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Friday, November 21, 2014

Lots of signs pointing in a similar direction - A big 2014-2015 !!

Winter 2014-2015 is coming in with a vengeance, the Green Mountains have been whitened and the mood is clearly appropriate for some preseason prognosticating. This is year 10 of the SCWB. It started with sporadic emails to Eric in 2004 and it continues now with blogspot, twitter and our own urban dictionary of weather terminology designed specifically for the avid New England powder hounds. The blog continues this year and will at least start with plenty of anticipation. The pattern, currently anchored by a magnificent looking positive PNA structure has delivered widespread cold to eastern North America. But even more intriguing is the combination of what appears to be at least a mild El Nino, a positive PDO and another impressive expansion of snow/ice in the Northern Hemisphere. Are these the ingredients for a round of 1977-1978 style fireworks ? Or will the welcomed addition of the southern branch of the jet stream deliver all of its goodies to Pennsylvania, New Jersey and points south like 2009-2010 or even worse, will the winter STB like 1991-1992 after a chilly November. Worthy of some discussion at the very least. I have added some links to further explain some terminology. Most of them go to Wiki (And I encourage contributions to them). 

After a 4-year hiatus, El Nino has returned, and over recent weeks has been gathering strength. The state of the ENSO continues to be one of the more reliable ways to predict behavioral patterns of weather in a given season, particularly winter. It can be particularly useful when trying to pinpoint the frequency of specific occurrences. And it is especially relevant  this year given its 4-year absence and the general presence of La Nina at varying degrees of intensity over the past 4 years. The strength of an El Nino is determined by the strength of positive sea surface temperature anomalies in the equatorial Pacific Ocean. There has been much discussion from the National Weather Service's Climate Prediction Center about the potential ENSO event this year and in spite of much discussion, El Nino, has been slow to manifest.  In the last 30 days however, sea surface temperatures in key regions of the equatorial Pacific have warmed to about 1 C above normal. Typically this is about the threshold where we see miner ENSO impacts become more significant. These impacts include a suppressed and active jet stream in the Pacific fueling a potent southern branch of the jet stream. A jet stream capable of relieving California of its 2-year drought, bring significant snows to the southern Rockies and heavy rainfall from Texas to the southeastern states. Quite often, these many juicy weather systems culminate their trip across North America by evolving into major East Coast weather systems. Temperature impacts have also proven to be substantial, especially during the bigger ENSO events. The persistent throng of energy in the Pacific can counteract the southward advance of arctic air. Much of the Northern Plains, Upper Midwest and most of the eastern two thirds of Canada typically see above normal temperatures during a significant El Nino. The most egregious example was 1998, where water temperatures warmed to over 2C above average. Mad River enjoyed a fun-filled winter with plenty of juicy winter storms, but it was warm and the season was interrupted by a damaging ice storm in early January which required herculean efforts by many coop volunteers to clean the woods of debris. This year's El Nino, like all the others since,  won't even be half the strength of 1998 and the result of this, we hope, is that we see more of the typical impacts of precipitation as opposed to temperature. I will point out that 1998 is often used as a reference point for many global warming/climate change deniers. "We haven't globally warmed since 1998 !" you will hear, or some version of that. Statistically that is true when using 1998 as a reference point since the 3-Sigma El Nino of that year provided for additional and significant temperatures perturbations globally and thus many years since have been cooler than that globally. Statistics have been and will continue to be manipulated to suit all sides of every argument but that is one that has always bothered me. Back to our present weather situation. Many might remember 2009-2010, the last El Nino winter and the heart-ache in northern Vermont as the snow piled up well to our south while General Stark enjoyed mostly dry weather for long stretches of time.  Yes that could happen again but the adverse storm track of  2009-2010 was primarily due to the extremely negative Arctic Oscillation that accompanied that El Nino that year. We like negative AO's but not that negative ! It is unlikely that combination will prevail again this year.


The PDO has earned its place in the preseason discussion and like the ENSO, the index is defying the most recent 4 year history. The PDO (Pacific Decadal Oscillation) describes the configuration of sea surface temperatures in the mid-latitude Pacific, above the equator where we monitor ENSO. Around the time of 2007, the PDO had its multi-decadal shift going from an index that was mostly positive to an index mostly negative. It is not a rule without exceptions, the PDO can be in a negative decadal phase and have a positive year and vice-versa. It just has decadal tendencies, which is how the phenomenon was discovered and although we are in a negative tendency, it appears we will have a positive year. We went into last season with a negative index but it turned in January and the index remained positive through March. The index could switch this year but will at least start the season positive. The sign of the index does have a tendency to be correlated with the ENSO but not entirely. In the case of this year, the development of the El Nino seems to have gone hand in hand with the evolving positive PDO. Why is the positive PDO significant ? The long wave or jet stream pattern can often have a difficult time locking in place with a negative PDO as illustrated during the winters of 2011-2012 and 2012-2013. Often times, cold and snowy weather can occur only to be abruptly interrupted by a intrusive thaw. A positive PDO favors the ridge west/trough east regime and these regimes can remain in place for longer periods uninterrupted.


I want to bring up an important side note related somewhat to the PDO. I want to aknowledge an important driving force behind last years cold weather, the repeated PV invasions and I also want to aknowledge an individual who pointed this fact out on more than one occasion last year. We mentioned the persistent Alaskan jet stream ridge as the catalyst behind much of the cold, but an old colleague of mine, Joe Bastardi, referenced that a driving force behind that particular weather feature was a large area/bubble of warm water in the Gulf of Alaska. This very large bubble of warmth blossomed in December of last year as the PDO switched from a negative to a positive phase. Bastardi style of self-promotional forecasting is something I take issue with at times and I particularly don't like his invariable bashings of the National Weather Service, which employs many talented meteorologists, who by law are not allowed to defend themselves against his repeated attacks. That said, Bastardi did point out the importance of this bubble of warmth in the Pacific and I think he's right, it was significant. This large area of warmth remains positioned in the Gulf of Alaska but it has shifted eastward slightly and the configuration of water temperatures now resembles the classic "red-horseshoe" look of the traditional positive PDO. 


The real fun begins when we start talking about the autumnal expansion of snow and ice in the Northern Hemisphere this year. It wasn't going to be easy but it was achieved. We actually bested last year's monster October snow cover number of 21.01 millions of square km and chimed in this year with a 22.88 number this year. The 2nd highest in the very brief 47-year history of recorded data. Some of the recent November weeks are running almost 4 millions of square km above last year according to our friends at the Rutgers University Snow Lab - . Lets dumb this down a bit this year and look at some big years in this 47 year history of recorded data. Here are a list of big autumn snow cover years (at least 1 STDEV above normal)  in the Northern Hemisphere and the corresponding result in Vermont. 

Year    Snowcover      Temp         Snow 
1970    21.84          Cold         Totally Epic
1971    21.53          Cold         Snowy
1972    21.52          Mild         Sucky
1976    25.72          Ext/Cold     Snowy
2002    23.24          Cold         Snowy
2009    21.01          Warm         Normal 
2013    21.01          Ext/Cold     Slightly Snowy 

By contrast here are some low snow cover years and the corresponding result. 
1979    14.68          Normal       Sucky
1980    13.61          Cold         Normal 
1987    13.35          Warm         Sucky
1988    12.78          Warm         Beyond Sucky
1990    15.58          Warm         2x Beyond Sucky
1994    14.23          Warm         Sucky

There are a few exceptions on both sides of course and it is hardly a perfect relationship. In fact, there are some recent years where snow cover was within a standard deviation of normal (but still above or below the normal) in the northern hemisphere and the weather in Vermont did the opposite of what this illustration is trying to display. That said, the 22.88 number is a strong signal and if past history is any indication, the colder weather will win a majority of the battles this year as it did last year. 

In a poker game, players use the term "tell" to describe a behavior or a demeanor that might give competitors a clue as to the nature of a players hand. I look at the weather the same way. By early November, it starts to exhibit a behavior that typically foreshadows the nature of the winter. It doesn't work all the time but it earns it's place as a contributing variable in the preseason discussion. This year it is a pretty obvious one. We have just been slammed with the strongest attack of November cold in more than a decade across the eastern half of North America. Lake Effect snows totaling over 8 feet and temperatures that are as much as 25 below average all supported by a classic positive PNA structure. It is a pattern I would expect we should see repeated a few times this winter since it is one that we do typically see in positive PDO years. What we need to really hope for is the support from the active southern branch of the jet stream, which will be catalyst for several monster east coast events 1978 and 1994 style. 


Needless to say I am pretty stoked about the upcoming winter. And yeah I am inherently stoked before every winter and I try and provide that disclaimer to account for the bias, but this to me is the most impressive collection of indicators we have seen in the last 10 years for a cold and snowy winter. It will also likely be a collection of indicators  not surpassed in the 2nd 10 years of the SCWB, if we can achieve that longevity. El Nino is providing us with some needed added southern stream juice, PDO is having a counter-tendency positive year, there is a mammoth amount of early Northern Hemispheric snow cover and the weather pattern has already exhibited a need for a big positive PNA outburst. Could we crap out like '91 ? Anything can happen and preseason forecasts can be inaccurate. I am probably about 60 percent confident about this forecast as opposed to my normal 55 percent preseason confidence. 1991, by the way, was a stronger El Nino year with a widespread and rather strong attack of November cold. The winter then proceeded to fall completely flat and turned out to be quite warm with a glaring lack of snow. The snow cover number that October though was 15.58 millions of square km. We were over 7 million square km above that this past October. With all that said, I expect, only for the 2nd time, for Vermont to see a colder than average winter and I expect some above average snowfall along with that. An additional shorter term update will follow in the coming days which will include some snow around Thanksgiving and a temperature moderation for the early part of December as we lose teleconnection support.  

1 comment:

Bhaskar Kumar said...
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