Overview:
The past year has been characterized by an atypical amount of global heat, as the Earth continues to recover from the positive ENSO extremum of 2015-16. The vestiges of the warmth significantly impacted the winter of 2016-17, producing a top 10 warm winter in innumerable regions. The ENSO extremum acted adjunctively with a retracted Pacific jet and abnormally potent stratospheric vortex to yield a tropospheric reflection featuring widespread ridging across the continental United States. Surface temperatures responded unsurprisingly as a consequence. During spring and summer 2017, the weak La Nina of winter 2016-17 transitioned into a preferentially neutral ENSO phase; however, this period turned out quite ephemeral as La Nina conditions reemerged in the autumn of 2017. Over the past couple of months, the thermocline structure in the Pacific has become increasingly suggestive of a La Nina – an east based La Nina – with deepening cold sea surface temperature anomalies in the eastern tropical Pacific. The orientation of the La Nina event has ramifications as far as the global walker cell circulation pattern, which governs upward motion/subsidence and concomitant tropical convection dispersal. Furthermore, Rossby waves emanating from tropical convection can modulate extra-tropical ridge/trough action centers. As such, mid-latitude weather is often a function of tropical phenomena, which is invariably the case given the enormous energy storage and production in the tropical latitudes. With that being said, high latitude oscillations will dampen or enhance signals such as ENSO depending upon the geographical region under consideration. As we move forward into winter 2017-18, the weak, east based La Nina should persist. Since ENSO is not overpowering, other variables must be assigned more weight. Northeast US and continental US weather is multifactorial, and thus, myopic analyses will indubitably fail. In the following, a cursory explanation of methodology is provided, followed by index expectations, and finally, forecasted pattern progression.
Methodology [Examination of Integral Factors]
• Geopotential height anomalies near the Taymyr Peninsula and Barents-Kara Seas in the autumn season have been shown to portend wintertime northern annular mode modality and thus the overall strength of the polar vortex. High geopotential height signatures have a distinct proclivity to engender enhanced vertical wave driving into the stratosphere which can positively feedback with a weakened tropospheric vortex. The geopotential height signature for this autumn has generally resembled that of a negative northern annual mode (Arctic Oscillation) precursor.
• Various indicators were utilized to diagnose the polar night jet / stratospheric vortex strength; stratospheric temperature profile, zonal mean winds, as well as the active ozone transport associated with the Brewer-Dobson Circulation; certain proxies were used to ascertain the relative strength of this circulation. A stronger BDC implicates greater ozone transport from the tropical stratosphere into the polar regions, which aids in producing stratospheric warming, and thereby usually resulting in a weaker stratospheric vortex. These signals – overall – for the month of November were suggestive of slightly stronger than average stratospheric vortex, which would argue for an ensuing winter slightly positive Arctic Oscillation.
• Snow cover advance in the northern hemisphere was less impressive than previous years; however, the extent of snow cover ended up being at the highest levels of the past 12 years in late October / early November. This – incontrovertibly – will positively feedback and aid arctic air reservoirs in the northern hemisphere. It is hypothesized that this might have a slight to moderate effect on the Arctic Oscillation, tilting it more negative.
• Proxies for solar activity were analyzed and compared with prior years since 1950 in order to determine tendencies with NAM/NAO covariation. Solar flux levels, geomagnetic activity, solar cycle duration, intensity, and trends were included in the analysis. The current solar cycle continues to head toward overall minimum. Research revealed that the probability of protracted blocking episodes this winter is not as high as one would have expected upon perfunctory glance at the rapidly declining sunspot numbers. With that being said, the combination of suppressed solar parameters yields a fairly auspicious signal for blockiness in the northern hemisphere.
• Precursor sea surface temperatures were analyzed in the Atlantic since 1950; this signal produced success 77% of the time in prognosticating the DJF NAO modality, and success 88% of the time when extracting strong ENSO years (thus, even more applicable for the current year). This indicator suggested a mean positive NAO for the upcoming winter.
• Another NAO indicator developed has shown very high predictive ability, analyzing data in the September-early November period. Retrospectively, since 1950, the method failed only 9 times out of 66 years, yielding success in accurately predicting the NAO modality for DJF 86% of the time. Over the past 20 years, the indicator has only failed in 1 year. The signal for this autumn suggests a mean positive winter NAO, concurring with the aforementioned correlation.
• The QBO has entered a strongly negative tendency over the past few months, with rapidly neutralizing values at the 50mb atmospheric level as well. This increasing easterly shear stress in the stratosphere will destructively interfere with vortex intensification events, especially in the early part of winter.
• ENSO and associated tropical forcing patterns were analyzed to ascertain walker cell behavior and potential extratropical influence
• Global atmospheric angular momentum this autumn was suggestive of a retracted Pacific pattern and overall La Nina atmospheric propensity; analog years were evaluated on the basis of similarities in AAM mean patterns as well as poleward transport tendencies. A highly retracted jet is likely to be the dominant pattern in the Pacific with mid-level ridging aligned near the longitude of the Aleutian Islands/Dateline.
• The October inverse NAO correlation rule was utilized as a tool; this specific correlation indicated a mean negative NAO winter
• 500mb geopotential heights were noted during the October-early November period and juxtaposed retrospectively with years since 1950.
• PDO and AMO trends were used adjunctively with sea surface temperature anomaly profiles to determine potential feedbacks on the atmospheric circulation for the cold season. The analyses corroborated the notion that the mid latitude jet would likely be stronger over the north Atlantic and weaker over the north Pacific.
• Recent hemispheric and global warmth – due to its anomalous nature – was taken into account in terms of adjusting the analog year output and final outlook numbers.
Index Expectations
In light of the examination of indicators, it is noted that a capricious atmospheric pattern may be a mainstay of the upcoming winter in the northern hemisphere, with significant / dramatic reversals both intra-monthly and inter-monthly. The following is forecasted for the mean values of the Dec-Jan-Feb (meteorological winter) indices:
ENSO: Weak, east based La Nina
PDO: Near neutral
AMO: Positive (strongly)
EPO: Positive
WPO: Negative (strongly)
PNA: Negative
AO: Negative (slightly negative overall, high variability, occasional deeply negative dips)
NAO: Positive (slightly positive overall, high variability, occasional dips strongly negative and strongly positive)
Given the anticipated teleconnection modalities, the forecasted DJF z500 pattern will be highly reflective of these circulations.
Forecast/Progression:
This winter will be significantly colder than the past two, but still milder than normal for a large percentage – if not majority – of the United States. The final temperature anomalies for DJF in the Northeast US will certainly feel cold compared to the past two winters, even though the numbers still average warmer than normal. The coldest part of winter relative to normal is expected to be the front third, which will be entirely antithetical to the past several winter seasons featuring 60 degree weather deep into December. While 60 degrees cannot be ruled out in a transient burst, I do not expect sustained warmer than normal temperatures to dominate the month for the Central and Eastern US, especially the Mid-west and Northeast US. Research has indicated that there is a heightened probability of stratospheric vortex perturbation in the early part of winter. While zonal mean winds are strengthening, and the Arctic Oscillation should neutralize in the longer term, upwelling wave amplification from the tropospheric pattern and down-welling effects of the easterly shear stress and largely suppressed solar activity will invite modest ozone maintenance early in the season. The vortex strength may fluctuate near normal, so I am not anticipating blocking to extent of 2009 or 2010 in terms of magnitude. It should be sufficient blocking to countervail lower geopotential heights dipping into the W Canada/NW US region, as a consequence of the retracted Pacific jet. December’s pattern should thus feature a negative NAO, negative AO, and the analogs suggest periods of positive PNA are possible as well. It therefore might be the case that the period up through the holiday season will be wintrier than Northeasterners have seen since the inflection point of the early 2010s.
As December becomes January, the NAO should become increasingly positive with a contemporaneous decrease in PNA values, yielding an atmospheric circulation more reflective of benign/mild La Nina years. This circulation could very well be the mainstay – to varying degrees – for much of the remainder of meteorological winter. Snowfall should be above to much above normal across the northern tier into northern New England this month, with precipitation running above average. The variation in the Pacific jet will continue to favor Aleutian ridging with periods of poleward ridging into the Arctic. This will permit arctic blasts to sweep through the Northeast, but the spasmodic pattern will preferentially favor warmth to the southeast of the Great Lakes.
In February, the pattern will be fairly similar to January, except with the potential for more negative dips in the AO and or NAO, as indicated by both research indicators and analogs. This could implicate increased opportunity for snow threats along the I-95 corridor of the Northeast, when also considering climatology peaks during this month. However, the interludes of favorability will be fairly transient, especially compared to the generally more protracted propitious period in early winter. Snowfall will be near or above normal from the Pacific NW and Rockies into the Plains, and also in the Lakes and New England. Precipitation throughout the winter will be lower relative to normal in the southern tier of the US.
March is outside the period of meteorological winter, and thus no confidence is expressed for this month. However, the analogs overall indicate that winter could potentially make a “comeback” in the coastal Northeast, relative to the performance of mid to late meteorological winter.
Analogs:
Based upon an examination of the multifarious variables discussed in the methodology section, the top analog years are as follows:
1. 2005-06 [first tier analog – quadruple weighted]
2. 1955-56 [second tier analog – triple weighted]
3. 1973-74 [third tier analog – double weighted]
4. 1983-84 [fourth tier analog – single weighted]
One will note that the index forecasts are fairly close to the mean of the analog seasons.
November of 2005 featured a dip of the NAO below -1.5 SD, much like this year. The mean DJF NAO was slightly positive, with a negative NAO in December.
2005-06 and 1955-56 had quite a few striking similarities to this year, and to a lesser extent 1973-74.
It should also be noted that the z500 analog composites will appear with lower heights than reality would produce due to the time period against which they’re compared (1980-2010 normals).
Outlook:
Outlook Temperatures
For the Local New York City Region
Dec-Jan-Feb Temperature Departure Outlook: +0.5 to +1.5; Warmer than normal
Expected evolution is colder than normal December (likely -1.5 or colder), warmer than normal January (likely +1.5 or warmer), and warmer than normal February (likely +1 or warmer). There is potential for one strongly negative temperature departure month; the highest probability of this occurring is in December. There is also potential for one strongly positive temperature departure month; this expectation has less confidence as far as timing, but January has a higher probability than February to see this.
Dec-Jan-Feb Precipitation Departure Outlook: Slightly above / wetter than normal
An active storm track is favored through the Ohio Valley and Northeast, with plentiful precipitation from 39N latitude northward along the East Coast.
Nov-Mar Snowfall Departure Outlook: Near normal for the PHL-NYC corridor
Due to the difficultly in prognosticating snowstorms, and possibility that one occurs in an overall warm pattern, the winter distribution of snowfall is lower confidence. However, December should produce above normal snowfall, January below normal, and February near normal. One major (12” or greater) snowstorm is possible this winter. This would likely occur between December 10th-30th or February 5th-15th.
Snowfall guesses for various CONUS locations:
Burlington, VT: 90-97”
Boston: 50-57”
New York City: 27-34”
Philadelphia: 17-24”
Baltimore: 12-19”
Washington DC: 9-16”
Richmond, VA: 3-10”
Raleigh, NC: <5”
Atlanta, GA: <2”
Houston, TX: <1”
Chicago, IL: 50-55”
Denver, CO: 60-65”
Seattle, WA: < 5”
Note: The analog z500 composite is reflective of high variability in the Arctic / Atlantic domains, leaning negative in the AO domain and slightly positive in the NAO domain. This will still be a noticeable increase in blocking compared to the previous couple winters. The below composite is indicative of strong poleward central Pacific ridging which will maintain the cold source in Canada throughout the winter.
VERIFICATION
Meteorological Winter Temperature Departures:
Meteorological Winter Precipitation Departures:
Local Statistics:
Dec-Jan-Feb Temperature Departure Outlook: +0.5 to +1.5; Warmer than normal
Actual DJF Departures:
NYC: +1.1
Newark, NJ: +1.4
LGA, NY: +1.2
Snowfall Forecast: New York City: 27-34”
Actual Snowfall for Winter 2017-18:
NYC: 35.2″
Newark, NJ: 33.9″
LGA, NY: 32.0″
*Note* – snowfall was significantly greater immediately south of NYC, and east of NYC over Long Island with totals of 40-50″+.
Commentary and Grading
This winter could not have gone much better as far as overall temperature departures – values fell directly within the ranges provided from the winter outlook, circa +1 for the DJF period. The way in which we arrived there was quite volatile, and expected to a significant extent. The cold/much colder than normal December verified, as did the very warm second half of the winter. The second half of January 2018 turned very warm, but the cold in the first half of the month led to a colder than normal January departure, an unexpected curve-ball. Nevertheless, the warmth was more than sufficient in the second half to countervail the negatives, and produce a solid positive/warm anomaly.
Nationwide, the idea that the South and the East would be warmer than normal w/ the core of the cold over the Rockies/N Plains verified well. One flaw was that the Southern Plains ended up cooler than forecasted.
Precipitation wise, the general idea of wetter than normal over the inter-mountain West, and the Appalachians/Northeast-Mid-Atlantic with drier conditions across the South verified fairly well.
Select snowfall for other cities:
Boston: 57.5″ / Forecast: 50-57”
NYC: 35.2″ / Forecast: 27-34”
LGA: 32″ / Forecast: 27-34”
JFK: 31.8″ / Forecast: 27-34”
Newark, NJ: 33.9″ /Forecast: 27-34”
Islip: 61.3″
Bridgeport: 39.9″
Philadelphia: 29.8″ / Forecast: 17-24”
New Brunswick, NJ: 40.6″ / Forecast: 27-34”
Wash DC: 7.8″ / Forecast: 9-16”
Baltimore: 15″ / Forecast: 12-19”
Dulles: 11.9″
Worcester, MA: 86.6″ (+/- 3″)
Hartford, CT: 48.3″ (+/- 3″)
Providence, RI: 45.9″
Overall, the snowfall forecast verified very well, with the forecast for less snow than normal south of the Mason-Dixon line, and snowier than normal in New England, working out correctly. NYC itself verified well. The one flaw/unexpected area was from portions of central NJ through Long Island, which had quite a bit more snowfall than I expected (40-50″+). Favorable storm tracks benefited these areas. Snowfall is always the most difficult part of a long range forecast due to the non-linear/stochastic nature of snow storms.
Final Grade – this, overall, will go into the “hit” column, bringing my long term accuracy rate since 2006-07 up to 82%.
Precipitation: A-
Temperatures: A
Snowfall: A-
Winter 2017-18 Final Grade: A-