does best after an extension of the North Pacific Jetstream, allowing for atmospheric river events and higher precipitation/snowfall on the mountains. This is best done with a Phase 7/8 MJO.
  • The Eastern US receives it’s best snowfall during the MJO Phases 8/1.
  • Phase 3 + 10 days promotes a +NAO (good for Northern Alps snowfall, bad for Southern Alps/Europe and UK) and Phase 6 + 10 days promotes a -NAO (basically the reverse).
  • Japan receives the most snowfall (by the strengthening of the East Asian Winter Monsoon) around Phase 7.

Either way, the MJO’s impacts are to be unhelpful for snowfall prospects earlier in winter, and more helpful later in winter, as more westerly bursts enter the Pacific.

Sunspots

SILSO data/image, Royal Observatory of Belgium, Brussels

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

QBO

The westerly/positive QBO has popped back into the seasonal outlook, after a rather malformed easterly QBO phase this year.

This is of benefit to a +AO/+NAO state, and therefore snowfall in the Western and Central US. And of less help to Europe, the UK, Eastern US and Japan, in terms of producing cold and above average snowfall (less weakening of the SPV and less correlation with a -AO).

The easterly phase on top at 10mb is present, but it won’t descend far enough to be of importance to us this winter. More information pertaining to the dynamics of the QBO is in this below quote from last year’s outlook (obviously technical):

“However it gives the false impression that the QBO is this one number index, which isn’t what it really is. It really is an Oscillation that goes from 5-10mb all the way down to 90-100mb within the equatorial region in the stratosphere, bounded by more anomalies associated with ground conditions and the Hadley cells below, and the Semi Annual Oscillation (SAO) above it. This means when the QBO is positive and descending at 30mb, it may for example be negative zonal wind anomalies below 50mb to 100mb. This shows that the effects of the QBO are not strictly linear, because of this inconsistency within the same Oscillation, creating different effects depending on the height and phase of the descending QBO. There is also the strength of the QBO to contend with, and whether the trend is for a normal QBO phase progression, or a sudden change in progression. These are all factors which play into the QBO’s effect polewardly towards the Stratospheric Polar Vortex, therefore on it’s tropospheric counterpart and then the general hemispheric circulation”

If you wanted to pick the ENSO analogs above that best fit the current QBO progression, you get this:

Obviously looks good for Northern US and Canada and even over much of Europe with a -NAO setup. Too much troughing in the East Asian region for better than average snowfall for Japan.

Stratosphere Polar Vortex

Looking at the extended range forecasts for the stratosphere, we are looking at a pretty average and normal beginning to the rise of the SPV.

EC starts with the stratospheric vortex weaker, but largely charts a pretty average course.

The bias-corrected CFS model, charts a similar course if a little weaker than the average increase through November and December. So a pretty neutral look through the first parts of the winter from the available modelling.

The only other datapoint we have in relation to SSWs and the stratosphere is to do with the transport of ozone from the SH to the NH.

Tropical upper stratosphere is currently colder than average, that indicates a stronger than normal Brewer-Dobson circulation, pushing ozone towards the NH Strat polar vortex. So again we certainly have the capacity for inducing a SSW, it’s just whether the tropospheric factors allow for it.

Another analog for the ENSO years discussed above, this time for the SPV:

So interestingly we see a capacity for the creation of SSWs in the month of February under these years, so something worth thinking about.

North Pacific

We see a very warm North Pacific. This certainly may help with precipitation a bit for the West Coast, with the Nina not helping in this aspect. But it also means ridging in the middle of the North Pacific, helping to reinforce the Nina-esque Aleutian High. So basically reinforces the focus on the Northern US for snowfall and cold.

North Atlantic

We see a clear contrast between the warm SSTAs south of the line between Newfoundland and the Iberian Peninsula, and the cool SSTAs south of Greenland. One will find the jet-stream develop along this line. The cool SSTAs south of Greenland will help to induce troughing over Greenland, or cause a +NAO setup with a strong North Atlantic jet stream. This correlates with a milder outlook for the British Isles, more snow for the Western/Northern Alps and less for the Southern Alps.

Conclusions

To summarise the factors discussed:

  • A basinwide moderate (possibly strong) La Nina this winter.
  • A positive QBO
  • Solar Minimum
  • Atlantic SSTs favourable for a +NAO
  • North Pacific SSTs favourable for a Aleutian Ridge.
  • Average growth of Stratospheric Polar Vortex into winter, potential for SSWs later.
  • Phase 3-6 MJO centred tropical base state
  • Low sea ice in Barents-Kara Sea.
  • Average snow coverage in Siberia.

To apply this to the major snowfall regions of the NH:

  1. We see a better than average season for the PNW/BC in terms of snowfall, including into the Northern Rockies (MT, ID, WY)
  2. We see an average or slightly above average winter for Utah and Colorado.
  3. We see a below average season for the Sierra Nevada (CA).
  4. A strong season for the Midwest and Northern US.
  5. A mildly above average season for New England.
  6. An average snowfall season for the Mid Atlantic to NYC.
  7. A below average snowfall season for the Southern half of the US.
  8. Moving onto Europe, we see a colder December for the UK, and a pretty average winter overall, with a weaker backend of the season.
  9. The Northern Alps should see an average season, maybe slightly better than normal.
  10. The Southern Alps (and the rest of Southern Europe) should see a slightly below average season, with the best conditions early in winter.
  11. Honshu and Hokkaido are likely to see a poor snow season this year, the latter half of the season is likely to be the better one. Possibly a bit better relative to the average for Honshu, than Hokkaido.

Thank you for reading this winter outlook! Stay tuned for the individual regional outlooks, where we look these conclusions and details in.

2 thoughts on “October Preliminary 2020-21 Winter Outlook

  1. how’s this prediction playing out for the Alps? Not very well so far it seems. No snow anywhere. Any chance of an updated forecast for the Alps?

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s