Flash Weather: 2014-15 Winter Weather Forecast

September 26, 2014

Flash Weather: 2014-15 Winter Weather Forecast

Well…it’s that magical time of year again when I switch into “winter weather forecasting” mode and spend a couple hours a day immersed in regional weather forums, computer model output and teleconnection analysis.

Now, I’ll admit…I’m just an amateur. I don’t consider myself professionally qualified nor do I trumpet this forecast as more valid than others out there. ‘Cause truth is: I’m just another voice in the chorus of passionate weather freaks hoping to accurately play-call the most entertaining yet most unpredictable season of all.

However, unlike the past two years, I have greater confidence in my understanding of what will happen during the upcoming meteorological winter. Despite tasting the bitter dregs of busting on a forecast (i.e. 2011-12), I’ve also triumphed in the victory of calling it right-on (i.e. 2013-14). And with my accuracy score improving every year, I have high hopes that this favorable trend will continue.

Although there are numerous meteorological criteria by which a meteorologist bases a winter weather forecast, I will focus on the five that I believe will be the biggest difference-makers this winter. In addition, I will grade each criteria with respect to their potential effect on a colder, snowier winter here in middle Tennessee.

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ENSO

The El Niño Southern Oscillation (ENSO) is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean once every three to four years on average. The Southern Oscillation refers to variations in the temperature of the tropical eastern Pacific Ocean (warming and cooling known as El Niño and La Niña respectively) and in air surface pressure in the tropical western Pacific. The two variations feature a warm oceanic phase, El Niño, which accompanies high air surface pressure in the western Pacific, and a cold phase, La Niña, which accompanies low air surface pressure in the western Pacific. It’s important to note that what actually causes the oscillation is not completely understood at this time and remains under study.

After consecutive neutral years (i.e. no El Niño or La Niña), it looks likely we’ll have a winter impacted by a weak El Niño pattern. Latest guidance by the CPC (Climate Prediction Center, part of the National Oceanic and Atmosphere Administration) indicates the Region 3.4 SST Anomaly will remain somewhere between 0.5-1.0°C through next spring. Even though this sounds like a minor ordeal, small fluctuations in ocean surface temperature can dictate what kind of weather we see during the winter months.

However, this particular El Niño comes with a unique twist. While a typical El Niño pattern features warmer waters progressing along the eastern equatorial Pacific all the way to the coast of Peru, some El Niño’s feature strong anomalous warming in the central tropical Pacific and cooling in the eastern and western tropic Pacific.

This version of El Niño is known as an El Niño Modoki, and is often associated with west-based El Niño’s (west-based meaning the progression of above average temperature anomalies move westward along the equator, rather than eastward). With respect to the SSTA graphics, note how the core of the greatest positive temperature anomalies shifts westward as we transition from the summer months into the upcoming winter months. This difference could lead to a cooler winter for portions of the southeast and mid-Atlantic states.

Although each El Niño Modoki varies with respect to impact, a typical Modoki setup promotes a colder eastern US and a warmer western US during the winter months, with much drier conditions affecting the Pacific coast. Hopefully, our friends out west can defy the odds in the coming months and receive some much needed rains to aid the worsening drought.

Now, to better understand the ENSO, it’s important to be familiar with the four El Niño regions: Niño 1+2, Niño 3, Niño 3.4 and Niño 4. Although each region is of interest, perhaps the most significant region when it comes to forecasting winter weather in the US is the Niño 1+2 region. With the Modoki effect in play, the greatest positive temperature anomalies should remain west of region 1+2 and stay confined to region 3/3.4. This is good news if you’re a snow-lover in the southeastern/eastern states.

According to WeatherBell forecaster and winter weather expert, Joe Bastardi, El Niño’s with stronger positive temperature anomalies in Region 1+2 have been found to trigger warmer winters, while El Niño’s with milder positive temperature anomalies have often coincided with cooler winters. Thus, if the Modoki effect kicks into gear, one has to wonder if the eastern US will reap colder consequences.

Of course, there’s still time for the El Niño to change course compared to what’s being forecast (currently the National Weather Service (NWS) forecasters are suggesting a 65% that the emerging El Niño Modoki will hold serve). But what is more certain is the unlikelihood of this El Niño maturing into one similar to 1997-98, where the sub-tropical jet dominated and kept the polar jet confined to the US/Canadian border.

Now as far as general impacts are concerned, with a weak to moderate El Niño, one can expect the chance of the southeastern/eastern US seeing more cold and precipitation than normal. This is primarily due to the pattern favoring a mobile polar jet dipping further south and interacting with a consistent subtropical jet. When the two jets merge, and colder continental air is allowed to engage the moist, subtropical air, this generally leads to more snow chances for many who live east of the Mississippi River; however, it's worth mentioning how the jet interacts with other atmospheric features (i.e. domes of ridging, the location of strongest air temperature gradients, the warming/cooling of various El Niño regions, etc.) is also just as important when determining where favorable low tracks will ultimately set up.

As many Nashville snow-lovers learned last year, just because you have the cold air in place doesn't mean you're locked in to see snow (#northwestflow, cough). In fact, the strength of a given El Niño is always key in monitoring, as stronger El Niño's typically deliver drier air to the Tennessee and Ohio River valleys, whereas weaker ones keep the drier areas confined further north. If neighboring areas of high pressure, which set up in the northern plains during El Niño's, extend far enough eastward, storm tracks are shunted east and/or southward, which knocks middle Tennessee out of the game (though this scenario often works out well for the Carolinas).

Ultimately, time will tell if all goes according to plan on the ENSO front (pun intended), but given the ingredients currently on the table, and given almost half of Nashville's top 20 snowiest winters of all-time occurred during El Niño winters (as opposed to neutral or La Niña winters), it’s hard to give this grade anything below an ‘A’.

Grade: A-

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PDO/QBO

The Pacific Decadal Oscillation (PDO) is the most influential function of monthly sea surface temperature anomalies over the North Pacific. The PDO has two phases: a positive and negative phase. During a positive phase, west Pacific waters north of 20° N cool to below average temperatures, while the east Pacific waters north of 20° N warms to above average temperatures; the exact opposite holds true in a negative phase.

With a developing weak El Niño and a warmer pool of east Pacific waters near the Gulf of Alaska and Pacific Northwest, I like the chances of a +PDO to continue into the winter months. The combination of these two features will certainly bode well for a colder, troughy pattern in the east, while pumping a ridge over the intermountain west and western Canada.

Concerning the PDO’s phase moving forward, experts at the University of Washington believe the chance of a +PDO verifying is ~80%, partly motivated by August ranking in the top 82^nd percentile for +PDO readings going back to 1900.

Thus, a consistent +PDO should help increase middle Tennessee’s odds of receiving more arctic blasts and snow.

The Quasi-Biennial Oscillation (QBO) is another important feature associated with the equatorial zonal wind. Essentially, the QBO is the measure of stratospheric wind speed and can help a meteorologist understand its effect on the troposphere. Like the PDO, the QBO has two phases: a positive phase and a negative phase.

A positive phase favors the progression of westerly winds and a negative phase favors the easterly winds. So if you want snow in middle Tennessee, then you root for a –QBO, since easterly winds promote a –NAO (which we will define shortly) setup with high latitude blocking, opening the floodgates for colder air to stream down from Canada into the lower 48. A –QBO matches up well with the ENSO, since an El Niño Modoki is 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.

When we take the current +PDO & -QBO into account, it’s hard to find a more favorable setup for a colder, snowier winter for our neck of the woods.

Grade: A-

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PNA

The Pacific North American (PNA) teleconnection pattern is one of the most recognized climate patterns when discussing weather in the Northern Hemisphere mid-latitudes. Again, like the PDO and QBO, the PNA has two phases. A positive phase means above average geopotential heights are experienced over the western US, and below average geopotential heights are experienced over the eastern US. The end result is warm air moving farther north than normal over the western US, while cold, Canadian air is forced southward over the eastern US leading to below normal temperatures. In a negative phase, the exact opposite occurs, leading to above normal temperatures in the southeast, mid-Atlantic and east coast.

Generally, a +PNA is associated with El Niño’s and a –PNA is associated to La Niña’s. Thus, one can imagine how the combination of a +PNA alongside an El Niño Modoki will benefit the odds of a snowier winter. Also, it will be interesting to see if ENSO values with remain on the stronger side of ‘weak’ (i.e. 0.5-1.0°C temperature anomalies in Region 3.4 as opposed to be closer on the neutral side (i.e. -0.5 - 0.5°C).

Perhaps, by now, you are starting to see how many of these teleconnections are intermingled with one another, caught up in some sort of atmospheric snowball effect (again, pun intended). Fact is, it’s hard for one atmospheric element to change without affecting another. Yet, I would still like to have a little more confidence on how the ENSO will behave before upgrading the PNA score higher, especially considering this teleconnection pattern can oscillate between phases multiple times in a given month.

Grade: B

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AO/NAO

As renowned meteorologist, James Spann, often says, “There is no skill to forecast a specific event so far in advance.” In other words, with regard to any type of forecast, there’s no point in pinpointing exact details months in advance. This perhaps is no more relevant than with the Arctic Oscillation and North Atlantic Oscillation (AO/NAO), although there may be some new evidence suggesting the performance of these teleconnection patterns in September and October foreshadow its behavior during meteorological winter (December 1 – February 28).

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.”

The NAO, as a subpart of the AO, is “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 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 (which has partly resulted in cooler than normal temperatures). Currently, we’re seeing a spike into above average territory for the remaining of September, which should translate into warmer-than-normal temperatures for most locations; however, a crash back to below average territory looks likely by early October. Thus, one can expect temperatures to plummet as a more active pattern sets up.

At this stage of the game, it's all about deciphering trends and seeing how they will influence weather patterns 3-6 months down the road.

Grade: Incomplete (though I'll lean in the B- direction for now)

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

Okay, okay. So I’m cheating here by weaving multiple factors into my final point. I suppose you could say I'm lumping the "contributing factors" together, with the first four being more "driving factors". Nevertheless, it’s important to address the intangible issues that may provide notable impacts, of which, I'll discuss four: the TNH effect, sunspots, arctic snow pack and the Polar Vortex.

The TNH isn't talked about much, most likely because it's a relatively new topic surrounding winter weather forecasting; however, it's worth talking about, since an understanding of its negative and positive phases can be beneficial in making short and long term predictions.

First off, the TNH teleconnection pattern should not be confused with the PNA, though these two patterns may appear similar on the surface. True, some of the consequences of +/-TNH and a +/-PNA may overlap, but this doesn't mean they are exactly the same.

With a +TNH, one can expect ridging in the east Pacific, just to the west of the Pacific coast. This contrasts with the PNA, where ridging stretches out over the western third of the conus. In the same light, the toughing axis sets up west of the east coast, mainly over the midwestern states. This is due to a) the riding axis setting up further west and b) the stubborn southeast ridge compromises the position of the jet stream so that the brunt of the cold air remains north of the Mason-Dixon line and west of the Mississippi River.

Remember December 21 last year? Chances are you don't, but hey...no worries; this just means you're normal. However, just to toggle your memory, December 21 featured a muggy, 76 degree day with heavy rain and thunderstorms (definitely squashing any dreams of a white Christmas). Now, check out the map above. See the southeast ridge dominating the eastern Atlantic (off the coast of Florida) into the Caribbean? That's a key signature with a +TNH. It keeps the troughing axis west of our area and as a result, the cold dumps out over the Four Corners and western plains states.

Like the AO/NAO, it's hard to get a solid read of how the TNH will behave months out in advance, but what we do know is the AO/NAO has the power to overwhelm the TNH if the AO/NAO is strongly negative.

Moving on, we come to the topic of polar snow pack. At this time, there’s not much snow pack developing across the northern hemisphere, granted, this is something that doesn’t normally take off until October. We note slightly positive anomalies showing up in northern Alaska, as well as other areas along the periphery of the Polar Circle. Even though it may not make much sense to analyze weather many thousands of miles away, if we can see more positive anomalies develop in North America, this will act as a cool filter when air masses interact with the landmass. In other words, greater snowpack will modify the air (in the colder direction) and in turn, affect the temperature of any air mass that progresses closer to our part of the country.

Bottom line: A colder Canada can only be good for those in central and eastern US hoping for a snowy winter. So this feature certainly bears watching.

In addition to polar snow pack, solar activity is another key intangible meteorologists look at when making a winter weather forecast. Despite some recent solar upticks, sunspot activity should trend downward as we approach winter 2014-15, granted it’s difficult, if not impossible, to predict when major solar flares will occur. Essentially, lower solar activity helps sustain colder air in play, whereas higher solar activity can throw a wrench into a colder winter setup, regardless of the teleconnections patterns in play.

Interestingly enough, researchers in the UK has recently identified a connection between sunspot activity and atmospheric conditions on earth through a phenomenon called “blocking”. As previously alluded to, blocking can have a serious influence on the movement of the jet stream in the northern hemisphere. If the jet stream is blocked, then the jet buckles and lifts up over the area of high heights (whether over western Canada or near Greenland) and allows colder air to move southward into the eastern US. If solar activity increases, then this can have a modulating effect over the duration and strength of the blocking.

At any rate, when we look at climatology and study the coldest, snowiest winters in play, almost all of them had a prolonged “blocking” period, which kept freezing temperatures in the picture for extended periods of time. Pretty remarkable how the sun can shape a winter forecast, eh?

Finally, I'll only briefly mention the Polar Vortex, as it has become a more popular topic of conversation going back to January 2014. The Polar Vortex isn’t what you watched in “The Day After Tomorrow”. Contrarily, the Polar Vortex is a surface phenomenon composed of a large-scale upper level cyclone near or both of the earth’s poles. Sometimes, when the polar vortex is weak, as the case last winter, a piece of the vortex will break off and move southward with the jet stream into the lower 48. It’s hard to predict if we’ll see an episode similar to what happened earlier in the year; however, I wouldn’t be surprised if we saw a similar event unfold given the other teleconnection patterns in play.

Grade: B+

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Factoring in all the information above, overall, I believe this winter will go down as third coldest of the 21^st century and the snowiest since 2010-11, with a chance for Nashville to see more snowfall than the past three winters combined! Given the state of the ENSO and PDO, not to mention the increased blocking threat due to above-average stratospheric temperatures (which ultimately benefit a -AO/-NAO), a solid foundation is in place for a decent winter.However, as we saw last year, hope must be tempered given each storm is unique and carries the potential to whiff at the last minute. Remember last year, a dry, northwest flow and inconveniently placed low tracks helped eat away at our most promising snow opportunities. While we saw many snow days, all nine of them (at least for Nashville) were at 0.5” or under, as we found ourselves stuck inside the snowdome. Interestingly enough, the same cities that cashed in on the most snow during the 2010-11 winter were also much more fortunate last winter (see graphic below).

But truth is: a snowdome sets up somewhere in the US every winter for various reasons. Thus, if I were a betting man, I’d have to go against the snowdome returning for 2014-15.

At any rate, I believe there will be plenty of wild temperature swings to compliment the abundance of snow opportunities. I also believe we’ll see at least one two-week period with strongly below normal temperatures, with a few thawing stretches in late January and February. Also, for some reason, I believe one of our biggest snows this winter will occur between Christmas and New Year’s, granted this is probably a result of how my relationship with Murphy’s Law goes (I’ll be away in California/Washington December 26-January 5).

Overall Grade: B+

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In my next weather post, I'll discuss some other concepts such as the Lezak's Recurring Cylce (LRC) and the October Pattern Index (OPI) Index, explore other potential intangibles such as stratospheric warming signals (including its relationship to the Polar Vortex), the Madden-Julian Oscillation (MJO) and the Atlantic Multidecadal Oscillation (AMO) and provide model updates to see if current projections are still holding (the JAMSTEC & CFS weeklies for instance). I'll also look at some other long-range guidance to see if we can further unravel the clues as to what we can truly expect for the 2014-15 winter weather season.

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References