Early September Seasonal 2020-21 Winter Preliminary Outlook

Welcome to another seasonal forecasting season for the NH, under albeit unfortunate circumstances worldwide. Today we will take an initial look at some of the factors involved with the season ahead.

Seasonal Models

In this section, we are going to look at a selection of freshly minted seasonal model predictions.

We start December on EC Monthly, with a strong Siberian High, an Atlantic Ridge and a Aleutian High setup. This causes lower snowfall for most of the US, except perhaps the Northern Great Plains. We could see a decent early season for Japan come off, with Siberia looking the way it is. Europe looks pretty average in terms of temperatures, and potentially more precipitation for parts of the Alps.

In January, EC sees the Siberian High stick around for good snowfall potential for Japan. Europe starts to see the influence from a +NAO, which means wetter conditions for the UK, and more snow for the Northern Alps. We see the SE US/Atlantic Ridge and the Aleutian High push against a Central US trough, benefiting Colorado and the Inland PNW, as well as bringing cold to the Great Plains.

In February, EC sees a similar situation for North America, with the Nina-esque Aleutian High dictating the pattern. The +NAO strengthens, which means the impacts for the UK and Northern Europe only increase with a mild and wet outlook. This means it will be dry for other parts of Europe with a European ridge. The Japanese season slows down with a more mid-latitude belt of ridging affecting snowfall for the Japanese Alps.

March presents limited opportunity for Japan, expecting a poor late season on EC. We see a vague -AO, but still a +NAO, meaning similar effects for Western Europe, and the downstream ridging makes it warmer for the Eastern half. Northern US sees troughing and potential late snowfall, the Southern half doesn’t get to see much cold with a persistent ridge.

CFS for the entirety of the NH winter focuses on an Aleutian ridge as well with troughing for the North-Central parts of the US and Canada. It also features a +NAO meaning more snowfall for the Northern Alps, and wetter/mild conditions for the UK and Northern Europe. It features a substantial Siberian High, which would be of benefit to Japan.

CANSIPS (Canadian) focuses on a strong Northern US snowfall setup with a Canadian vortex. We also see the same +NAO effects that are seen on the other models. The ridging around Japan, rather than towards Siberia is of little benefit to Japanese snowfall, and may hamper that season. We see a very +AO, Arctic-driven outlook for the winter from it.

ENSO

If we take the model average from all of the seasonal models at play here (graphic below), we get a borderline La Niña possibly. You could argue under the definitions set by the Australian Bureau of Meteorology that it doesn’t even qualify as a Nina. One thing you could note in addition to the actual mean, is that there are outliers towards both ends, of course NASA going for a strong Nina, but a number of models including the Korean, Chinese and a few of the minor, sometimes odd American models are strong outliers for a warm neutral state or even a Nino in the first half of next year.

So if we instead take the median bunch of models around the average, we see a cooler trend towards a firmer Nina. So in practical terms, we are looking at a borderline-weak La Niña, peaking in November and December.

But of course the depth of the anomalies is only part of the story, we also must look at the location of these anomalies. We are going to take some EC modelling from last month in this instance:

We are arguably looking at a more Central Pacific or Modoki based Nina if we are going to take this for face value. There is some research backing stronger precipitation in the Japan region during the winter during a Nina Modoki. But in general we see conditions from a Nina that favour snowfall in the Northern parts of the US and Canada. And for Europe, we see a +NAO type set-up, so wet for the UK and Northern Europe, snowy for the Northern Alps, and less snowy for the Southern Alps.

I took a wide selection of Nina Modoki/Central Pacific years, and we see pretty similar themes. A strong Aleutian high, which pushes troughing into Northern and Western North America. A strong +NAO and therefore jet stream coming off the NA Continent towards the UK and Europe. And finally a +AO, indicated by the trough over the Arctic. All this points to a strong snow season for the Western and Northern US, as well as the Northern Alps.

Sunspots

SILSO data/image, Royal Observatory of Belgium, Brussels

We are at 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.

Tropical Forcing

The tropical base-state over these past few months has been focused on the Indian Ocean, or around MJO Phases 1-3. The current forecast from CFS regarding tropical forcing is shifting towards a more Maritime Continent dominant tropical base-state, and suppressive of the MJO phases that traditionally benefit the Eastern US, Europe and Japan in terms of widespread cold and snowfall.

The most suitable SST region for tropical forcing over winter this season is Maritime Continent, so CFS guidance makes sense. In general, leading up to a Nina you don’t see a great deal of MJO action deep into the Pacific, but at the peak of the event (during this winter in this case), you see greater MJO amplitude. This could be of benefit to the Western US in terms of trying to break through that Aleutian High for some jetstream action (specifically referring to California here).

QBO

The westerly/positive QBO has popped back into our lives, 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. But the QBO is obviously a dynamic factor at play here, so best to keep track of it over the next few months, and I will clarify in the final seasonal outlooks.

Snow & Ice

Not a great deal of news on the freezing season that is useful to us in the seasonal outlook, but it is worthy of note that Arctic sea ice is at the second lowest minimum in recorded history. In general, of course there is more heat and instability in the polar region and this is just a demonstration of that. Ice cover on the Russian side is particularly low, which actually may help a more unstable -AO pattern develop later in the Spring into Winter, but it is really too soon to tell. We need data from October for this particular process.

Local SST patterns

Finally we will take a brief look at the forecast for NH SSTAs.

Both observations that can be deducted are inline with the rest of the outlook.

Warm SSTAs in the North Pacific assist with ridging in the Aleutian region, in line with a Nina climatic response.

And the Atlantic has a large warm patch of SSTAs that enhance ridging off the US Coast, and the interaction with that small patch of Colder SSTAs south of Greenland encourages a +NAO pattern.

Conclusions

These are just really rough initial conclusions to be made, these will be amended and played with over the next month or so. Stay tuned for further outlooks.

  • Good snowfall season for the Central parts of the US (Colorado, Inland PNW/Alberta, Great Plains), and for the Northern Alps.
  • Some potential, but still under the influence of a ridge (Utah, California, PNW Coast)
  • An average, perhaps early season for Japan.
  • Probably a poor snowfall season (Southwest US, Eastern US, UK, Southern Alps).

Thank you so much for reading this seasonal outlook for the Northern Hemisphere.

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 are meant to be interesting information, that can help to see what the season might be like.

Thanks again for reading, follow me on Twitter @longrangesnow and subscribe to my email list by clicking on the tab on the main header above.

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