Happy New Year to everyone!
A front come forces itself into BC/PNW on the 18th with moderate-heavy snowfall. The Rockies will start to see moderate snowfall over the 19th and 20th, before winding down over the 21st. This is from the EC Monthly Model.
Some moderate snowfall occurs in British Columbia and the PNW over the 21st, with lighter falls over 22nd and 23rd, setting the tone for a dry week succeeding it.
A trough heads over BC/PNW, as the next month of the year starts, with heavy snowfall on the 1st of Jan, on EC Monthly.
- We are in Phase 6, and I expected a strong MJO signal to carry through Phase 6 into 7 by next week, and an eventual orbit to Phase 8 by the end of the first fortnight of the year. This progression shown by the BoM’s model is quite similar to my thoughts on the pattern progression indicated by the tropical signal.
Phase 6 supports the currently ongoing SSW, and later Phases 7-8-1 of the MJO supports troughing on the Eastern US. GEFS shows a Phase 7 stall in the next two weeks, which I think is plausible, but we would be more likely to see a Phase 8 stall in my opinion. EPS shows a weaker Phase 7-8 orbit, than I expect. It has been largely off the tropical signal, and a stronger Brewer Dobson Circulation (colder tropical stratosphere ), helps amplify the MJO signal.
- In terms of the GSDM (Global Synoptic Dynamic Model), We should see a pretty big load of positive East Asia Mountain Torque (+EAMT) over the next 10 days, EPS is signalling it’s end by then. EC Determ looks settled in for a little more. GFS doesn’t look great for a good +EAMT beyond the 5th, which is a change in tune from the “add more momentum” model we have seen for a week. Positive North American Mountain Torque (+NAMT) will be around for about a week or so, starting from around the 5th. So if the EAMT drops, the NAMT should keep the Global MT figures up for a little while. We see that +AAM (Atmospheric Angular Momentum) peak around the 3rd Week of Jan, with +PNA effects well and truly settled in before that (begin probably the 2nd week of Jan). It could hang around for a little while into the 4th week of Jan (just look at how long this “negative phase” of the cycle has been, the equilibrium formed by the GSDM will give a turn to positive AAM). And we will eventually see a drop in AAM with a GWO(below) Phase 8 for a little while in early Feb, maybe a little later if we are lucky.
What does this all mean for the North America continent? We will see a North Pacific Jet extension bring West Coast troughing and snowfall over the next week, but the growth of the Aleutian Low should spark a change to Eastern US troughing and Western US ridging by the tenth day of January. This should only continue into the second half of the month stronger, with SSW effects taking hold of a -AO by the 10th of Jan also, with extratropical effects going along from there. In general, GWO Phases 6,7 & 8 are pretty good for cold prospects in the Eastern US.
- We now have an official Sudden Stratospheric Warming. What is important now is what those impacts mean for North America.
This chart shows NAM effects on the stratosphere and troposphere together, showing the lower and upper levels of the atmosphere. The upper level warming or negative Arctic Oscillation index regions are slowly descending, with -AO impacts just starting to come into effect at the end of the NASA GEOS run. GEFS is showing a weak -AO, but doesn’t display important mid-level coupling. I believe the strongest effects from this SSW in terms of high level blocking, and a colder Eastern US (and unfortunately warmer Western US) will be in the last two weeks of Jan and the first week of Feb. It’s making it obvious that it will be a slow downwelling event in terms of atmospheric impacts. EC-46 shows a negative phase of the AO/NAM beginning from Jan 18, peaking on Feb 4th.
- There’s a pretty overwhelming pattern towards a reasonable pattern for troughing for the Western US in the next 10 days, maybe even out to 15 days if they are lucky. But given the +AAM and the SSW’s slow but decisive impact, we should see a +PNA, Eastern US trough and Western US Ridge beginning around the 10th of Jan, and slowly establishing to Jan 18-20, with a strong +PNA period from then until the end of the first week of February, maybe even with a second week extension involved. The best conditions for the next 10 days in the West will be for the Coastal ranges in California up to British Columbia. Not so much for the US Rockies, unfortunately.
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Disclaimer: There is low skill asssociated with using long range model forecasts to find snow systems.
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