Late September Northern Hemisphere Preliminary 2019-20 Winter Outlook

The latest outlook from Copernicus is out, which uses modelling from ECMWF, UK Met Office, Met France and a few other European models.

What it shows us is:

  • Greenland Low and Azores High = +NAO, according to it.
  • Aleutian/Kamchatka Low, according to it.
  • -EPO ridging across Alaska and south into the Eastern Pacific.
  • European ridge, according to it.
  • Strong ridging over Japan, according to it.
  • Ridging over Eastern US and less ridging over the Western US, according to it.

What does this raw modelling mean for the snowy regions of the Northern Hemisphere:

  • An average season for Hokkaido, and a less than great season for Honshu.
  • An average, maybe slightly above average season for the Northern Alps, a below than average for the Southern Alps.
  • A decent season for the Western North America, a poor season for the Eastern US.

What do I think of this:

  • Obviously this is early days, so don’t take the modelling to heart.
  • The models have been reasonably consistent with a Greenland /Icelandic trough, alongside a responding Azores high. So the chances of a +NAO is growing for this winter. However many drivers are against this idea, and prefer a -NAO.
  • A +NAO in the way the models depict this event, would mean a wet and largely mild winter for the UK.
  • It also reduces the chance for snowfall bearing events in the Southern Alps, and focuses snowfalls on the Northern Alps.
  • Another consistent model feature is an Aleutian Low. I favour the scenarios thrown around that involve a low around Kamchatka and then further south. This is when the SST map comes into play:
  • The warm sea surface temperature anomalies will likely bring ridging to the Alaskan region. This means the Aleutian low will probably slide in southeast of Kamchatka.
  • Given that particular Pacific SST setup, you could see the jetstream following the less warm anomalies in between the two regions of warm SSTs, because of the contrasts between the temperature gradients. This would be particularly good for Tahoe/Mammoth snowfall and to a lesser extent Utah snowfall.
  • The warm SSTAs around the SE and Eastern US could assist with ridging in the region.

Other important things to note:

  • The Siberian High is expected to strengthen into it’s winter form earlier than usual.

This is expected to allow the lake/sea effect snowfall occur for Japan earlier than normal. It would also expand snowfall towards Eastern Europe and Scandinavia earlier than normal. Overall early snowfall cover over Siberia serves to enhance later snowfall prospects for Japan, the Eastern US and much of Europe.

  • The early snowfall cover over North America would allow deeper cold to be developed early, which would potentially lead to an earlier snowfall season for both the Western US (which is already beginning to see snowfalls) and closer to winter, for the Eastern US as well.
  • The drop in temperature in the polar stratosphere could allow an increase in ozone transport to the Northern Hemisphere Polar Vortex, via the Brewer-Dobson Circulation. For those who aren’t familiar with the stronger BDC, it means that a Sudden Stratospheric Warming is more likely, and that increases snowfall prospects for Europe, Eastern US and Japan.
  • The last thing I want to talk about is the extratropical and tropical signals.

These signals from CFS (above) and also models such as ECMWF, show in the case of the former Phase 1-2 MJO forcing through December and January, and in the case of the latter, it shows it throughout winter. What this means is there is continued correlation with troughing in the Eastern US associated with Phase 8 & 1. These developments are hopeful. But there is more to it.

We are currently in a strong -AAM owing to an underlying Nina-esque background state in the atmosphere. This will cycle through, but the current progged charts show a -AAM like environment, particularly in the first two months of the winter season.

I am not a massive fan of analogs, but I will use one in this case to demonstrate the effects the -AAM has upon the atmospheric circulation.

Above is a -0.5 and below -AAM composite for DJF. It shows us rough conditions for what I think will be featured in the 2019-2020 winter.

  1. Alaskan Ridging
  2. Deep low in the Central US.
  3. Greenland High
  4. European troughing/-NAO
  5. Japanese troughing.

So there would be a favouring of snowfall for the Central and Eastern US, based on my predictions, especially earlier on. The eastern side of the Rockies would do well according to the above chart, but I at this point favour more of a shift to a -EPO domain for the Aleutian High, with some development of the Aleutian Low. I could see instances with the Subtropical jet heading south of the Alaskan high, bringing strong snowfall to California and the Southwest.

Troughing also favours more snowfall for Europe and Japan. It also rebukes the idea of a +NAO I suggested above, according to the models. Models can be wrong, and many signals lead me to think a -NAO is certainly possible, perhaps not a particularly strong one however for this winter.

So my overall current thinking:

  • Early, good season for Japan, Eastern US and Europe.
  • A decent season for California, a poor season for the PNW, Colorado slightly above average perhaps.
  • Early Siberian High development, with a potential SSW later in the season.
  • Nina-esque atmosphere, cool neutral-weak oceanic La Nina.

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.

One thought on “Late September Northern Hemisphere Preliminary 2019-20 Winter Outlook

Leave a Reply

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

You are commenting using your 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