North America on the long term 15th December

Hello all, here is my first North America on the long term update. I will take a look at the models for the Western North America (where the best skiing is :), and then sum up the drivers for the whole continent.

Models

Christmas Day

EC shows a moderate-heavy snowfall event for Whistler and the PNW on Christmas Eve, moving inland into Christmas Morning, as well as south to Tahoe. GFS agrees for the most part, perhaps about six hours ahead of EC.

27 Dec-2 Jan

EPS Control and GFS both suggest a 27th Dec start to this system, with moderate-heavy snowfall to the PNW and BC. EC Monthly suggests a 29 Dec start, with the focus on the PNW and Tahoe in terms of very heavy snowfall on the 29th and 30th. On the 31st, it continues down in Tahoe and Mammoth, with moderate snowfall moving into the Inland PNW (Idaho, Montana and Wyoming). On the 1st and 2nd, it moves down into the Four Corners region, and the rest of Colorado and Utah, bringing moderate-heavy snowfalls, before fading on the 3rd into showers, along with a second trough behind it, that brings heavy snowfall to Tahoe and Mammoth on the 2nd.

4-8 Jan

EC Monthly shows a low bringing heavy snowfall to the Sierra Nevada on the 4th, which moves down to Arizona on the 5th and 6th. It then balloons to a large snowfall event for CO, UT and the Four Corners Resorts on the 6th, continuing with moderate-heavy snowfall, until it fades on the 8th.

8-11 Jan

On the 8th, another system on EC Monthly hits Tahoe and Oregon from the northwest, moving with moderate-heavy snowfall inland over the 9th to Colorado and Utah, as well as Tahoe until the 10th. The snow in Colorado and Utah continues until late on the 11th.

Climate Drivers

We are currently experiencing an El Niño Central Pacific based event, which is forcing upon us a generally +AAM atmosphere (more westerly momentum).

However with a -FT currently in place (losing momentum in the tropics), we will eventually lose East Asian Mountain Torque, retracting the North Pacific Jet. By the time that happens (22-24 Dec), we will have an Aleutian Low established (which may move West towards Russia) allowing a Sudden Stratospheric Warming precursor(good for a Eastern US snowfall). We will also see +PNA/-EPO ridge in the Western North America fully established by the end of the NP Jet extension around the 23rd. So beyond this week, I expect a poorer outlook for the Western US ski resorts unfortunately. We may see some troughing develop between Xmas and NY, if the AAM dips low enough towards GWO Phase 8/1.

The MJO is heading for Phase 4, which is good for Western US snowfall, and may give rise to a Atmospheric River event for the West Coast, if we have a North Pacific Jet extension behind it, which isn’t likely again until the New Year. This is a reason why the inter-holiday period shouldn’t be that bad, but the overall pattern still isn’t great.

The Stratosphere is heading towards a SSW by technical standards. It’s not overestimating it, to say it will be a massive event. I don’t think we will see a total collapse, but we will see a reversal of zonal winds.

This will induce a -AO effect, and from the last days of 2018, reduce snowfall opportunity for the Western US, and create a potentially massive snowfall period for the Eastern US for the first two weeks of January. I believe an improvement for the Western US will be due in the 3rd week of Jan.

So the pattern is aligning towards a relaxation for the Western US from the 22nd Dec for a few days, and then more snowfall options from the 26-29 Dec or something like that. Then a relative lull in activity for the Western US in the first two weeks of Jan, and a big two weeks for the Eastern US. And than an improvement in Weeks 3 & 4 January. The models give much of the snowfall to CO/UT and the Four Corners region, plus California, rather than the PNW and BC ski resorts, which would be expected in a more Niño-like pattern.

Thanks so much for reading. I hoped you enjoyed reading it.

Disclaimer: There is low skill asssociated with using long range model forecasts to find snow systems.

Thanks again for reading this North American long range snow forecast, 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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