Flash Weather: 2018-19 Winter Weather Forecast


Well, folks. We’re a month removed from the autumnal equinox…and I think we all know what that means…

…break out the fall décor (if you haven’t already), have yourself a very merry pumpkin-spice latte, and check Cameron’s blog for yet another preliminary winter weather forecast.

Now, I admit: while last year’s winter weather forecast verified for the most part, I underestimated the magnitude of late December-early January cold fearing a multi-week blowtorch would skew monthly temperature means (as it did January and February 2017); however, as it turned out, there were very few stretches December 2017 to March 2018 featuring consistent +5-10 departures. So while I caught the overall pattern in advance, I belittled the margin on account of recency bias. Had I removed such predisposition from the equation, my results would have been more accurate. But as they say, you live and learn, right?

As far as what we can expect in the winter ahead, while we can only speculate off trends and unfolding phenomena, confidence is increasing we’ll see an active, entertaining winter based on current teleconnection probabilities.

But before I present my preliminary teleconnection grades, here's a brief breakdown of some of the signals we'll be discussing in this post (courtesy of DT WxRisk):


In addition, here's NOAA's DJF temperature/precipitation forecast (issued October 18, 2018)...



As we'll examine, the temperature probabilities, though understandable, are likely on the conservative side.

But enough talk. You got the context. Let's dive in...
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ENSO – We start off by checking the sea surface temperatures in the Pacific Ocean where things have continued to warm since last winter. Note the increase of organge and red colors on the animation below.








After consecutive weak La Niña campaigns (albeit 2016-17 fell a hair short of meeting the threshold), it’s impressive to see how much equatorial Pacific waters have moderated this year. Last year, we still had a fairly large cool pool of negative SST anomalies sandwiched between 10°N and 20°N; however, this year, that SST pool has reversed as evidenced by the increase in orange and red 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). Accordingly, it will be interesting to see how long this warming pattern holds and strengthens heading into meteorological winter.


Worthy of note: As of October 11, 2018, CPC (Climate Prediction Center) has issued an El Niñowatch in light of current ENSO SST anomalies and their forecasted progression through early next year. Remember at this time last year, CPC was issuing a La Niña watch as negative eastern Pacific SST’s anomalies advanced late in the summer.

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 slightly above average departures in Niño 1+2 and much above average departures in Niño 3.4, a stark contrast to last fall when both regions experienced cooler departures. Furthermore, we note the eastern Pacific beginning to warm after spending much of the past year below normal. While much has been said in weather circles about the El Niño Modoki (see graphic below) potential, it will be interesting to see if the core of the warmest water will remain in the central Pacific or if the eastern Pacific warming trend will throw a wrench into NOAA's ENSO forecast.

If the waters off the coast of Peru remain cooler in comparison to the Central Pacific and a west-based El Niño establishes itself (west-based meaning the progression of above average temperature anomalies move westward along the equator, rather than eastward), then the probability of a cooler winter for the southeast and mid-Atlantic states will increase.


As for now, it’s worth wondering how strong will this winter’s El Niño 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 Niño'sand La Niña's are death nails in the coffin since ENSO of this magnitude can override favorable teleconnections (like a +PNA/-EPO/-QBO) as we'll see in a moment); however, once you get down to ‘moderate’ category, 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).

In our case, we see a weak El Niño flirting with moderate status, but ultimately falling short. Based on middle Tennessee climatology, this boasts very well for our wintry weather prospects as this generally translates into cooler temperatures as noted by the graphics below (per Huntsville NWS).





Bottom line: Unlike previous years, the ENSO setup looks increasingly favorable with strong signals pointing to a weak (potentially moderate) El Niño.


Granted, El Niño’s are notorious for bringing drier conditions to our region (due to the subtropical jet diving further south); however, all is forgiven if the primary precip type is frozen. Either way, if you’re a middle Tennessee snow fan, a weak El Niño is the best ENSO teleconnection of the six…and that alone is worth being giddy about.

Prediction: Weak El Niñ

Grade: A
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PDO – As mentioned in prior years, while ENSO can hog the winter weather headlines this time of year, perhaps the most underappreciated telecom 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, since 2006, we've seen a strong -PDO (2006-2013), a strong +PDO (2014-16; credit the super El Niño of 2015-16), and now a weak -PDO (2017-present) that in recent months has begun to increase.


With respect to ENSO, this makes sense as an increasing PDO can enhance the effects of El Niño and subdue the effects of La Niña due to proximate warmer SST anomaly pools (see July 2015 graphic below)...


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 graphics in next section) 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 and/or –EPO despite the PDO state?

Based on ENSO state, I lean towards ‘yes’, especially since regions where stronger Pacific SST anomalies exist have a more influential role in the pattern. Whatever the case, I’m not expecting the PDO to be a primary driver in this year’s winter in light of where current indices are at and since I don’t expect readings to drastically fluctuate by time the overall winter weather pattern establishes itself.

Bottom line: Like last year, the PDO will not be a primary teleconnection driver this winter.

Prediction: Slightly below to near neutral PDO

Grade: B-
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PNA/EPO – Since both the PNA and EPO pertain to western ridging, I figured I’d combine the two into one category this year.

Though PNA and EPO are similar, the differences go a long way in determining what parts of the country see Canadian chill or cross polar flow. 

For instance, with a +PNA, the jet pops a ridge over the west coast as it slices down through the Rockies to the Gulf Coast while -PNA triggers reciprocated trough and ridging axes bringing warmth to the east and cold to the west.


Meantime, on the EPO side, a +EPO locks a low near the Gulf of Alaska allowing mild Pacific flow to dominate the conus. Conversely, with a -EPO, a ridge of high pressure sets up in place of the aforementioned low, keeping the ridge west of the Pacific coast and a trough centered over the Midwest.



Essentially, if you're a southern snow fan, you want either a +PNA or -EPO to develop and lock in a multi-week period of arctic air in hope it merges with an active subtropical jet. And while there's little skill predicting months in advance, given eastern Pacific teleconnections have a tendency to be stubborn in amplified patterns, it is possible to gauge probabilities of certain setups.

Bottom line: Based on November projections and the typical evolution of weak El Niños (per DJF ENSO analogs), I believe we see more +PNA/-EPO weeks than –PNA/+EPO within the December 1 - February 28 time frame.

Prediction: More +PNA weeks than -EPO weeks, though either way, the outlook looks favorable for the southeast.

Grade: A-
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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 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).


Taking a look at the current AMO, we see a slightly positive index having dipped below the '0' line earlier this summer.


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.

Prediction: AMO will remain slightly positive (combined with a slightly negative PDO, we can work with this combo)

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 recent QBO history, it's worth noting a +QBO was a major player in the warmer winters of 2015-16 and 2016-17 while last year's -QBO helped usher in cooler temperatures. Unfortunately for the upcoming winter, the QBO, though currently negative, is increasing rapidly. Assuming the trajectory continues, we could be looking at a +QBO by December 1, which by itself, would promote an increase in westerly winds and a strengthened polar vortex (unfavorable for cold air intrusions, especially when merged with a +NAO). Accordingly, it wouldn’t surprise me to see the QBO rise and remain in positive territory throughout the winter. 

If there's any good news, it's the ENSO. For instance, if the emerging El Niño fizzles or parks closer to 0.5, the QBO would have more of a voice; however, since ENSO is projected to be north of 1.0 during DJF, the QBO will likely not have as much influence.  

For now, check out how fast the QBO is rising. Last month, if we compared September 2017 to September 2018, we'd be talking about how much more negative the QBO is this year compared to last; however, when we compare the last two October's, we see an entirely different narrative unfolding...


From here, it's important to discern whether the QBO is west-based or east-based since east-based QBO's often coincide with sudden stratospheric warmings, weaker Atlantic jet streams, and eastern cold whereas west-based QBO's often coincide with stronger jets and western cold.

Lo and behold...we find the QBO, like last year, is east-based between 10-45mb (mainly 30-45 mb) which should favor blocking patterns and troughiness in the east. Not a bad sign if you ask me.


Bottom line: If the ENSO fades, the QBO will be a key piece on the winter weather chessboard; however, if the ENSO verifies, the QBO will play a minor role as the yielded telecom.

Prediction: Negative east-based QBO will rise and become slightly positive; however, unlike last year, the QBO will not play as big a role in the overall pattern (assuming it levels between -10 and 10.

*Spoiler*


As we'll see under 'Intangibles,' we are currently in a solar minimum. Combined with an east-based, QBO, you can see a notable lack of red in the chart above.

Grade: B
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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 alarge-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 next month. ‘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.

That said, it's encouraging to see a -AO/-NAO combo heading into November since cold Novembers translate into cold winters ~65-70% of the time for our region (per Nashville's CF6 data).



While it's too early to know if this AO/NAO dip is transient or the start of multiple-month stretch of negativity, based on the current teleconnection composite (i.e. +PNA/-EPO/+AMO/-PDO/-QBO/ENSO =  weak El Niño), I can't help but think we'll see longer periods of mid-latitude blocking this winter compared to the past three.

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, AAM, TNH, etc.)

As far as last year’s intangibles go, no question I should have downplayed the after effects of a disappointing 2016-17 campaign. Clearly, I tailored my gut, in part, to climo trends as opposed to going off present signals entirely. Consider this my official confession.

As for this year’s intangibles, it’s worth noting the past two months of Siberian snowpack buildup and how it has translated into North America. 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 remains evidence we’ll be 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; however, with below normal solar activity in clear view, I'm not concerned about a brief spike affecting the upcoming winter weather pattern. 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 worth mentioning 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.


Per the animation below, I believe when the MJO is active, we’ll have a couple weeks where it resides in between phases 1-2 and 7-8. Consequentially, temperatures in the east will be colder than average, in part, due to the effects of El Niño.


Lastly, let me say a few things about the TNH and AAM patterns.

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).

Per CPC, "the positive phase of the TNH pattern features above-average heights over the Gulf of Alaska and from the Gulf of Mexico northeastward across the western North Atlantic, and below-average heights throughout eastern Canada. The positive phase of the TNH pattern is associated with below-average surface temperatures throughout the western and central United States, and across central and eastern Canada. It is also associated with above-average precipitation across the central and eastern subtropical North Pacific, and below-average precipitation in the western United States and across Cuba, the Bahama Islands, and much of the central North Atlantic Ocean."


While it’s impossible to forecast amplitudes and placement of troughs this far out, I will say it’s quite possible we see a -TNH this winter as the ENSO overrides it. Accordingly, if you like snow/cold in the southeast…rejoice.

Finally, we conclude matters with the AAM, a measure of global wind torque/momentum...basically how fast the atmosphere is spinning relative to the Earth's rotation. Like most teleconnections, there's a negative and positive phase where a -AAM is often associated with La Niña/western troughs and an +AAM is associated with El Niño/eastern troughs. As we alluded to with the MJO, these states are largely influenced by the locations of western Pacific SST warm pools and thunderstorm activity. 

As you can see, the AAM, on cue, has flipped from negative to positive as we've progressed in October. Based on current ENSO, this makes sense as the impacts of El Niño begin to snowball (pun intended).

Bottom line: With a weak El Niño to compliment these intangibles, the atmospheric stage is set to score the southeast a cooler pattern...perhaps a midwinter snow or two.

Grade: A-
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First Call: In light of my analysis above, in conjunction with signals coming off the JMA, UKMET, and ECMWF weeklies (see below), I believe this winter will go down as the fifth coldest and fourth snowiest since 2000, with Nashville seeing more snow days and prolonged cold shots than last winter. Furthermore, for the first time in my winter weather forecasting 'career', I'm calling for below average temperatures for December, January, and February. A bold take, no doubt, but one I'm willing to roll the dice on. 

Precipitation-wise, I am concerned east Tennessee/North Carolina will see more snow opportunities based on climatological storm tracks in similar setups...


...however, given the signal for colder statewide temperature departures, the future is bright for Tennessee snow lovers.




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

Overall Grade: A-/B+
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References
  • National Oceanic Atmospheric Administration
  • National Climatic Data Center
  • Climate Prediction Center
  • State Climate Office of North Carolina
  • James Spann, ABC 33/40
  • Chris Bailey, WKYT
  • AmericanWx (Shout out to the Tennessee Valley forum)
  • DT WxRisk
  • GG Weather
  • NWS-Huntsville
  • Kirk Mellish
  • GensiniWx
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