Flash Weather: 2017-18 Winter Weather Forecast

Well, folks. We’re one week past the autumnal equinox…and I think we all know what that means…

…break out the fall décor, have yourself a very merry pumpkin-space latte, and check Cameron’s blog for yet another preliminary winter weather forecast.

Now, I’m not gonna lie: my forecast last year busted badly due, in part, to a screaming Pacific jet and lingering 2015-16 Super El Niño side effects. If you recall, outside of a few days in January and March, we absolutely blowtorched from December 23 through the end of March with top 5 positive temp anomalies for the stretch.

Just look at how nasty these CF6's look...





While it’s hard to imagine a worse winter this season, given the nature of winters since 2011, it’s only fair to wonder what to expect in the months ahead, specifically between Thanksgiving and the start of spring 2018.

Thus, as I always do this wonderful time of year, I present you my preliminary teleconnection grades for the winter ahead...
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ENSO – We start off by checking the sea surface temperatures in the Pacific Ocean where things have continued to cool down since last winter. Note the decrease of red colors on the animation below.





After a stubborn La Niña campaign riding her brother’s (i.e. El Niño) coattails, it’s impressive to see how much equatorial Pacific waters have cooled again this summer. Last year, we still had a fairly large warm pool of positive SST anomalies sandwiched between 10°N and 20°N; however, this year, that pool is less pronounced as evidenced by the increase in blue shading (typically defined by NOAA as sea surface temperature anomalies less than -0.5° C for the Niño 3.4 region of the east/central equatorial Pacific). Thus, it will be interesting to see if this cooling pattern holds or reverses course between now and the end of year.



Worthy of note: As of September 14, 2017, CPC (Climate Prediction Center) has issued a La Niña watch in light of current ENSO SST anomalies and their forecasted progression through early next year. Remember at this time last year, CPC was cancelling their La Niña watch as negative eastern Pacific SST’s anomalies started to modify late in the summer. More on that in a second…

For now, let’s recap our four primary ENSO regions (Niño 1+2, Niño 3, Niño 3.4 and Niño 4) as the magnitude of SST anomalies in these zones can influence winter weather outcomes.



Overlapping SST anomalies over these regions, we find Niño 1+2 containing greater negative SST departures than Niño 3.4, a notable flip from last year when the greatest negative SST anomaly concentration occurred in the central Pacific. With a La Niña modoki1 unlikely to verify, it’s fair to say we could see more classic La Niña conditions in the months ahead. The question is: how strong will La Niña be?

To answer this, we must look at the Oceanic Niño Index (ONI), the de-facto standard applied by NOAA for identifying El Niño (warm) and La Niña (cool) events in the tropical Pacific, especially the Niño 3.4 region. Events are defined as 5 consecutive overlapping 3-month periods at or above the +0.5o anomaly for warm (El Niño) events and at or below the -0.5 anomaly for cold (La Niña) events.  The threshold is further broken down into Weak (with a 0.5 to 0.9 SST anomaly), Moderate (1.0 to 1.4), Strong (1.5 to 1.9) and Very Strong (≥ 2.0) events. 




For those in middle Tennessee, typically strong El Niñoand La Niña signals are death nails in the coffin since ENSO of this magnitude can override other pro-winter teleconnections; however, once you get down to ‘moderate’, the ENSO is dialed down enough to the point other signals (like the Arctic/North Atlantic Oscillation) can play a more prominent role (see 2009-10 & 2010-11 winters).

Bottom line: The strength, expanse, and timing of La Niña is going to play an important role in our winter weather outcome for 2017-18. If current mean ensembles are correct in a suggesting a weak La Niña folding by early spring, then chances are ENSO could help us more than hurt us this go-around.

Prediction: Weak “La Niña

Grade: B-
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PDO/EPO – As mentioned in prior years, while ENSO can hog the winter weather headlines this time of year, perhaps one of the most under-appreciated telecoms is the PDO (Pacific Decadal Oscillation), a pattern of Pacific climate variability similar to ENSO but more enduring in nature.

For those keeping score at home, the PDO has been quite positive most of the decade after spending the majority of the 90’s and 00’s in negative territory; however, as we noted last fall, the PDO has been on the decline in El Niño’s wake. Thus, we’ll need to factor in a more negative PDO setup this season given where the needle is currently pointing.

Generally, when dealing with a declining PDO, we find it can enhance the effects of La Niña and subdue the effects of El Niño as a result of proximate colder SST anomaly pools. Contrarily, when the PDO is positive, as the case back in July 2015 (see graphic below), the effects of El Niño are strengthened.



In the case of a +PDO, western ridges ignited by a +PNA (Pacific/North American oscillation) can become primary delivery mechanisms for colder/winter weather even when the (Greenland/Arctic) blocking potential and other telecoms (like the AMO, ENSO, etc.) are unfavorable.

However, in the case of a weakening PDO, prevailing negative SST anomalies can trigger a –PNA/western troughing assuming a +EPO (Eastern Pacific Oscillation; see graphic below) is in play and Atlantic blocking is off the table.

The question is: will we see a blocky setup east of the continental divide to give us a +PNA despite the PDO state?

If yes, we could very well see a 500 mb Dec-Feb ENSO (-.5 to -1.5), PDO (Weak positive to moderate negative), QBO (Moderate to Strong negative) setup (see graphic below):



Bottom line: Despite the PDO lining up “cool-cool” with the ENSO, it won’t be as big an influence this winter.

Prediction: Declining PDO (Part 2)

Grade: C+
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AMO – The AMO (Atlantic Multi-decadal Oscillation) is basically the cousin of the PDO with positive (warm) and negative (cool) phases occurring every couple decades or so. While the main drivers of the AMO aren’t entirely known, the thought is the AMO is a major influence on the behavior of northern blocking.


For instance, with a positive AMO, the tendency for blocking increases (Note: I’ve had this backwards in my mind the past few years…you live and learn the more you study these things ;) as areas of low heights set up south of the Greenland high block; contrarily, in a negative regime, low heights encroach over Greenland, in turn, flatting the flow in zonal fashion across the Atlantic (i.e. not good for cold and snowy weather in the east). If it helps, think of the AMO as the near inverse of the NAO (i.e. +AMO à - NAO, -AMO à +NAO; see two points down).



Bottom line: Although the transition between phases can occur rapidly, the direction of the AMO has been trending positive lately, which could mean better blocking potential this winter. Still, given the Atlantic’s decadal track record, I’m hard pressed to get my hopes up despite the AMO compass pointing in a colder direction.

Prediction: AMO trending positive

Grade: B+
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QBO – Like the PDO and AMO, the QBO (Quasi-Biennial Oscillation), the mean zonal winds of equatorial stratosphere, has a positive phase and a negative phase, with a positive phase favoring the progression of westerly winds and a negative phase favoring easterly winds. Since a -QBO typically weakens the polar vortex with easterly winds promoting a -NAO setup with high latitude blocking, it’s no surprise a -QBO is often linked to cold, snowier winters for eastern conus residents.
Recapping the past few years, we’ve noticed the QBO has been a major player in our back-to-back warmer winters temperatures; however, this year, the QBO is trending negative, a favorable signal for cold as it triggers a decrease in westerly winds and helps weaken the polar vortex (i.e. promotes a more ‘mobile’ vortex and conditions suitable for cold air intrusions, especially when merged with a –NAO).




As some of you may recall, back in January 2014, a -QBO helped unleash the polar vortex southward into the Great Lakes region, which drove multiple arctic shots into in the eastern 2/3rd's of the conus, resulting in many states experiencing a "Top 10 coldest January".

Will this happen in 2017-18? Honestly, your guess is as good as mine.  But while my expertise with respect to this telecom is limited, I will say it wouldn’t surprise me to see the QBO fluctuate (albeit, in mostly negative territory) throughout the winter, especially if the developing La Niña remains in ‘weak’ territory.

Bottom line: When it comes to the current teleconnection chessboard, the QBO is one of two main pieces I’m watching that could significantly alter whatever pattern establishes itself late fall/early winter.

Prediction: QBO continues to fall, becomes factor in mid-winter pattern change

Grade: A-
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AO/NAO – I know I said I wouldn't dive heavily into definitions, but due to the importance of the AO/NAO with respect to winter weather forecasting, I'll make an exception. Just to review, the NAO, as defined by NOAA, is defined as a “large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland...where the surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe as far eastward as central Siberia and eastern Mediterranean and southward to West Africa.”
In large part, the NAO is tethered to the AO, “a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and Western Europe [often helped by some measure of high latitude blocking], and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East.”



So how does this apply to the upcoming winter? Honestly, we won’t really know until November.

‘Cause generally speaking, while AO/NAO trends are beneficial when determining temperature pattern potential in the 8-14 day range, forecasters can only know how the AO/NAO will behave a few weeks in advance.

Still, despite great uncertainty, my gut suggests we could have longer periods of mid-latitude blocking compared to last year.  Granted I struck out with a similar sentiment last year as I underplayed the jet’s response to a weakening PDO and the upside-down omega setup (EPO ridge, western trough, eastern ridge); however, given how poor Atlantic blocking has been in recent years, I expected a reshuffled teleconnection deck can only improve our chances.

Bottom line: While I’m not calling for sustained blocking like we saw during the 2009-10 and 2010-11 winters, I do think we’ll have more prolonged cool spells lasting 10-15 days as opposed to a 3-5 days.

Prediction: On average, an east based +NAO (with longer –NAO periods scattered within)

Grade: I (Incomplete)
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The Intangibles (Polar snow pack, solar activity, MJO, TNH, etc.)

As far as last year’s intangibles go, no question I should have sided with the experts who suggested post-El Niño warmth would dominate the winter months. Of course, I didn’t want to believe them then, but as I know in hindsight, it’s always better to go with the gut as opposed to hope deferred. Consider this my official confession.

As for this year’s intangibles, I’ll be carefully monitoring the October Siberian snowpack to see if it maintains an above-average course. While Siberian snowpack isn’t a primary driver of winter weather in the US, it can enhance the intensity of any arctic air that decides to move our way (Think of it as a cool filter when air masses interact with the landmass).



Concerning solar activity, there’s still evidence we’ll in minimum territory this winter; however, this doesn’t mean a flare is out of the question. Granted, I don't have much knowledge in the area of solar activity forecasting. Yet, with below normal solar activity on the docket, I'm not concerned about a brief spike affecting the trajectory of the upcoming winter weather pattern as a whole. Again, we simply note trends and see how they correlate to the present state of other telecoms, such as the easterly/negative QBO’s mergence with a potential –NAO (though I default to AO/NAO being positive per decadal norm).


Another enhancer I didn’t start mentioning until last year is the MJO (the Madden-Julian Oscillation), a mode of atmospheric variability marked by the behavior of convection and Rossby wave propagation from Indian to Pacific oceans. While precipitation anomalies on the opposite side of the globe may seem like a non-factor, meteorologists now have a better understanding of the correlation between Kelvin waves and areas of up/downwelling, which as mentioned earlier, are indicative of where negative and positive SST anomalies setup. And since Pacific and north/west Atlantic SST anomalies, in addition to AO development/high latitude blocking, are fundamental drivers in establishing a given winter weather pattern, you can bet there’s good reason meteorologists pay attention to the MJO this time of year (click graphic below for animation).



Real quick, let me say a few things about the TNH pattern...



After one of the strongest +TNH patterns on record in 2013-14, the TNH was basically a non-factor during the 2015-16 Super El Niño before returning just in time to signal the 2016-17 blowtorch. Will it be a prominent feature this winter? Remember earlier when I mentioned how a +PNA helps promote ridging in the west, troughing in the east?  Well, with a +TNH, the axis of ridges and troughs shifts westward, which results in a ridge peak in the eastern Pacific (not the west coast), a mean trough over the plains/Midwest (not the east), and a parked southeast ridge east of Florida (which keeps the southeast rather mild). While it’s impossible to forecast amplitudes and placement of troughs this far out, I will say it’s quite possible we see some textbook +TNH this winter. Thus, if you like snow/cold in the Great Lakes, rejoice; if you like snow/cold in the southeast, better hope Greenland blocking…or at least a 50/50 low can squeeze those below-average heights southward.

Bottom line: With a weak La Niña in play as well as the high latitude blocking potential (as hinted by the QBO and PDO), even if the overall pattern is warm-biased, I believe we’ll see enough atmospheric maneuvering to help us score a midwinter snow or two.

Grade: B-
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First Call: In light of my analysis above, I believe this winter will go down as the ninth coldest and eighth snowiest since 2000, with Nashville seeing more “pure” snow days and prolonged cold shots than last winter, yet still above-average temperature wise for the December 1 – February 28 time frame.

For a month-by-month breakdown, please check out my YouTube winter weather forecast video below.

Overall Grade: B-/C+
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Footnotes
1)      “Modoki” meaning anomalies in the central Pacific that have not engaged the class Peruvian upwelling regions, which in turn, limit the trade winds’ effect on SST anomalies; see winters 1998-99, 2008-09, 2010-11
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References
  • National Oceanic Atmospheric Administration
  • National Climatic Data Center
  • NOAA Office of Satellite and Product Operations
  • Climate Prediction Center
  • State Climate Office of North Carolina
  • James Spann, ABC 33/40
  • Chris Bailey, WKYT
  • AmericanWx Forum
  • DT WxRisk
  • GG Weather
Cover photo creds: Pinterest

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