Flash Weather: 2015-16 Winter Weather Forecast

Well, folks. We’re only 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 winter weather forecast for middle Tennessee last year held up rather well. Granted, I normally don’t toot my own horn considering I’m just a humble, amateur meteorologist in it for the love of the science; however, after the multiple ice storms and ankle-biter snow events we experienced between January 23 - February 21, I’d have to say my “B+” grade, in terms of activity and magnitude, was the correct call.

So as the leaves start changing and the days grow shorter, I’m sure many of you are wondering: what can we expect this winter. Well, I’m so glad you asked.

Last year, I itemized and graded certain atmospheric criteria to obtain an overall grade for the winter. This year, I’ll follow a similar approach (with slightly less technical jargon and definitions) so you can better see how each teleconnection works together.

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ENSO – We start off by checking the sea surface temperatures in the Pacific Ocean where things have really heated up since last winter. Note the extensive coverage of red (which represent positive SST anomalies (departure from average) in °C) on the animation below.


After some underachieving El Niño episodes in recent years (i.e. the neutral ENSO winters of 2012-13 and 2013-14, plus the borderline weak El Niño episode of last year), it’s impressive to see this one maintaining its intensity. In fact, the CPC (Climate Prediction Center) has already issued an El Niño advisory with a 95% chance of El Niño continuing through Northern Hemisphere winter before weakening next spring. Thus, with a strengthening moderate El Niño already in play, it’s no question Mother Nature has reshuffled her deck.

Of course, you’re probably wondering what this all means. Well, for starters, a moderate to strong El Niño can provide more emphatic impacts for various regions, such as more rainfall in California, more persistent drought in the northwest and Great Lakes, and stormier, unsettled weather in the southern plains and southeast. But as the case with snowflakes, no two El Niño’s are ever alike and as the 2015-16 El Niño comes into focus, it’s becoming clearer this year’s version will be far from typical. 


Perhaps the most pressing ENSO-related issue right now is if and when the warmest water in the Central Pacific will shift west over the coming weeks. While it may seem trivial, the retrogression of the warm core goes a long way in determining winter weather impacts in the lower 48. As the case with an El Niño Modoki, if waters off the coast of Peru cool 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 will increase. If an east-based El Niño wins out (though based on SST progressions, it's looking more likely we won't be seeing a 1997-98 repeat), then we could be looking at a warmer winter.

 

As for now, we play the waiting game and monitor trends in each of the four El Niño regions (Niño 1+2, Niño 3, Niño 3.4 and Niño 4) in hope to unlock clues as to how ENSO will influence the 2015-16 winter weather landscape.

Grade: C+

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PDO/PNA – I’m linking these two telecoms together due to their interconnectedness in this setup. While ENSO (El Niño) and the AO/NAO hog the winter weather headlines this time of year, perhaps the most under-appreciated telecoms in recent memory have been the PDO (Pacific Decadal Oscillation)/PNA (Pacific North American) pattern...as well as the EPO (Eastern Pacific Oscillation), which I won't harp on in this post since its a better short term indicator as opposed to a seasonal forecasting tool.

For those keeping score at home, the PDO has been quite positive the past couple winters after spending the majority of the 90’s and 00’s in negative territory. With the new regime in its third year, odds are, the positive trend will continue, considering the PDO shifts in decadal oscillations, as opposed to the PNA, which operates on a more mesoscale level. 

At any rate, the ramifications of an established +PDO/+PNA (note the +PNA signal for January/February coming off the Canadian SIPS) for our part of the world is huge as they can boost our snow chances,  even when the Atlantic telecoms (like the AMO) are unfavorable. 

Take the past two winters, for example, where a generous +PDO/+PNA combo and a  favorable Northern Pacific Mode helped offset the lack of Greenland/Arctic blocking, as well as the brisk nature of the long-wave pattern.

In both cases, western ridging (which unfortunately deepened droughts west of the Rockies) fueled by warmer-than-normal waters near the Gulf of Alaska promoted troughing in the eastern states, paving the way for polar plunges into our neck of the woods. Yet, despite the arctic intrusions, the lack of blocking ultimately kept the cold air in check by allowing it to propagate quickly into the Atlantic without any resistance. If anyone in middle Tennessee is curious why it has been hard to capitalize on snow opportunities during this stretch, no doubt, it’s been the transient nature of the cold air.

At least as long as we can hold to a +PDO/+PNA, hope lives…specifically in the form of a cold-air delivery mechanism. ‘Cause at the end of the day, you can’t have snow with cold…and you can’t have cold without a blazed trail for it to follow. With a +PDO acting as a dominant driver in the overall winter weather pattern, I’m fairly confident in the idea of this winter NOT being a blowtorch (i.e. above-normal in temperature), which isn’t a bad place to start if you’re a snow-weenie like me.

Note: The only reason I'm not giving this telecom an 'A'  or 'A+' grade is due to the possibility of the warm SST anomalies in the warm +PDO region backing off some by winter's arrival (though how much this happens remains to be seen). My prediction, however, is that the +PDO will hold serve and mitigate north Pacific SST cooling/Aleutian Low influences (which didn't in 1997-98 and one of several big reasons why the eastern two-thirds blowtorched).

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.

With a positive AMO, the tendency for blocking lessens as areas of low heights set up south of Greenland; contrarily, in a negative regime, high heights build in, which can ignite the blocking needed for cold and snowy weather in the east.

Although the transition between phases can occur rapidly, the direction of the AMO has trended in a cool direction, which means an enhanced chance of blocking this winter. Now, as we’ve learned the past few winters, you can experience a colder-than-normal winter without blocking; however, if anyone remembers the brutal winters of 2009-10 and 2010-11, northern blocking can trap cold shots over the same area for an extended period of time. If you prefer mild winters, then you’ll want to hope the AMO reverses course soon.

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 in the US.

Last year at this point, the QBO was trending negative, which lined up well with the developing El Niño Modoki (an ENSO feature driven by easterlies which allows the greater positive temperature anomalies to move westward (or from the east, hence the term, easterlies), away from Region 1.2.

This year, the QBO is trending positive, an unfavorable signal for cold, snowy winter prospects since it triggers an increase in westerly winds and helps strengthen the polar vortex (i.e. keeps it locked near the poles as opposed to dislodged away from it in times when a -NAO moves into northern Canada/Hudson Bay).

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

Will this happen again in 2015-16? Honestly, your guess is as good as mine. But while my expertise with respect to this telecom is limited, I do expect the QBO to fluctuate throughout the winter as the El Niño begins its descent from near-record territory down into the moderate to weak range.

Grade: C-

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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, forecasters can only know how the AO/NAO will behave a few weeks in advance; however, knowing the trend of AO/NAO phasing can have a substantial impact on predicting temperature trends in the 8-14 day range.

Looking at the latest AO results from the CPC, we can see recorded observations going back to June 1, which tell us the AO has been slightly negative for the majority of the summer, only recently rising into positive territory. Interestingly, this is how the AO behaved last year, with an above average spike occurring just in time to issue in the fall season; however, as one can see on the graphic below, a crash back to below average territory looks likely for the first half of October. With all the hoopla going on concerning the strong Mid-Atlantic low colliding with Hurricane Joaquin, it’s no surprise the AO will drop some as an unsettled pattern sets up, in turn, setting the stage for below-normal temperatures for much of the east (though I wouldn't be surprised if we saw a couple more warm spells between now and November 1). 

Grade: Incomplete (rising towards ‘B’ territory)

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The Intangibles (Polar snow pack, solar activity, TNH, etc.)

As far as intangibles go, I’m going to wait and address these issues in greater depth in a later post. What I will say for now is I’ll be carefully monitoring the October Siberian snowpack to note any anomalies in its coverage throughout the month. 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 is evidence of a decline 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 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 westerly (positive) QBO’s mergence with a potentially negative NAO.

Real quick, let me say a few things about the TNH pattern. While the strongest +TNH pattern helped magnify the arctic outbreaks of 2013-14, I don't expect the TNH to be as prominent this winter. For those who may be wondering what the shrek a TNH pattern is, 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).

All that to say...with a moderate to strong El Niño and a warmer presence of eastern Pacific waters, I believe the ridge/trough axises will resemble positions closer to last winter, as opposed to 2013-14. So if you don't hear much about the TNH this winter, this is why.

Grade: B+
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Factoring in the information above, overall, I believe this winter will go down as the fifth coldest and fourth snowiest since 2000, with Nashville seeing more “pure” snow opportunities than last winter (and less ice…fingers crossed); however, as we’ve seen in recent winters, hope must be tempered given each storm is unique and carries the potential to whiff at the last minute.

With that said, I'm still confident we'll see our fair share of cold (apparently, NOAA agrees; see graphic) and snow this winter, given the decent blocking potential, a weakening El Niño (by the time winter arrives), and an amplified longwave pattern driven (and a latter half split flow?) by warm eastern Pacific waters.

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

Overall Grade: B
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References
  • National Oceanic & Atmospheric Administration
  • National Climatic Data Center
  • Climate Prediction Center
  • The Weather Centre
  • Weather Willy
  • AmericanWx Forum
  • DT WxRisk
  • Griteater
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