Winter 2025-2026 Offers Return to Normalcy

[Baroclinic Zone]

Nice write ray.

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Plowable Snowfall Still Favored Sunday

Devin In Details As Considerable Model Uncertainty Remains

Rainfall Likely Tonight

Regardless of the ample supply of cold throughout the first third of the month of December, the storm system approaching the region today still looks to fall as primarily light rainfall for the majority of the region.

 

 

The rising temperatures throughout the overnight period last night were the first indication of this outcome, as an innocuous looking wave over the Ohio River Valley manages to phase in enough with a lobe of the PV to amplify and track through southern Canada, which has already turned the winds out of the southwest across much of the northeastern CONUS.

 

Although some light snowfall is possible across the higher terrain of the Berkshires, this particular system will prove more met than white. This is a very easy forecast since the system is traveling so far to the north, however, when that is not the case, modeling will be very sensitive to any subtle change with respect to phasing attempts is an extremely fast flow. This will be on full display with regard t the weekend forecast. 

Complicated Weekend Scenario Unfolding

As of this morning, there were two more distinct parcels of energy over the arctic that are modeled to be on a collision course over the Canadian prairies late this week.

 

The system over the Arctic Ocean, and the one entering Alaska are forecast to rapidly funnel SSE in the flow between the PV and west coast ridge, before ultimately coalescing in the vicinity of Alberta, Canada on overnight on Friday.

 

 

The system will then careen around the southern side of the PV on Saturday, at which point guidance diverges concerning the precise nature of this crucial interaction. 

 

 

The European cluster of ensembles favors a scenario in which there remains enough space between the PV lobe and the developing system to allow to amplify sufficiently as to eventually impact much of the region with a plowable snowfall on Sunday. The GFS camp, on the other hand, keeps the PV in close enough proximity to greatly impede the development of the wave to the south by largely shearing it apart, which results in little if any snowfall across the area.

 

The version of the EURO guidance that is derived from Artificial intelligence (AI) is even more aggressive than the traditional model, and implies a warning event of 6" or greater throughout at least a portion of the area.

 

The Eastern Mass Weather hedge remains in favor of a plowable snowfall for at least a portion of the forecast area on Sunday given previously discussed analog support and teleconnector convergence, in addition to robust AI support in addition to a death of high latitude blocking at present to pin the PV slightly further to the south. First Call is likely either Thursday or Friday.

 

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Festive Light Snowfall Likely Sunday

Plowable Possible South & East

Compromise Solution This Weekend

On Wednesday, Eastern Mass Weather discussed subtle differences between the GFS and European suites, respectively, with the latter favoring a plowable snowfall for  large portion of the region and the former merely flurries. 

 

 

The preference was for the European blend, which entailed a significant snowfall for most of the region; however, as is often the case, it appears as though a compromise will ultimately be in order.

 

Synoptic Overview

It appears as though a rather modest parcel of energy congealing over the prairies of Alberta, Canada later today and tonight will race east-southeastward and begin interacting with a lobe of the polar vortex (PV).

 

This particular randevu will be a complex, and somewhat noncommittal one, at least initially. The two pockets of energy will initially flirt with one another in that they will reman in close enough proximity to hunter the amplification of the smaller system.

 

 

However, any phasing will occur at a rather lethargic pace owed to the shallow amplitude of the western ridge, which will lack the depth to trigger a more aggressive and committed union.

 

The end result will be a light snowfall, which a some moderate totals likely well to the south, where plowable amounts remain possible. 

 

 

Anticipated Storm Evolution

Light snowfall will begin to break out after midnight Saturday night, during the predawn hours on Sunday from south-southwest to north-northeast.

 

 

The "peak" of the storm will occur in the vicinity of dawn on Sunday, when low pressure will begin to slowly strengthen south of Long Island, and snowfall becomes more steady and even moderate at times the southern and southeastern third off the area.

 

 

Snowfall will begin lighten up later in the morning and during the midday hour, but just as that happens, the energy from the polar vortex lobe to the west will begin to drop in to initiate phasing as the system exits.

 

 

This will likely serve to protract light snow across especially the eastern half of the area through the afternoon, which will entail the potential for very light additional accumulations before ending during the early evening hours.

 

 

Stay tuned for important updates on the holiday period to be issued early next week-

FIRST & FINAL CALL

[rimetree]

I was curious about previous winters around here when Washington had significant December flooding. Using Concord, seems all over the place but some good years in there like '07-'08, '77-'78, '70-'71 and '55-'56. Looks like some of these were mentioned in Ray's write-up.

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Verification for Festive Sunday Snowfall

Strong Forecast But Slightly too Conservative

The season's first significant snowfall for the southeast coast of New England was identified at a very extended lead time using a combination of seasonal analogs, long range ensemble guidance and pattern recognition. In fact, forecast for the storm itself actually ended up being slightly too conservative.

 

The 1-3" forecast for the central portion of the region worked out well, however, there were two primary problem areas with the overall forecast. The most obvious issue is that the 2-4" portion of the forecast over the southeastern quarter of the area actually verified in the 3-6" range, given amounts up to 5" over southwestern, Connecticut (Norwalk) and 6" on the outer half of cape cod (6" from Hyannis to Chatham). There is also a strong argument to be made that the western portion of the 1-3" area should have been extended slightly further to the north, as evidenced by the 3" total reported in Great Barington, MA in the southwestern corner of the state. This is a shortcoming that likely would have been alleviated had a follow up map been issued on Saturday, when it had become clear that the storm would track slightly further to the north.

Final Grade: B+

 

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Mid-Month Warm Up Arrives On Schedule

Path to White Christmas Remains Despite Friday Grinch

December Forecast Largely On Track

Thus far the month of December has behaved largely as expected, primarily owed to the similarity of the anticipated deviation from the predominate Maritime Continent forcing of the past decade, to the analog periods of December-January 2017-2018 and 2024-2025.

 

December 1-13 2025:

 

 

December 2017-January 2018:

December 2024-January 2025:

 

 The primary differences being that this month has been more extreme in terms of the magnitude of negative temperature departures, possibly owed to the extreme Bering Sea blocking (-WPO) that has been observed  (-3.48 WPO peak on 12/13). The polar vortex disruption that occurred late last month has behaved remarkably similar to the early December 2000 analog event in that it fell just shy of triggering a full  850MB zonal mean wind reversal in the arctic. 

 

 

This has afforded the vortex the opportunity to recover to something that will approach climo levels by the holiday week, as forecast last fall per research conducted Lee et al (2019), which indicates that the arctic high regime is favored for less than 20 days following a weak PV (roughly November 28th). This despite the insistence of initial modeling and social mediaologoists alike that it would remain weak through the new year. 

 

This consistent recovery following the lack of a zonal wind reversal has allowed the high latitude blocking to relinquish it's grip by mid-month, as forecast.

 

 

The increasingly hostile polar domain in conjunction with lowering heights on the west coast as reflected by a descending PNA value will allow the storm track to shift west just in time for the expected "Grinch Storm" this Friday, which will eradicate all of the snowpack throughout southern New England. 

 

 

This is event is similar to periods prior to the Christmas holiday in the referenced analog months of December 2007 and 2008, especially the former.

 

Thereafter, there is going to be a battle waged throughout the holiday period as the MJO remains shallow and variable, while the Pacific Trough regime begins to assert itself.

White Christmas Odds Perilous As Pacific Trough Regime Establishes

One of the primary assertions made in the Eastern Mass Winter Outlook last fall was that the aforementioned recovery of the polar vortex and persistent RNA pattern would team up to induce a Pacific trough pattern during the second half of the month, as a potential pre-cursor to a January stratospheric reflection event. 

 

Current long range guidance now reflects such a pattern.

 

While the forecast for the balance of 2025 would appear rather unchallenging, things are seldom as they appear in weather. In this particular instance, the caveat is an MJO wave that is expected to remain rather meager and variable after having largely dissipated over the course of the past week following its brief flirtation with phase 8 near the outset of the month.

 

 

 

Note the tendency for the MJO to stagnate and become incoherent enough in phase 8 as to actually be reflected as having reemerged into the MC phases as byproduct of the continued warmth in the western Pacific that was referenced last fall.

 

 

The consistent struggle for the MJO to remain a coherent wave in the western Pacific has been a theme over the course of the past decade that any seasoned forecaster should not have neglected to consider.

 

 

The disjointed signal that results from the continuation of the battle for proxy between the MC and western Pacific will allow for an occasional element of western Pacific expression to assert itself into the pattern as a byproduct of the areas of dual forcing. This will be especially prevalent during the final week of the month, which is when some potentially colder intrusions may periodically infiltrate the northeastern quarter of the country 

Here is the Christmas Eve 500mb depiction per EURO Weekly product, EURO ensemble package, GFS ensemble and Canadian ensemble.

 

Having a supply of cold near by means that the likelihood of a white Christmas across the southern New England forecast area, while unlikely after the visit from the Grinch on Friday, can not be discounted since there may be some weak northern stream energy passing through in the vicinity of the holiday.

 

 

There is also some suggestion amongst guidance that Scandinavian blocking could develop by the New Year, which could represent a precursor to an episode of some Greenland Blocking prior to the development of any subsequent refection event later in January.

 

 

That being said, it would not at all be surprising if this -NAO signal waned at least somewhat in future guidance. Although the pattern is unquestionably turning milder on average, it would be advisable to stay tuned for updates as the all important holiday period approaches because the devil is often in the details.

 

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White Christmas Likely For Portion Of The Area

Weak System Impact On Tuesday

Synoptic Overview

It now appears as though the possibility of a White Christmas has increased for at least a portion of the area, despite Friday's "Grinch" storm having scoured the region of any residual snowpack. This is due to a weak system that will break off of the Pacific energy out west during the day on Tuesday and subsequently slide to the east-southeast in the flow between the polar vortex and central US ridge.

 

Although this energy will begin to amplify upon reaching the coast as it descends the eastern periphery of the ridge, it will do so too far to the east to produce a significant storm for the forecast area. Had this ridge been centered a at the longitude of Montana as opposed to Minnesota, far greater travel disruptions would be likely given that a faster rate of amplification would yield a much more significant storm further to the west, and closer to the coast. While that will not be the case,  a period light snowfall will leave light accumulations across especially the interior.

 

 

However, the track of the system will be to the north of the area prior to a full transfer to the coast, which will allow winds to switch to the south. 

 

This will eventually induce a transition to rainfall prior to precipitation ending Tuesday evening across much of the coastal plain, which will obviously affect which locales manage to observe a White Christmas (1" or greater of snow on the ground at 7am Christmas morning).

Anticipated Storm Evolution

Light Snow will begin overspread the western half of the area during Tuesday morning and reach all but the eastern third of the area by midday. The morning commute should be fine given the late start of the snowfall, meager intensity and the reduced volume of traffic given the holiday period, however, commuters in Connecticut may be wise to leave some extra time, especially southwestern areas.

 

The south coast of Connecticut will have transitioned to rain b early evening, as the storm begins to intensify and the storm moves closer to peak across the area, which will slow a Tuesday afternoon commute that will likely already have a heavier volume owed to holiday traffic. 

 

The storm will begin to redevelop off of the coast by midnight into the early predawn hours of Christmas Eve, but not before precipitation ends as rain roughly inside of I 495.

 

Snow and rain showers will taper off during the predawn hours on Christmas Eve, with the first light yielding a festive blanket of white across the interior, a barren, puddle-filled earth near the coast.

 

Stay tuned for more information on what maybe a similar system on Boxing Day.

FIRST & FINAL WHITE XMAS CALL:

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First Call For Weekend Snows

Major Accumulations "Blocked" From Region

Latter November Stratospheric Warming Impacts On The Forecast

The time has come for an Eastern Mass Weather mea culpa of sorts as it pertains to the stratospheric warming this past November. The winter outlook published during the early portion of November referenced the early December 2000 analog in postulating that this warming would not result in a reversal of the 850 zonal winds across the arctic. It was also (incorrectly) asserted on December 16th that this event did in fact barely fall short of the reversal criteria.

 

This was an honest mistake, as it has been brought to my attention by the very meticulous Larry of the Americanwx.com forum, that there was barely a marginal reversal on November 28th.

 

This is in more in line with the December 4th, 1981 warming that was cited as a potential alternative scenario that would in fact yield a full reversal. Ultimately, while the warming episode in-and-of-itself was relatively well forecast using the 1981 and 2000 analogs, the ramifications of it were certainly understated in that high latitude blocking was not expected to materialize prior to the new year, following the mid-month relaxation. However, it is now clear that this will be incorrect given that NAO blocking has rapidly burgeoned into existence on modeling in the period centered around the holiday week.

This is taking place approximately 30 days after the 11/28 reversal and consistent with the time lag of the development of high latitude blocking following a reversal per research, and this will have major ramifications on the potential weekend storm.

Synoptic Evolution Yields North Atlantic "Traffic Jam"

The emergence of the north Atlantic block this Christmas, on the eastern side of the NAO domain in response to the previously referenced stratospheric warming late last month, is what sets in atmospheric "traffic jam" of sorts. 

 

Normally, high latitude blocking helps to facilitate snow storms for the area, but in this instance the opposite is actually the case. Northern stream energy dumping into the Maritimes pools underneath the developing block once off of the coast and amplifies. At the same time, the energy for the weekend potential ejects out of the Pacific trough careens east-north-east over the top of the central US ridge and on a collision course with the Great Lakes by Boxing Day.

 

The lead wave has developed into a deep, closed 500mb low near the 50/50 position by this time and is largely impeded from exiting to the NE by the nascent block to the southeast of Greenland. This results in yet another zone of confluent flow over New England since this system can not "get out of the way", so to speak, which shears and redirects the weekend system to the southeast as it approaches the region after descending from the peak of the ridge.

 

 

This should relegate any significant snowfall to the southwestern quadrant of southern New England, primarily impacting portions of the state of Connecticut. 

Expected Storm Evolution

Snowfall should begin to overspread the region from southwest to northeast late in the evening on Friday night.

 

Reaching the Boston area during the predawn hours on Saturday morning.

 

However, as this point it will begin to encounter resistance from the block and associated confluence to the northeast, thus flurries will struggle to reach far northeastern Massachusetts by late morning.

 

Snowfall should begin too taper region wide by late afternoon and early evening.

 

FIRST CALL:

Final Call will be Thursday night or Friday.

 

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 Strong Forecast for Tuesday Snows

White Christmas Winners & Losers Accurately Conveyed

Marginal Conditions Made For Relatively Complicated Forecast

Some of the more difficult forecasts take place in the lighter events, as small deviations from the expected amounts are often more discernible when it means a slushy coating versus 3", as opposed to 12" versus 15". This is especially true when the stakes are a bit higher as they were yesterday, since not only were early cancellation decisions looming, but the timing of the storm carrying into the predawn hours of Christmas Eve meant that it had immense implications on the prospects for a White Christmas. There were also some complicating factors at play given that the storm had to overcome both a very dry antecedent airmass, as well as marginal temperatures given the path of the disturbance tracking to the north of the region.

 

Be that as it may, the forecast ended up verifying quite well.

 

Forecast Results

Here is the Eastern Mass Weather Final Call versus the actual outcome. 

 

Note that the amount of snowfall reports exceeding 2" are relegated to outside of I 495, as specified in the forecast map from Saturday night (left). Given that temperatures today are expected to rise well into the 30's, a white Christmas will likely be relegated to this locales having received at least approximately 1.5" of snowfall. Thus only the northwest edge of the "White Xmas possible" area is likely to meet the official criteria of having at least 1" snow depth on Christmas morning. The weaknesses to the forecast are that 2" reports from the western part of the state are lacking, and there are some 4" amounts in the southeastern corner of New Hampshire, thus coating to 2" and 2-4" ranges would have been more advisable in these areas, respectively.

 

Final Grade: A

 

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Final Call For Saturday Snows

More Significant Impact Region-wide 

Synoptic Overview

There have been subtle changes from First Call issued on Monday that will have fairly significant impacts across the region. The general evolution remains the same, as the northern steam disturbance approaching from the west will encounter confluence over northern New England and weaken while being deflected to the southeast.

 

However, it now appears that said confluence will be slightly weaker and further to the northeast, meaning that the approaching system makes it slightly further north and remains coherent enough for sizable impacts into the region.

Note that by 4pm this afternoon, the system has closed over the eastern Great Lakes.

 

 

After midnight, the mid level low makes it into New York State, at which point it begins to encounter the confluence from the block to the north east, and in response weakens and veers more to the southeast. 

 

 

Expected Storm Evolution

Snowfall should begin to break out across southwestern Connecticut this evening.

 

Overspreading central Massachusetts by midnight.

 

 

And the balance of southern New England by shortly thereafter, although it may struggle to reach the ground initially due to the antecedent dry air.

 

During the predawn hours, a heavy burst of snow moving northeast in association with the 700mb warm front will begin to slow over Connecticut river valley of Massachusetts into the far northeastern corner of Connecticut, as the warm push aloft meets resistance from the confluence stationed over Maine. 

 

This will prolong a few hour stretch of up to 1-2"/hr snowfall rates in this area.

 

 

 

It is at this point that the system will begin to weaken, and the dynamics will before more diffuse as the storm progresses eastward.

 

This is evidenced by the disparity in the forecast soundings across the state of Massachusetts, as the profile is much more conducive for a period of heavy snow at Westfield, for instance, than it is at Lawrence.

 

DEC 26     Final Call For Saturday Snows Final Call For Saturday Snows More Significant Impact Region-wide  Synoptic Overview There have been subtle changes from First Call issued on Monday that will have fairly significant impacts across the region. The general evolution remains the same, as the northern steam disturbance approaching from the west will encounter confluence over northern New England and weaken while being deflected to the southeast.   However, it now appears that said confluence will be slightly weaker and further to the northeast, meaning that the approaching system makes it slightly further north and remains coherent enough for sizable impacts into the region. Note that by 4pm this afternoon, the system has closed over the eastern Great Lakes.     After midnight, the mid level low makes it into New York State, at which point it begins to encounter the confluence from the block to the north east, and in response weakens and veers more to the southeast.      Expected Storm Evolution Snowfall should begin to break out across southwestern Connecticut this evening.   Overspreading central Massachusetts by midnight.     And the balance of southern New England by shortly thereafter, although it may struggle to reach the ground initially due to the antecedent dry air.   During the predawn hours, a heavy burst of snow moving northeast in association with the 700mb warm front will begin to slow over Connecticut river valley of Massachusetts into the far northeastern corner of Connecticut, as the warm push aloft meets resistance from the confluence stationed over Maine.    This will prolong a few hour stretch of up to 1-2"/hr snowfall rates in this area.       It is at this point that the system will begin to weaken, and the dynamics will before more diffuse as the storm progresses eastward.     This is evidenced by the disparity in the forecast soundings across the state of Massachusetts, as the profile is much more conducive for a period of heavy snow at Westfield, for instance, than it is at Lawrence.

Westfield, MA

Lawrence, MA

It is around mid morning that the focus of the storm will shift to the north and south shore of the Boston area, as ocean effect and enhanced snow becomes the predominate low level forcing mechanism as the mid level dynamics decay.

 

 

 

Snowfall will taper off across the region of the region during midday hours. and ultimately during the afternoon across the north and south shore.

 

 

FINAL CALL:

 

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Friday-Night Saturday AM Snows Verification

Strong Overall Forecast With One Major Exception 

Failed Hedge Against Data For Best Banding

The vast majority of the forecast was very accurate, as can be seen by the juxtaposition of maps below.

 

 

However, the one glaring fault with the map is unfortunately where it mattered most, which is respect to the all-important best banding. As it turned out, the 6-10" "bonanza area" should have been placed approximately 50-60 miles to the southwest, over the southwestern half of Connecticut, as the consensus of guidance had emphatically signaled. The rest of the forecast was perfect with the exception of the north shore of Massachusetts, where the ocean enhancement did not come to fruition, so 1-3" would have sufficed as opposed to the chosen 2-5" range. But this issue pales in comparison to the aforementioned discrepancy with respect to the erroneous placement of that extremely heavy band that contained snowfall rates of up to 3-4" for a short period of time.

Overemphasis Placed on 700MB Warm Front

The main forecast premise in Eastern Mass Weather's Final Call yesterday was that guidance was not emphasizing the placement of the 700mb warm front enough as it pertained to where the band of exotically heavy snowfall would materialize. This was obviously crucial since this band ultimately led to major accidents despite a lower holiday volume of traffic. The forecast rationale was that the band would focus where the 700mb warm front stagnated, which was over the upper Connecticut river valley of Massachusetts, much of Worcester county and into the hills of extreme northeastern Connecticut and northwestern Rhode Island. 

 

However, the reason that guidance was not targeting this area where the mid level front stagnated, as it would under conventional synoptic circumstances ,is that the dynamics of the system were eroding and being displaced southeast at that point due to the resistance it was met with by the confluent flow to the northeast. 

 

This is an issue that the forecast took into account and contemplated when considering the disparity between the ominous forecast soundings over western Connecticut and, the tamer scenario being depicted over Worcester in central Massachusetts. 

 

However, Eastern Mass Weather incorrectly took a leap of faith that guidance was eroding the dynamics too quickly, which was the ongoing trend. This would have allowed central Massachusetts to better avail of the stagnating midlevel warm front to endure a protracted  period of enhanced snowfall rates, but ultimately these rates occurred instead further to the south, over Connecticut, as the consensus of guidance had been indicating. 

 

Final Grade: B-

[SJonesWX]

B-?
I respect your forecasts and you putting yourself out there, I really do. Had you not put in the 6-10” area you would have had an A for sure. But you owned up to that mistake which is admirable. I guess I’m not sure that I agree with your grading, but as I type this I’m not sure you were that far off with your grade. C+ maybe?

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1 hour ago, SJonesWX said:

B-?
I respect your forecasts and you putting yourself out there, I really do. Had you not put in the 6-10” area you would have had an A for sure. But you owned up to that mistake which is admirable. I guess I’m not sure that I agree with your grading, but as I type this I’m not sure you were that far off with your grade. C+ maybe?

I agree that is a big miss on that central area, but I still sent B- bc the rest of the forecast was essentially perfect...but yea, I had pause. I am usually a pretty tough grader, but threw myself about bit of a bone there...many outlets were also too robust in the max area, too...hell, NWS went 8-12". I was torn between B- and C+, so that is reasonable.

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December Colder Than Forecast

January Poised to Deliver Increased Volatility & Storm Chances

December Review

The month of December has featured near normal to slightly above normal snowfall throughout the majority of the region, although some areas near the coast are going to finish slightly below average given that the month was not as active as anticipated.

 

Although the rate of storm's was not as frequent as theorized, the region's first widespread snowfall did in fact occur near the end of week two, as implied by the analogs in the winter outlook. The month behaved largely as expected in terms of the evolution of the pattern, primarily owed to the similarity of the anticipated deviation from the predominate Maritime Continent forcing of the past decade, to the analog periods of December-January 2017-2018 and 2024-2025.

 

December 1-25 2025:

 

December 2017-January 2018:

December 2024-January 2025:

 

However, the month has been much more extreme than forecast in terms of the magnitude of negative temperature departures by approximately 5 degrees F, as the range across the region is between about -4 to -6F, as opposed to -1 to +1F forecast. This is largely due to the extreme Bering Sea blocking (-WPO) that has been observed (-3.48 WPO peak on 12/13), which has also amplified the positive anomalies that were expected across the western CONUS. 

 

 

In fact, the magnitude of the mean WPO this month has been so extreme that it has entirely negated the consistently negative RNA that was anticipated to modify the pattern to an extent.

 

 

This extreme nature of this -WPO block is also likely at least in part why the pattern remained drier than expected, as northern stream clippers were the predominate storm type as opposed to the anticipated SWFEs. There are also additional idiosyncrasies with regard to the polar domain that have conspired to bias the forecast consistently colder than expected, despite a fairly well forecast upper level pattern.

Behavior of the Polar Domain During December

The polar vortex disruption that occurred near the end of November was similar to the December 4, 1981 alternative scenario that was laid last fall in that it barely managed to trigger a full 850MB zonal mean wind reversal in the arctic, where as the 2000 event missed by a narrow margin. Thus it was slightly more significant than the December 2000 analog event.

 

 

However, the vortex still recovered to something approaching climo levels by the holiday week, as forecast last fall per research conducted Lee et al (2019), which indicates that the arctic high regime is favored for less than 20 days following a weak PV (roughly November 28th). This despite the insistence of initial modeling and social mediaologoists alike that it would remain weak through the new year. 

 

 

 

This consistent recovery of the PV following the major disruption one month ago did in fact result in a reprieve from the high latitude blocking by mid-month, as forecast, however, the blocking has unexpectedly rematerialized over the course of the last week given that the stratosphere and troposphere are uncoupled.

 

The hostile polar domain during the mid-month interval in conjunction with lowering heights on the west coast as reflected by a descending PNA value did in fact allow the storm track to shift west just in time for the forecast "Grinch Storm" to verify , which eradicated all of the snowpack throughout southern New England. 

 

 

This is event was similar to periods prior to the Christmas holiday in the referenced analog months of December 2007 and 2008, especially the former.

 

However, there were idiosyncrasies with respect to the Polar vortex that acted to mitigate these warmer interludes. First of all, it has been consistently stationed not only on this side of the hemisphere, but as nearby as southeastern Canada, which in conjunction with the potent Bering Sea ridge has ensured that the mild intervals were always shorter in both duration and magnitude than the colder intervals given that the cold air source was always so readily accessible. Additionally, even when the stratospheric vortex had recovered to climo level and beyond as the holiday period approached, it remained uncoupled from the tropospheric vortex, which has allowed the blocking to redevelop, further attenuating what was expected to be a prolonged stretch of milder weather across the northeast.

 

This has been especially evident during the latter portion of the month, which has acted to mitigate the moderation of the pattern despite the development of the expected Pacific trough regime.

Mid-Month Pacific Trough Transition Well Forecast

There has been a battle waged throughout the holiday period, as the MJO has remained shallow and variable, while the Pacific Trough regime asserted itself right on schedule. Note the disparity between the first and second half of the month.

 

The first half of the month was remarkably similar to the aforementioned MC mismatch analog periods.

 

 

And the latter half baring a striking resemblance to the Pacific trough data set, despite not being mild across the east in the mean due factors previously discussed.

 

 

Be that as it may, the development of this Pacific trough regime further evinces one of the premises of the Eastern Mass Weather winter outlook from last fall in that the MJO would continue to struggle to accrue residence time in phase 8 because of the west Pacific warmth.

 

This has indeed been the case.

 

The consistent struggle for the MJO to remain a coherent wave in the western Pacific has been a theme over the course of the past decade that any seasoned forecaster should not have neglected to consider.

 

 

The disjointed signal that results from the continuation of the battle for proxy between the MC and western Pacific will allow for an occasional element of western Pacific expression to assert itself into the pattern as a byproduct of the areas of dual forcing. This has been, and will continue to be especially prevalent during the final week of the month, as arctic intrusions have managed to periodically infiltrate the northeastern quarter of the country and limit what would have otherwise been a mild couple of weeks in the mean during this ongoing Pacific trough regime. The final sequence of 2025 is a prime example of this, as the system poised to impact the area tonight and tomorrow drives west of the region and induces a warm up for Monday propelled into the region on a southwest flow. 

 

However, this warm up is met with resistance from antecedent cold, which produces a period dangerous icing early Monday morning before a surge of warm air transitions precipitation over to plain rain.

 

The departing storm is then rapidly followed up by yet another invasion of arctic air the very next day.

 

This is the type of pattern that heralds in the new month, and new year, and as will be discussed, also may serve to trigger a chain of events in the stratosphere that will ensure that the latter portion of the season is anything but characteristic of a typical cold ENSO season.

 

January Replete With Volatility & Ample Opportunity For Storminess Amid Colder Risks

Pacific Trough & MJO In Conflict With Polar Domain

The Eastern Mass Weather winter outlook asserted that an intensifying polar vortex and Pacific trough regime would bias the first half the month of January significantly milder than average, which would then be largely negated by mid month transition to a +TNH regime that would usher in a return to arctic cold and winter storm opportunities. It was to be a tale of two months that would average slightly milder (+1 to +3F) in the mean.

 

 

 

 

The forecast will be graded as is per the scoring guidelines that Eastern Mass Weather has abided by since its' inception. However, it is now clear that there are colder risks earlier in the month due to the same factors that conspired to bias December much colder than expected. These factors are the aforementioned close proximity of the mean  polar vortex position to the forecast area, as well as the tendency for the polar stratosphere to remain uncoupled from the troposphere during intervals of time in which it is stronger than average. Clearly the PV has strengthened to near and above climo levels, as forecast, but this has not been accompanied by a consistently hostile polar domain during this period as forecast. 

 

Note the presence of the seemingly omnipresent -NAO block, which in conjunction with the behavior of the PV continues to allow colder intervals during a Pacific trough regime that would ostensibly favor consistently mild conditions.
 

 

The first half of the month will be similar to the latter portion of December in that it will be a battle between a

 polar vortex that will be stationed near James Bay in the lower levels, while the stratospheric vortex consistently stretches in the direction of North America, and a meager MC MJO wave that will constructively interfere with the Pacific trough regime.

 

WEAK MC FORCING WILL ALLOW FOR SOME MILDER DAYS

 

 

In the mean this pattern is likely to yield a metaphorical "tug-of-war" between arctic air and warmth that will not want for storm opportunity. The first such opportunity looks to occur in the vicinity of January 6-7th.

 

Thereafter, during the second week of the month, there is likely to be another milder push before major changes ensue that will have colder shorter-term implications. This will also potentially trigger a feedback in the stratosphere that will determine the manner in which winter 2025-2026 concludes..

 

 

Latter January +TNH to Potentially Shuffle Stratospheric Deck

Looking ahead the second half of January, there are signs that the mid-month stratospheric reflection event theorized to take place in the Eastern Mass Weather winter outlook may indeed come to fruition.  This would not be a surprise given that the onset of the Pacific trough regime in mid December did in fact materialize, and this is the most conducive pattern to the development of a reflection event. Consider the following passages from the winter outlook issued last fall for added clarity: "Research by Kretschmer et al (2018) illustrated the importance of planetary wave reflection for anomalous cold across North America. This expounded on earlier work by Kordera et al (2016) that found that wave reflection born of Pacific blocking tele-connected to a down stream trough over North America. Thus the implication here is that the Alaskan ridge pattern, which is not at all connected to the PV and is actually accompanied by a +NAO, as alluded to earlier, is most conducive to reflection events and is this correlated to the most severe arctic outbreaks in the US. Lee et al (2019) refers to this type of pattern as the "Alaskan Ridge Regime" , which is similar to the Tropical Hemisphere Pattern (+TNH)".

 

 

Here is a composite of seasons that fit this +TNH description over the past decade or so.

 

 

Note the similarity to the -EPO predominate extra tropical Pacific pattern that is favored this winter season.

 

 

The implication here is that there is an increased likelihood for both anomalous cold outbreaks, and deviation from the predominate MC regime of the past decade during winter 2025-2026, which corroborates with previous forecast methods considered. 

"Lee et al (2019) identified the transition from this Pacific trough regime to the Alaskan ridge regime as most conducive to the type of wave propagation needed to trigger a reflection event, which renders the Pacific trough regime the precursor pattern".   

 

Note the Presence of  the Central US Ridge in PT Regime (Lower Right) That Has Been Consistent Throughout Latter December

 

Lending confidence to this notion is the fact that the European ensemble guidance implies the possible development of an Alaskan ridge regime beyond mid month, as the EPO and WPO are expected to descend, and the PNA and NAO are forecast to rise.

 

 

While this does not absolutely ensure the development of a +TNH pattern later in January, if one were to develop, this is precisely the type of behavior from these teleconnections that any forecaster would expect.

"The mean length of the process is 20 days, with 10 days being the minimum, and 60 days the maximum, per Lee et al (2019). Considering the length of the 2001 (33 days) and 2018 (20) events, it as expected that a reflection event will begin between approximately January 13th and 16th 2026, and end between about February 2 and 18th. This is consistent with the climatologically favored time frame per the research that was referenced previously". 

"The expected midseason progression is illustrated in the schematic below that details the typical evolution of reflection event. Note that the pattern begins to reverse in earnest approximately 5 days after the event onset, during which time the anonymously strong PV reverts to normal strength, and begins to stretch due to the building Alaskan and Aleutian ridging that retrogrades from North America. 

This is why it is often very mild at the onset of these events, during the antecedent Pacific trough pattern, but by ten days post reflection the pattern has reversed and is characteristic of an Alaskan ridge regime (bottom right above). Once the process terminates and run its course by early February, a more canonical La Nina pattern will likely ensue in the absence of the expected SSW. However, should one materialize as anticipated, another arctic high interval is possible for potential grand finale to the season".

More on this possibility later, but this particular update shall conclude with an acknowledgement of the resemblance between the modeled latter January (21st) pattern, and the wave 2 type precursor pattern to a an eventual PV split.

 

The February 4th date that is implied using the 14 day window noted in the above annotation is well within the January 17th and February 17th window identified last fall for a SSW and accompanying PV split. 

Conclusions To Be Drawn

Lots to consider, but in summary, the early January Pacific trough regime will be largely negated by a polar domain that will continue to be conducive (lower PV that is close by and uncoupled from stronger stratospheric vortex, and stratospheric vortex that often stretches towards northeast US) to allow for colder interludes into the northeastern US. The current Pacific trough regime is then forecast to act as the precursor to a mid-January reflection event, which will induce the Alaskan ridge regime necessary to ultimately trigger a wave 2 split of the PV in February, which will all about ensure an ending to this winter season that will be nothing like the tame ending that we have come to expect from cool ENSO winters. Needless to say, the month of January should be anything but mundane, so stay tuned-