North American on the long term 30th Dec

A deep dive into the climatic drivers heading into the New Year for North America.

  • So among the takeaways from the latest EC Weeklies, we have the proliferation of a strong -NAO, as one can see from the consistent ridging from this week. We also see a largely -AO setup for the next month and a half according to the model.
  • The Aleutian Low gets strong, like really strong, over the next two-three weeks, before winding back up in the way of ridging in the North Pacific into February.
  • The first 10 days of the forecast looks very nice for the Western US, particularly the PNW.
  • As we move into the latter 2-3 weeks of January, we see a period of favourability for the Eastern and Southern US in terms of cold and snowfall.
  • EC contends that troughing in early February will retrograde back towards the West, possibly setting up a good period for them in that early-mid Feb period.

Now there’s two reasons for the really good forecast for Eastern and Southern North America for January for snowfall.

Stratosphere/Polar

We are seeing the beginning of an Sudden Stratospheric Warming, which means that the temperatures in the stratosphere or the upper levels of the atmosphere are warming up, and the polar vortex is disturbed and stops moving in a predictable elliptical fashion. This has the usual effect of bringing a weaker polar vortex at the surface, also known as a negative Arctic Oscillation.

EC Weeklies (above) argues that one is about to occur (a proper reversal of the 10mb zonal wind at 60hPa), there is also growing evidence for it on GEFS and CFS, otherwise showing at the very least a very strong hit at the polar vortex.

We see this in the chart below, as the upper levels become more negative or blue, but we are certainly helped out by the fact that blocking has already started to develop in the high latitudes (NH polar region), and pushed the lows down to the continents.

This sets up the possibility for an exciting forecast for snowfall and cold to be brought to the Eastern and Southern United States, as they are traditionally the regions of North America that benefit from the negative AO. An SSW (or a moderate-major stratospheric disturbance) in this case is brought about by Ural blocking contributing to a positive East Asian Mountain Torque (more about that in a minute) and a strong Aleutian low downstream. This Ural blocking pushes back towards Greenland, as the Aleutian low matures, and helps lock in a trough over the Eastern US (known as a -NAO).

The one potential problem here to watch out for is where the stratospheric polar vortex lands, as the SSW progresses. For the cold and snowfall to follow the disturbed vortex for the East and the South , it needs to be over the North American side of the Northern Hemisphere.

Pacific

The other major factor in the mix here is the growing potential setup in the Pacific. We are potentially going to see a:

  • Strong, possibly record-breaking East Asian Mountain Torque develop over this week, and possibly proliferate deeper into mid-January. This helps to add momentum to the global circulation.
  • A very strong (again possibly record-breaking) Aleutian low and strong North Pacific jet stream.

This is quite possibly the best possible circumstance for one to be in to help bring about snowfall. This often sets up a chain reaction with a -EPO/+PNA ridge push against the expanded Aleutian low to try and achieve equilibrium, which then evokes a trough downstream in the Central and Eastern thirds of North America.

Initially the expanded North Pacific jet should help the Western US for the first 10 days of the new year with some snowfall. But as we start to see synoptic responses to that, we start to see ridging develop over the West Coast, in favour of cold and snowfall on the East Coast.


We are looking at a potentially good period in terms of snowfall risk for the Eastern and Southern US from approximately the 10th of Jan, maybe a little later, peaking towards the latter two weeks of the month, and then continuing into the first week of February, and possibly beyond, but more information is needed for February.

This Pacific setup, if it comes to full fruition, is certainly one of the best I have seen in my years looking at the Northern Hemisphere charts.

Whether it delivers or not, will depend on whether the Atlantic drivers powered by this -NAO/-AO blocking the path across the North Atlantic and the strong Pacific mechanisms that are possibly creating enormous synoptic tension, can work effectively together.

MJO

On that note, we have a fairly neutral MJO (yes I know the RMMs are not great), but it’s pretty conclusive that not a whole lot of driving is happening from the tropics at the current time.

We might see an orbit in the Indian Ocean, that could be a little destructive (for the Eastern US) to the current forecast if it comes to be later in January.

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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