Abstract
Historical accuracy has been 80% since inception of long-range forecasting in 2006. The dynamic interaction between the atmosphere and ocean will – to a certain extent – be congruous with the preceding cool season, though certain variables will derogate from that putative status quo. Background forcing is redolent of a continued diminution in solar activity, surrogates of which are sunspot number and solar flux; however, geomagnetic activity has not yet reached a minimum in solar cycle 24. Another exogenous forcing mechanism, the QBO, is in a transitional phase with descending, downward propagating easterly shear stress, but juxtapositions between the present pace and 10-30mb temperature analysis and other QBO phase vicissitudes indicate that the descent into negative will be modestly protracted. Although the QBO, adjunctively with suppressed solar forcing, will be more felicitous than the prior cold season, the descending QBO will not have a significant effect on mediating blocking genesis for much of the winter season. Walker, Ferrel, and Hadley Cell assessment / proxies, atmospheric angular momentum tendencies, Pacific oceanic signaling, my NAO formula, among other considerations, evince that the winter will be dominated by z500 mid-level ridging in the sub-tropical latitude belts, redolent of a Nina-esque structure. Spasmodic poleward ridging is likely in the North Pacific, though much less than multifarious, notable years of poleward ridging. The Atlantic will generally feature a stronger than normal mid-latitude jet with lower than normal geopotential heights near Greenland, though a one-month period of higher than normal heights is possible, if not more probable than not. The themes of the winter sensible weather wise will include plentiful snows across the Rockies, Northern Plains, Upper Mid-west, and potentially into the Eastern Lakes and Northern New England. Elsewhere, snow will be near or below normal. The temperature paradigm will project strongly on the climatological La Nina regime, with colder than normal anomalies in the Rockies and Plains, and warmer than normal in most other areas. Herein, the veridical data will be examined; apocryphal notions will be refuted; and, the overall anticipated patterns delineated.
Methods
One of the principal index forecasting techniques employed is my NAO formula, developed several years ago. It has a success rate of 89%; namely, for the period 1950-2018, the formula correctly prognosticated the ensuing NAO modality for DJF 89% of the time. This is a formula underpinned by significant research; the formula accounts for the multifactorial nature of the NAO via inclusion of several, heterogeneous proxies. Other methods for prognostication include data collated on sea surface temperatures, ENSO status, SOI, IOD, the PDO, AMO, geopotential height patterns and trends in the Arctic during the autumn, diagnosis of atmospheric angular momentum, torques, the GWO, tropical forcing, QBO direction/magnitude, solar forcing (including solar flux, sunspot number, and geomagnetic activity), analysis of Walker, Ferrel and Hadley Cell structure/behavior; and, apposite analogs, that is, years which comport with the present day paradigm.
Results
Prognosticated Teleconnection Index Modalities are as Follows [Note: these forecasts are for the mean values for DJF]:
NAO: Positive
AO: Near neutral, Slightly Negative to Slightly Positive [via significant shifts during the winter]
EPO: Positive
WPO: Negative
ENSO: Near-neutral
PNA: Negative
PDO: Negative
A more descriptive disquisition of the above index results follows.
Discussion
The atmosphere constantly strives to attain a balance it can never achieve. The periodicity of the ENSO cycle is an exemplar of this. A particular snapshot of several years may be dominated by cold ENSO conditions, but a macro-scale view of several decades reveals an increased spectrum of warm and cold ENSO. Thermodynamic disequilibrium drives weather. Ultimately, equilibration would cause energy available for work to approach zero. It is therefore axiomatic that disarray is a centerpiece of meteorology. One atmospheric pattern may be pleiotropic, in that it has a ripple effect of impacts.
This autumn, there are several integral variables to note. The atmospheric angular momentum budget has tended below parity, redolent of more easterly momentum deposits in the Earth system than average. The GWO, a proxy of total torque, tropical/extratropical wind-flow additions and subtractions, demonstrates that the atmosphere is behaving more like a La Nina than El Nino. While momentum has increased over recent weeks due to extratropical torque forcing, that forcing is insufficiently powerful to induce a surplus of angular momentum in the system.
The atmospheric pattern in November 2019 is responding to an interlude of intra-seasonal forcing via the MJO, much like November 2018, which operates to produce an ephemeral mask on the background, base state. Indubitably, the MJO forcing will yield a transient diminution in the Northern Annular Mode (NAM) and NAO values. Again, note that the MJO circuits this autumn are concordant with the circuits of 2018 – amplifying in coherence through phases 6-7, then declining subsequently.
The latest Pacific Decadal Oscillation (PDO) value for October 2019 is -0.81, indicative of an increasingly inauspicious regime for Southeastern US mean troughing. Warmer than normal water temperatures in the Western Pacific, the off-equator portions of the Pacific, and west of the West Coast USA, will act adjunctively and feedback with the neutral to negative AAM regime, favoring a retracted jet, and low-latitude Aleutian ridging. ENSO-QBO interplay and Hadley Cell modulation will permit occasional poleward proclivities to this Aleutian ridging, however, these episodes will increase polar transport into the northern tier of the CONUS.
Dovetailing from the aforementioned expectations, the eastern EPO domain will be dominated, often, by lower than normal geopotential heights. The Walker, Hadley Cell behavior will favor ridging in the North Pacific with concomitant trough in the West/Rockies of the United States. Prior years that featured extensive negative EPO patterns and potent mid-level ridging in the Northeast Pacific are generally distinguishable from the present year, either by way of GWO/AAM tendencies, sea surface temperature anomalies, and/or Walker/Hadley or Ferrel structure and behavior. Note that QBO-ENSO dynamical forcing plays a significant role, too, and La Nina seasons of modest to strong westerly shear stress may force more robust Pacific ridging. Some hints of those factors are present this year, which is precisely why I favor spasmodic poleward ridging, but the factors extant this season do not support a continuance/persistence of poleward ridge of the magnitude recorded in 2013-14, or 2014-15, for example.
The QBO easterly shear stress will eventually down-well to z30; however, the pace of descent as evinced by the slope, and the temperature profile, suggests that there could be a multi-month period of weak positives or near 0 values, prior to the full transition into negative. Concordantly, the true negative phase should not effectuate tropospheric vicissitudes until the second half of winter, particularly February and March. At which time, it is more likely that the NAM/AO tends on the negative side, countermanding the more positive NAM/AO of the first half of winter. It is – additionally – critical to cognize that the likelihood of the stratospheric polar vortex reaching zonal winds of 40 m/s at the 10hpa/60N level in November is very high. Since that event is relatively anomalous, analogs may provide a further hint insofar as auguring the ensuing mean stratospheric (and tropospheric) vortex state. The vast majority of the cases wherein zonal winds reached 40 m/s continued to feature stronger than normal vortices during the winter. Often, reversals occurred in late winter (late January onward). In this particular winter, I anticipate that the stratospheric vortex will weaken significantly in late January, potentially leading to more tropospheric blocking in February. Whether this results in a technical stratospheric warming event is indeterminate.
As noted hitherto, the provenance of the NAO forecast is a combination of disparate proxies that account for its multifactorial nature. The NAO is also an index of persistence and cyclic propensities. Although solar conditions are weak and purportedly near optimal, that is often a necessary but not sufficient condition for extensive NAO domain blocking. There are other factors forcing the mode as well. Furthermore, as a general and practical matter, the descent of the solar cycle tends to force a greater frequency of positive NAO periods. My NAO formula suggests that the mean will be positive for DJF. This is not a deviation from most winters of the past decade.
To recapitulate, the winter will be characterized by retracted jet propensities; neutral to negative AAM; null to 1-2-3, Nina attractor phase GWO orbits; a negative PDO/PNA, a variable AO via less blocking early-mid winter and more late winter; a generally positive NAO, though one month could be negative (favoring February); a neutral ENSO; a warm AMO; and, a QBO/solar combination, though seemingly not very unpropitious, that will tend to underwhelm in terms of its relative inability to induce significant, protracted tropospheric blocking.
Outlook and Progression
December
The month will be characterized by hemispheric forcing agents highly inimical to the development and persistence of high latitude blocking structures. The northern Hemisphere should feature sub-tropical ridging and a stronger than normal polar night jet, thus concomitant +NAM. This should create an overall lower than normal geopotential height anomaly in the Arctic/NAO domain, thus, teleconnecting to mid-latitude zonal westerlies.
The ramifications for sensible weather include a fairly benign pattern, in the context of normal winter conditions, across much of the United States, and likely Western Europe. There should be aperiodic injections of polar/arctic air into the Rockies and Northern Plains. Snowfall should be below to well below normal in the Eastern, coastal regions, and closer to normal the farther north one goes. Across the Lakes and Northern New England, snowfall departures are more correlated to precipitation vicissitudes than temperature anomalies, so for these areas, it is unlikely that snowfall will be below normal. Temperatures, a fortiori, given the z500, will be warmer than normal for most of the South and East.
January
Alterations should begin to initiate across the Arctic domain as the month progresses onward, due in part to downward propagating easterly shear stress via the transitioning QBO. Poleward Pacific ridging tendencies are possible, and, a priori, these ridges may connect with developing Arctic geopotential heights to facilitate a stronger transport of polar/arctic air into the Rockies and Northern Plains. However, the momentum backdrop should be neutral to negative with poor transports.
The resultant sensible weather should near to colder than normal temperatures in the N Rockies/N Plains/Upper Mid-west, and generally warmer than normal in the East/South. Snowfall departures, accordingly, should be below normal for most of the East outside of Northern New England. Central New England may see closer to normal snowfall in January due to increased jet suppression via spasmodic Pacific poleward ridging. Precipitation will be above normal in the OH Valley, interior Northeast, and Lakes, via an active SW-NE storm track.
February
Of the three meteorological winter months, February will differ most from the mean pattern. It is in this month that easterly shear stress will have a more significant effect on the AO and NAO domain, though mediated in significant part by anticipated, enhanced tropospheric receptivity to the genesis of blocking action centers. Higher than normal geopotential heights should dominate the Arctic domain, in concert with some North Pacific ridging. However, there will still be some persistence of SE US ridging via the angular momentum tendencies and backdrop. This should otherwise obviate would could have been very cold and snowy month on the East Coast. Nonetheless, the month should be the coldest and snowiest of the winter, compared to December and January. Still, the expectation is that these conditions will not be sufficient for many of the East Coast cities to reach their average snowfall for the season. Temperatures will be colder than normal across the northern tier and warmer than normal across the South, and near normal elsewhere. Snowfall will be near or above normal across most of the Lakes, OH Valley, Northeast and Northern Mid-Atlantic. There may still be a gradient across the Mid-Atlantic due to SE US ridging, thereby preventing much snowfall south of Washington DC.
March
Low confidence expressed for this month. A continuation of the expected paradigm and analog extrapolation indicates, potentially, a colder than normal month across the North and Northeast US with near or above normal snowfall.
It is expected that the majority of the snowfall for I-95 cities south of Boston will be derived from February and/or March events.
Outlook Specifics: Temperature, Precipitation, Snowfall
NYC Metro Region
Temperature Departures, DJF
Warmer than Normal: +1.0 to +2.0
Monthly Breakdown
December: +2.0 to +3.5
January: +1.5 to +3.0
February: -1.0 to +0.5
Caveat: Note that long range temperature departure prognostications are purposefully conservative in their magnitudes, with respect to the positive or negative direction forecasted. Thus, the implicit implication is that, for December and/or January, the primary risk is that the actual departures are even warmer than I am forecasting. For February, the primary risk is that the departures are colder than forecasted. As such, if the evolution is +5, +2, and -2.5 for Dec, Jan and Feb respectively, that will still fit the progression and overall means forecasted.
Snowfall
Below Normal: 16-24”
Caveat: Most of the analogs congregated between 16-24”. There was one lower outlier and one higher outlier (a single year above normal for snowfall, in the 30s). Thus, the likelihood is high that the total snowfall will be under 25”, but a scenario in which greater than 25” falls cannot be entirely ruled out.
Precipitation
Near Normal
Snowfall Projections for Select Cities:
New York City, NY: 16-24”
Boston, MA: 36-44”
Philadelphia, PA: 10-18”
Baltimore, MD: 7-15”
Washington DC: 5-13”
Richmond, VA: 3-11”
Raleigh, NC: 1-9”
Burlington, VT: 80-90”
Analog and Forecast Maps
The maps are displayed in the following order:
- DJF z500 geopotential height anomaly, analog composite
- December z500 geopotential height anomaly, analog composite
- January z500 geopotential height anomaly, analog composite
- February z500 geopotential height anomaly, analog composite
- DJF temperature departures – analog composite
- December temperature departures – analog composite
- January temperature departures – analog composite
- February temperature departures – analog composite
- My Temperature Departure Forecast Map for DJF
- My Snowfall Departure Forecast Map for the entire 2019-20 winter season
VERIFICATION:
DJF Temperature Departures – CONUS:
DJF 500mb composite:
Accumulated Snowfall Departures through Feb 19th:
Brief Analysis:
Snowfall departures across the CONUS were generally congruous with expectations spatially – with the storm track favoring the Rockies, Upper Mid-west, and Northern New England for near or above normal snowfall. Snowfall forecast for Mid-Atlantic was accurate with well below average totals. NYC-BOS snowfall totals were forecasted to be below normal, with the reality featuring well below average totals.
Overall snowfall forecast for the CONUS = A-
Temperature wise, departures were well above normal across the East, versus a forecast for solidly above normal. The Southeast US and SW US were well forecast. The Plains/NW US were warmer than forecast.
Overall temperature forecast for the CONUS = C/C-
Local NYC region temperature forecast = C/C-
Local NYC snowfall forecast = B/B-
As to the pattern: forecast was +NAO, +EPO, -PNA, and near neutral AO. The only flaw in the pattern outlook was a more positive AO than expected for the second half of the winter, otherwise the z500 coincided very well with expectations.
Overall pattern, 500mb forecast for the NHEM = A-
Thus, the overall winter 2019-20 outlook yields a grade of: B-/B.
This was a hit, and a very good forecast overall, with the primary flaw being the magnitude of the warmth, thus lowering the final grade .