Welcome to the start of this season’s long term outlooks for North America. This year we are trying to involve the entire continent with the usual models and climate driver analysis.
EC Monthly shows this week to feature troughing along the West Coast, and also shows some potential for New England and SE Canada. CFS likes Western US ridging and Eastern US troughing.
24th – 30th December
Again EC Monthly shows troughing in the Southwest US, benefitting California and the Four Corners states, and the opposite for the PNW with ridging. Ridging is forecast for the East Coast of US. CFS again shows the continuing of a Eastern US troughing and snowfall pattern, and Western US ridging.
31st December – 6th January
This week on EC Monthly shows ridging in the PNW region. It also depicts troughing for the Northeast US. CFS shows a strengthening SE ridge, Central US troughing and Pacific coast ridging.
Some troughing on the West Coast of US, but most of the continent is dominated by a Southern ridge, with some troughing in the far north of New England
This period shows troughing becoming more significant for the Midwest and the NE of the US. Meanwhile there is ridging in the SW and the PNW.
The GFS VP200 forecast shows the Indian Ocean standing wave remaining in place for the medium term, which isn’t as great for atmospheric rivers or trying to shift the pattern.
CFS keeps this theme running long term, so it is expected for the IO standing wave to stay this way throughout the month, amid some ups and downs of the signal. I’d expect it to slowly weaken this month, and complete its change into the new year, with the decline in the +IOD state.
This means that a deep cold pattern for the Eastern US is probably still not going to come until potentially early-mid January, as the tropical pattern starts to really evolve and get interesting.
The EC Weeklies are consistent with a strong IO tropical standing wave. The activity in MJO Phase 6 & 7 in January might make things more interesting for cold risks in the long long term, but for now, we are stuck with this base state. It all depends on the progression of the +IOD.
- We will remain in GWO Phase 8/1 for a few more days, thanks to the lingering +EAMT.
- From say the 12th-14th of Dec to the last week of the month, we should see a proper -GLAAM circuit.
- GEFS & GEPS show positive mountain torques around the Christmas period, with potential downstream impacts around the New Years period.
- We should see a more positive momentum circuit in the first two weeks of January.
- What this means is that we should see a strong Aleutian Low based on the current +EAMT over the next 10-20 days, pushing into the EPO domain, flooding the CONUS with Pacific air as the extension matures.
- We will see this impact continue until the progression weakens this after Christmas. We will see better chance for cold for the East in the New Year.
Apart from the biased GEFS, the stratospheric polar vortex per measure of 10hPa and 50hPa zonal wind is strengthening. There’s a decrease of large wave 1 amplitude and a mild increase of wave 2 amplitude.
So a chance of a SSW are unlikely for December, and will probably increase towards the latter parts of the winter. An inducement of a -AO state will probably have to wait until later in the season, again setting back a deep cold early winter for the Eastern US.
FWIW CFS shows a -AO for the late December and early-mid January period, after some ups and downs in the medium term. EC Weeklies show a +AO for the late December to early January period.
The jet in the North Pacific extends in the next week, and then normalises in the middle of the month, while shifting towards the pole. The focus of the snowfall in the 15th-30th December period will be towards the PNW IMO, thanks to this poleward shift. Later in the month, as the jet retraction peaks, it may get less conducive. But I’d forecast
Chances for snow-bearing systems for the Southwest are not particularly high at this point, thanks to not a very forceful Sub-tropical jet stream at this point. But nonetheless the positive Pacific Meridional Mode (warm SSTAs southwest of California), still provides potential when the STJ probably expands in early-mid January. Ultimately we need Nino forcing to be stronger, and this won’t be a thing until the IO standing wave declines.
- I expect a good run over the next 10 days for the PNW.
- This good pattern of snow-bearing weather will eventually expand out to the majority of the Rockies, with the exception of the far Southwest for the 10-20 day period.
- Tahoe and the Rockies should both do well out of this period, once the initial 10 days pass.
- After the New Year, we should see deeper cold risks per the GWO cycle for the Central and Eastern North American regions.
- The short term good pattern over the Eastern US will dissipate into ridging in the latter half of December. It might get more interesting in the days prior to January, with the aforementioned colder risk in Jan. This doesn’t mean there is no chance of snowfall, rather it is more limited.
- After the current short term Southern US snowfall risk, chances will be lower until January, or at least the last week of December.
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.
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