December Review
The month of December has largely evolved as anticipated, with a very active storm track and near normal to slightly negative monthly temperature departures throughout most of New England. These departures should not change much over the final few days given the current forecast.
Note our thoughts in relation to December that were expressed in the 11/13/19 Eastern Mass Weather Winter Outlook:
"The month of December should be both active and variable. A robust PNA should be the primary driver of the pattern, thus central and northern New England should see the most robust snowfall anomalies. While the PV may encounter some disruptions, it should recover, which may ultimately prove more inimical to the development of early season blocking, which is not climatologically favored, regardless. The NAO should have some increased moduality, and a transient negative phase could link with the PNA to create a major early season storm. Favored period being from 12-5 to 12-19.
-1 to -2 departures north of Concord, NH, around normal down to 40N, and up to +1 points southward".
-Eastern Mass Weather 11-13-19
Obviously the month did not verify as cold as the composite across the Eastern half of the nation, nor was it supposed to, as the normal to -2 departure that was explicitly mentioned implied. The potential for a major winter storm did indeed come to fruition, however it occurred two days prior to the outlined two week window. This system likely contributed to one slight discrepancy with regard to the monthly snowfall departures, as the heaviest snowfall anomalies ended up verifying over south central New England, as opposed to north central New England, as forecast. However the real proverbial "fly in the ointment" was the PNA forecast, which was predicted to be robust and was merely modestly positive, as evidenced by the cooler anomalies over the Pacific northwest than implied in the composite. .
The polar domain behaved largely as anticipated, with increased variability that manifested in an active storm track.
Although the polar vortex did endure some minor assaults early on in the month of December, as suggested in the winter outlook, it has also proven every bit as resilient as anticipated. Thus the notion that the first half of December would be biased cold, followed by a mild second half of the month that would herald in a rather protracted winter reprieve has also come to fruition.
Here is a review of the expressed thoughts from 11-13-19 in relation to the behavior of the polar fields.
"The seemingly favorable antecedent conditions for the disruption of the PV in conjunction with both the observed north atlantic tripole over the summer, as well as the anticipated favorable Hadley cell configuration in association with the ongoing modestly warm ENSO event all favor increased blocking. However they are interpreted as being suggestive of perhaps one month during the coming period featuring one or more major and sustained blocking episodes. This will likely occur later in the season because conditions should be hostile for the development of sustained blocking during the first half of boreal winter 2019-2020 due to the considerable initial intensity of the PV, in conjunction with the delayed descent of the easterly QBO phase. This does not preclude intervals of negative NAO and bouts of wintery early season weather, owed to the elongation of the PV that is conducive to periodic and transient cold intrusions, as well expected volatility of the NAO modality. It is also important to note that while conditions appear favorable for at least minor disruptions to the PV, its recovery from any such occurrence is expected to be both proficient and timely. This is due in part to climatology favoring only minor assaults early in the season, as well as the anticipated resiliency of a potent PV denoted by +AO conditions presently observed within the polar stratosphere. Such a recovery period after any potential assault would likely lead to a protracted mid winter thawing period as the vortex reconsolidates and possibly becomes more circular in nature.
Sure enough, the polar vortex is fairly stout, though elongated, and well centered in the vicinity of the pole at present, which is indicative of a positive AO.
And there are signs that it remain as such for a rather extended period of time.
January Preview
There are not significant changes to the though process regarding the month of January.
"January remains fairly active, at least throughout the first half. The PNA ridge begins to erode, however the PV may be undergoing some dysregulation, so the PNA relaxation may be offset to a degree. Departures normal to -1 NNE, normal to +1 for SNE and the mid atlantic." Eastern Mass Weather- 11-13-19
However it does now appear increasingly likely that the polar fields will remain fairy hostile to extended periods of cold and snow, as the latest forecasts have confirmed the Eastern Mass Weather notion of the polar vortex continuing to grow more consolidated and circular throughout the first third of January.
Predominately +NAO Indicative of Strong Polar Vortex Throughout January
Predominately +AO Indicative of Strong Polar Vortex Throughout January
In addition to the hostile polar domain, the expected development of an RNA has already ensued and is expected to remain prevalent through at least the month of January, all but ensuring a relatively mild month for the northeast.
RNA Favors Southeast US Ridge this January
The correction vector with regard to the slightly above normal January forecast issued this past fall would seem to be pointed in the milder direction. Snowfall in southern New England will likely be below normal this month, with normal snowfall across central New England and perhaps above normal in norther New England. This stratification of the monthly snowfall gradient will be owed to a strong polar vortex and RNA favoring a mean southeast US ridge in conjunction with an eventual relaxation of the deep trough currently positioned in the vicinity of Alaska. This abatement should allow cold to remain entrenched in Canada, thus providing a reservoir of cold to be tapped by storm systems for at least northern areas.
Relaxation of Alaskan Trough Reflected by More variable EPO
This progression is also favored by the MJO, which is forecasted to flirt with phase 7, before relaxing and remerging in phased 4 and 5 about a week into January.
Phase 4:
And Phase 5 of the MJO in the second week of January also teleconnect to the aforementioned pattern, which features a predominate west coast troughs and southeast ridge:
Finally, seasonal guidance confirms:
Both the Canadian SIPS:
And the European monthly guidance confirm:
Conclusions
The month of January looks fairly mild, and should not produce very much in the way of significant snows or severe cold for southern New England. However much like last season, what opportunities do present themselves should favor the interior and central/northern New England, as the cold will enter through the western US, and only methodically bleed eastward near the Canadian border in a modified state. The best chance for the foreseeable future appears to be with a system approaching around January 5th, as heights in the vicinity of Alaska and the polar domain appear to relax as much as they appear that they will for most of the next month. The interior and northern areas are of course still favored, but the system may feature some mixed precipitation for coastal southern New England.
Ensemble consensus for this system exists amongst the European ensemble suite:
The GFS ensemble package:
And the Canadian product:
Second Half Revival of Winter Likely
While it does appear that January may perhaps verify even milder than originally anticipated, the forecast for the balance of winter remains unchanged, as a more favorable polar field should negate what may remain a hostile Pacific. This is supported by the notion that the QBO (+5.07 as of November) should be neutralizing in February, the Eastern Mass Weather analog composite from the winter outlook, as well as some of the current seasonal guidance.
For February:
And especially March:
