Active Pacific Jet Finally Poised to Subside Later in January
More Wintery Pattern Delayed But Not Denied
Thus far the month of January has not evolved as anticipated. Although the RNA that was so prevalent throughout the month of December has in fact abated as forecast due to more el nino like tropical forcing, it has been very mild with a notable dearth of snowfall across much of the northeast, regardless.
This has been primarily due to a stronger than anticipated Pacific jet stream, which has eradicated the cold air supply across much of the continent.
While this was expected to be a recurrent theme over the course of winter 2022-2023, it was not at all expected to be so prominent throughout the month of January and has led to a very mild first half of the month.
However, there are signs that the active Pacific jet will relax over the course of the final third of the month.
Mid Month Pattern Changes to Ensue
There is a growing consensus among ling range ensemble guidance that the very active Pacific jet will finally begin to relax and allow the supply of cold air to be replenished, as a ridge develops in the vicinity of the west coast and aids in the delivery of said cold to the eastern US.
The relaxation of the mild Pacific jet is also apparent in the ensemble forecast of the EPO index, with a notable descent evident in both the European:
And GFS ensemble suites:
This period also denotes the onset of the window of opportunity for significant winter storms across the region, which at least initially, look to be primarily Pacific driven.
Neutral Signal from Tropics and Polar Domain
While a there is likely to be a reoccurrence of the early season episode of high latitude blocking, at least to some degree, later this season, it is unlikely to occur during the month of January. Tropical forcing appears to remain relatively weak, as noted with respect to the low amplitude MJO wave passing out of phase 8 and through phases 1 and 2:
And continued decay of the wave of el nino like regime that has been situated closer to the dateline during the majority of first half of the month of January.
What his likely entails is a continuation of the current + PNA regime, as evidenced by the aforementioned west coast ridging apparent on long range guidance, with a possible stagnation towards the end of January, and essentially an inconsequential signal from the high latitudes.
Given the lack of major high latitude blocking in conjunction with a progression towards modest MJO phase 2 wave progression, an inland track of low pressure areas, in addition to inherent coastal precipitation type issues, continues to be a risk factor moving forward.
However, even in the absence of any major high latitude blocking at least through the duration of January, there does appear to be a tendency for the polar vortex to become more elongated towards the western Hemisphere. This may act to redistribute more of the global supply of cold back closer to home as January continues to age.