Welcome back to the first (and late) European Snow Outlook. First we look to the models, and then we link the dots with the teleconnections, with a big look at the GWO and the AAM.
A trough towards the Northern Alps begins on the 21st on EPS and GFS, and brings moderate snowfall through the 22nd and 23rd, easing before the 24th. There are light-moderate snow showers on the 26th also forecast on EC Monthly.
31st Dec – 2nd Jan
A low to the North on EC Monthly, brings a trough into the Northern Alps with moderate-heavy snowfall through New Years Eve and New Years Day, easing on the 2nd. CFS shows a trough bringing moderate-heavy sudden snowfall to the North Western Alps on the 31st. Another trough hits the next day with moderate-heavy snowfall, continuing through the 2nd, moving to just the Eastern Alps on the 3rd.
EC Monthly shows a trough pushing over the Northern Alps, from West to East, with potentially heavy snowfalls on the 3rd and 4th. Moderate snowfall is brought by a smaller trough associated with the low behind it to the North on the 5th. CFS shows this period mostly as dry.
EC Monthly shows the beginning of an Atlantic pushed front into the Western Alps, with moderate snowfall. CFS shows a low from the south pushing snowfall later on the 6th, bringing heavy snowfall to the Southwestern Alps on the 7th. It moves east towards Austria on the 8th, and finally weakens on the 9th.
Here is an extract of my thoughts for the GWO:
“This is my GWO forecast. I forecast in a few days the AAM will be at a neutral phase, and the GWO will be in Phase 1. I expect the next week to feature this decrease from GWO Phases 7/8/1. Week 2 features a movement from Phase 1 to 4, via the COD, or via low amplitude Phase 2 & 3. The main point of this, is to state that there will be a neutralisation and/or a slightly negative anomaly in the global momentum budgets around the 15th of Dec. This would be the low point of the AAM. Momentum would be slowed, and the pattern would be in line with a cool neutral ENSO. This in in line with global anomalies acknowledging that current negative torques have respective lags against changes to the AAM in line with the GSDM framework (+FT > +MT > +AAM).
Then Week 3 shows the beginning of the strong torques coming into play, and with a 3-5 day lag, the AAM goes positive falling back into the atmospheric state it has set for itself. Late Week 1 (~10th) should feature a +FT, Early Week 2 (~14-15th) should feature +MT in both the North American and East Asia domains. Week 3 (20-27 Dec) should see the realtime effects of the increase in AAM, and after the 7-10 day NP jet extension lag, we should see +PNA effects come into play around the 23rd, give or take a few days. It’s going to take the AAM a week to respond to the torques and get itself through the GWO P1 > 4 stage, and into Phase 5.
Week 4 sees the peak of the +AAM. I expect a P6-7 GWO for roughly 2 weeks, given the Nino base state and the recent climatology, as well as the feedback loop with the stratosphere (more on that in a second). So we should see positive leaning PNA (and a downstream -NAO) effects for the first half of January, and any downstream stratosphere influences only serve to perpetuate these influences.”
This is a very global reaching statement, but it is important to state what a +AAM means.
(+AAM 0.5 and above, 500hPa Geo Height Anomalies)
This composite shows both what the +AAM can do, and what it can do with a -AO setup (together). The strong jet off the Eastern US works into a gradient between the predicted Greenland blocking (after Christmas) and troughing in the Atlantic towards Europe. And this delivers troughs and cold for the UK, and then the Alps inland. The Northern Alps would be the better position, over the Southern Alps, and the long term models reflect that.
This shows an attack on the Stratospheric Polar Vortex.
We have a +EAMT in the 10 day forecast (so by the 18th). The forecasts shown by the EPS in the Aleutian Domain, look like they are in the process of creating +EAMT with the pushing down of Ural Highs, by the TPV moving into Siberia in the 8-12 day period. The Aleutian Low gears up the +EAMT for it’s jet extension, and once the part of the TPV turns into a Okhotsk Low, we will see increased heights into the Barents-Kara Sea, and perfect conditions for a SPV attack. We also have a strong Brewer-Dobson Circulation (increased ozone transport > weakening of strat effects). This all gives us conditions for a SSW in the 18-23 Dec period, with the tropospheric impacts around Xmas. The actual reversal (which I do forecast has a 60-70% chance of happening in this period, despite my own, and various other expert’s seasonal forecasts) would take place around Xmas itself(the whole event may take a week or so from the 18th), with the greatest effects around New Years, and resulting in a -AO and a downstream -NAO at the troposphere level for a good two weeks into January at least.
The combination between the AAM and the (technical) SSW looks to bring cold and above average snowfall to the European Alps from the 25th Dec, becoming at it’s best around New Years, and continuing at least two weeks into January.
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