Tuesday, October 4, 2016

Flash Weather: 2016-17 Winter Weather Forecast

Well, folks. We’re two weeks 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 held up on snow, but not so much on temps, granted my overall pattern prediction was correct. If you recall, we absolutely torched in December with + ~13.0° positive temp anomalies (obliterating my December 2015 predictions) before a major pattern flip set the stage for Nashville’s most epic snowstorm in 13 years. By season end’s, most middle Tennessee locations had experienced a top 10 warmest winter, yet with more snow than the previous three winters combined.

Case and point: for Nashville, the winter of 2015-16 finished a full 7.0° warmer than 2014-15 mean temperature-wise, but with 5.2” more snow. I mean…you talk about defying some serious odds. I don’t know what’s more unusual: last winter or two winters prior (i.e. 2013-14) where we had 10” less snow despite the average mean temp being 7.2° colder.

While it’s hard to imagine us having a more improbable winter this season, after three wacky winters in a row, it’s only fair to wonder what to expect in the months ahead, specifically between Thanksgiving and the start of spring 2017.

Thus, as I always do this wonderful time of year, I present you my preliminary teleconnection grades for winter 2016-17...
ENSO – We start off by checking the sea surface temperatures in the Pacific Ocean where things have really cooled down since last winter. Note the decrease of red colors on the animation below:

After an unprecedented 2015-16 El Niño event, it’s impressive to see how much equatorial Pacific waters have cooled this summer. But perhaps what stands out more is the large warmer SST anomaly pool sandwiched between 10°N and 20°N.  Is this a leftover consequence of last year’s super El Niño? I tend to think ‘yes’; however, I expect more shades of blue to emerge in coming weeks on ENSO monitor maps as neutral conditions (typically defined by NOAA as sea surface temperature anomalies less than -0.5° C for the NINO 3.4 region of the east/central equatorial Pacific).

Worthy of note: As of September 8, 2016, CPC (Climate Prediction Center) has canceled the La Niña watch and has since not issued an ENSO alert system status. Remember last year at this time, confidence was unusually high concerning the impending super El Niño as evident by the El Niño advisory issued three months in advance.

This year, there’s no question Mother Nature has reshuffled her deck with “neutral” holding the slight advantage over La Niña for most likely ENSO outcome of winter 2016-17.

Before we continue, 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 3.4 to contain the greater negative SST departures with minor departures in Niño 1+2. With the greatest concentration of negative SST anomalies in the central Pacific, could this be evidence of a La Niña modoki1 in the making? Perhaps. Either way, given the presence of above average anomalies in the low latitudes and a limited cooler pool along the equatorial Pacific, it’s fair to assume classic La Niña conditions will not be in play this winter given ENSO will likely remain locked in neutral through next spring.

My prediction: A return to “La [Nada]” (see 2013-14 & 2014-15 winters)
Grade: B
PDO/PNA – I’m linking these two telecoms together due to their interconnectedness in this setup. While ENSO and the AO/NAO hog the winter weather headlines this time of year, perhaps the most under-appreciated telecoms are the PDO (Pacific Decadal Oscillation) and the PNA (Pacific North American).

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 fourth years, 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 are huge for our part of the world as they can boost our snow chances, even when the Atlantic telecoms (like the AMO) are unfavorable.
Take the winters of 2013-14 and 2014-15, for example, when 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 fueled by positive SST anomaly pools in the northern Pacific promoted eastern troughing, including a few polar plunges into our neck of the woods. Yet, despite the arctic intrusions, the lack of Atlantic blocking ultimately kept the cold air in check by allowing it to propagate quickly off the east coast without much resistance.

Last year increased warming near the Gulf of Alaska and off the California coast (above average SST pools referred to as ‘blobs’) merged with heating equatorial Pacific waters to form an unprecedented super El Niño event, resulting in persistently mild conditions across much of the country.

This year, with ‘super’ out of the ENSO vocabulary, we note the potential of the north Pacific SST blob regaining greater control over ENSO and delivering more frequent cold shots to the eastern 2/3rd's of the conus. In other words, I expect winter 2016-17 to have more features in common with 2013-14 and/or 2014-15 than 2015-16.   

Yes, I know I missed the mark last year by underestimating the magnitude of the anomalous warmth in our area; however, I am confident if we are to see a second straight year of significant above average temperatures, it won’t be because the  +PDO isn’t doing its job. And yes, I realize the PDO is decreasing as we speak; however, I am projecting the PDO to stay weakly positive, perhaps leveling off at the neutral/positive borderline. We'll see.

Prediction: Decreasing positive PDO (decreasing +EPO; will this go -?) leveling off by DJF
Grade: B+ (B- if neutral by start of meteorological winter)
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 negative 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 high latitude 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.

Given most seasonal models agree on the oscillation state in both Pacific and Atlantic, I don’t see any reason to contradict them here.

Prediction: Negative AMO to hold serve in 2016-17
Grade: A-
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 positive, which lined up well with the developing strong El Niño.

This year, the QBO is trending negative, a favorable signal for those hoping for a cold, snowy winter since 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 2016-17? Honestly, your guess is as good as mine.  But while my expertise with respect to this telecom is limited, I will say I expect the QBO to fluctuate throughout the winter, especially if the developing La Niña remains in ‘weak’ territory.

Prediction: QBO flips, becomes factor in mid-winter pattern change
Grade: A-
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, my gut suggests we’ll have longer periods of mid-latitude blocking than last year, just not sustained blocking like we saw during the 2010-11 winter. If we do see a –NAO setup, we may see it occur during winter’s first half before flipping positive in February, but honestly, there’s not much skill in trying to ballpark the AO/NAO this far out.

I will say I am curious to see if the northern Atlantic SST’s stay warmer than average heading into the winter. In general, colder SST anomalies in this region favor an enhanced longitudinal temperature gradient, a stronger jet stream, and +NAO; however, with warmer SST’s, opposite conditions tend to abound. Needless to say, it’ll be interesting to monitor this piece of the forecast as we inch closer to December.

Prediction: East based slightly +NAO on average (with longer -NAO episodes in December/January)
Grade: C+
The Intangibles (Polar snow pack, solar activity, MJO, TNH, etc.)

As far as intangibles go, I’m not sure what to think about the experts who suggest post- El Niño warmth will dominate the winter months. What I will say for now is, as usual, 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 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 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 QBO’s mergence with a potentially -NAO.

Another enhancer I forgot to note last year, but will feature this 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.

Real quick, let me say a few things about the TNH pattern. After one of the strongest +TNH patterns on record helping to magnify the arctic outbreaks of 2013-14, the TNH was basically a non-factor last winter; however, I wouldn’t be surprised to see it return as 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 in regions that saw the greatest plus temperature departments last winter.

All that to say, with La Nada in play as well as the high latitude blocking potential (thanks to noticeably warmer north Atlantic SST’s and –NAO potential), I believe we’ll see enough in the way of favorable ridge/trough axises set up for middle Tennessee (especially west/northwest/middle) to capitalize on some midwinter snow.

Prediction: Stronger MJO activity camping out in mild phases (i.e. 4-6) ~50-60% of time; limited effects from southeast ridging; minor benefits from favorable solar/snowpack activity
Grade: B-
First Call: In light of my analysis above, I believe this winter will go down as the fourth coldest and sixth snowiest since 2000, with Nashville seeing more “pure” snow opportunities than last winter (hopefully 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.
Still, I'm still confident we'll see our fair share of cold and snow this winter, given the decent blocking potential and an amplified longwave pattern driven by warmer than average central/eastern Pacific +northern Atlantic waters.

For a month-by-month breakdown, please check out my YouTube winter weatherforecast video.
Overall Grade: B

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
  • National Oceanic Atmospheric Administration (NOAA)
  • National Climatic Data Center
  • Climate Prediction Center
  • Bureau of Meteorology Research Division 
  • Oregon State Climate Office 
  • North Carolina State Climate Office
  • James Spann, ABC 33/40
  • Chris Bailey, WKYT
  • AmericanWx Forum
  • DT WxRisk
Cover photo creds: deviantart.net

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 United States License.https://www.youtube.com/watch?v=sVkJk3R9G2s

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