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Wednesday, November 25, 2015

Can this season's big El Nino wreck our winter ? Outlook 2015-2016 is here !

Mild weather has assumed control of the weather in New England. The prevailing warmth began in September, messed with our foliage somewhat, and has since continued to dominate the region through most of November. This represents a big change verses the previous autumn and it would be natural to be fearful of a such a trend continuing through the winter. From my perspective, there wasn't much point in tweeting and posting to the blog with little or no encouraging news for powderhounds. Thanksgiving upon us and the time has come to get started so away we go. There's also plenty to talk about. Many know about the intense ENSO event that has enveloped the equatorial Pacific Ocean - El Nino, which has already and will continue to have a loud voice regarding the behavior of the weather this winter. El Nino's of this magnitude should make us a little wary but as I will later show, they are also not to be automatically dismissed as wasteful seasons. We also have a few variables working in our favor which I hope will help the season reach some respectability in spite of a less than favorable short term outlook.

This short term outlook has not gotten better over the past few days. To put it simply, we are not getting any high latitude blocking to help mitigate the impact of what is a furious jet stream in the Pacific Ocean. I can look at the weather map and without even knowing it, tell you that we are contending with a big El Nino. Mild weather will thus be the result, but New England will not be as mild as our com-padres in the central United States. A storm during the first two days of December is likely to be either ice or rain. After that we should remain modestly above normal on the temperature front and there is a chance for an impact from a stronger southern stream system in between December 5-10. We are going to have to remain patient here by the looks of things. There are no indications of a major pattern change and I personally don't think we will see such until at least the time of the Winter Solstice.

Getting back to the seasonal aspect of our outlook we should discuss our current El Nino in detail. Yes, it's a big deal and hopefully not a big problem. We started to see evidence of this El Nino just as we were putting a wrap on last season. Subsequent strengthening over the summer and autumn has brought us to where we are now - an ENSO event that has, in magnitude, blasted through anything else we have seen since the great El Nino of 1997-1998. The critical "3.4" region of the equatorial Pacific which is a vast expanse stretching from 120-170 W (about the width of the U.S. from east coast to west) is now seeing water 3 degrees C above average as of mid-November. Statistically speaking this is putting the event toward the right edge of the bell curve or beyond 2 standard deviations from normal. We have been quantifying ENSO since the middle part of the 20th century and this is how it shakes out comparing each November.

Rank   Year    SST Anom

1)     1997    3.26

2)     2015    ~3.05

3)     1982    2.54

4)     1972    2.03

5)     2006    1.24

El Nino's featuring SST anomalies of more than 2 degrees C in this critical 3.4 region of the Pacific have a relatively clear behavioral distinction in the winter. The south gets extremely wet from Texas to the Gulf States to the Carolinas. The north-central region of the U.S. including the Midwest and Plains is typically mild and quite dry. Most importantly for California is an invigorated jet stream which typically positions itself further south verses other years and drenches the state with rain. This same jet stream also makes it difficult for Arctic air to maintain a grip on large portions of the U.S. east of the Rocky Mountains. New England typically is also enveloped by mild weather and although the region has at times stays dry, statistics suggest that storminess ultimately finds its way to the state and a healthy chunk of that precipitation is snow. The "big 3" ENSO events of '72-'73, '82-'83 and '97-'98 were all mild relative to average and all had some horrendous periods of warm and snowless weather. Somehow though, all three saw decent amounts of snow of either average or above average thanks to a few epic periods. In the case of '72-'73 it was an epic, nearly record breaking start to the season in November and December. This was followed by one of the worst January/February periods ever when the snow depth at Mt Mansfield actually dropped. March and April then proceeded to be very snowy. The winter of '82-'83 looked very much the opposite with warm and dry weather at the start and some snow and cold at the end of January and February to keep things respectable. The winter of 1997-1998 was quite warm but Vermont did quite well relative to many places, avoiding many thaws (one with a historic ice storm) and receiving above normal snowfall.

The "mild but sometimes stormy" behavior is not a statistical accident but a function of the ENSO. Arctic air has a difficult time gripping the region for any real duration but the massive and energized southern branch of the jet is sending some big and very juicy systems up the coast. The Valentines Day storm of 2007 is one such example, a storm that spared the I95 corridor of significant snowfall but dumped 15-45 inches across upstate New York and interior New England. The "Big 3" El Nino winters were all generally pretty snowless winters for the big east coast cities. The big "Megalopitan Storm" of February 1983 brought snowfall totals to respectable levels that season but aside from that, snow along the coast was sparse. 1972-1973 featured only traces of snow for a few east coast locations and 1997-1998 wasn't much better. The moral of this whole story is that although many snow lovers along the coast will be moaning and groaning, snowfall across the interior from the Appalachian Mountains all the way to New England, does occur and during favorable stretches of the winter, falls quite heavily. I expect this season to be very much the same across Vermont this winter in spite of a rough start.

The PDO or Pacific Decadal Oscillation has played such a massive role in each of the last two winters and deserves some detailed analysis once again. The PDO went positive during the winter of 2013-2014 after a long stretch of negative months. It subsequently turned very, very positive in late 2014 and become the strongest "positive PDO" every recorded during the aggregate of winter months through 2015. As of late autumn, it remains a very positive 1.47 which is very respectable though not quite a record. For some detail on the PDO here is some WIKI help - PDO detail. I also probably spent each of the last 10 Seasonal Outlooks describing it so you can check out that as well. It's safe to say that it will remain positive for at least a good part of this upcoming winter and this is fairly typical during an El Nino, particularly one of this magnitude. We have noted each of the last two winters that though the PDO has been a very strong player in some of the strong outbreaks of cold, it has been the specific nature or characteristic of the PDO which has, in my view, been critical. Take a look at the SST anomaly map in the Pacific Ocean in early February of last year.

Note the large area of warmth in the northwest Pacific. This "red blob" was very good to us, promoting big surges in the PNA or frequent large upper air ridges in western North America.  The same map taken during November of this year still depicts the "red horseshoe" which characterizes a positive PDO event but the differences are nonetheless evident.

The "red blob" in the northwest Pacific, although still present, is not the dominant feature. The warmth in the northern half of the Pacific is actually spread out over the eastern Pacific. The biggest feature there of course is the massive area of red near the equator. That is what this year's "El Nino" looks like on this map. No doubt it is not messing around this year !

The PDO should therefore continue to make a more positive contribution to our weather, at some point but I am not expecting it to be the factor it was the past two seasons, in spite of its strength.

For the third consecutive year, we had a big time build up of snow in the autumnal months across the northern hemisphere. Looks like we will need the help but at least we have it. This past October chimed in with 21.4 millions of sqkm of snow across the NH which was the 7th highest of the 48 years recorded. Last years 22.5 number was the third highest over the same period. A rapid expansion of snow and ice in the autumn months helps the arctic air across the northern latitudes pool more efficiently and attain a greater strength. The prevailing weather pattern still needs to support the transport of cold into New England for us to feel the effects of such a variable but eventually I think we will get that support.

During the last several seasons we have spent the last chapter of these prognostications discussing whether or not the weather has been displaying any "tells" to borrow some poker terminology. Particularly in November, the weather pattern will exhibit behaviors that will foreshadow the upcoming personality of the season. During the last two winters, big surges in the PNA brought particularly intense shots anomalous cold into Vermont. Both were statistically eye popping and were indicative of much of the weather that was to follow. During the fall of 2011 several shots of cold weather failed to come in as advertised and this also was a pattern that repeated itself during the ensuing winter. 2009-2010 was another, albeit, weaker El Nino winter but was actually dominated by repeated high latitude blocking events. One large event actually occurred in October and largely foreshadowed other, similar such events that occurred both in December and in February.

This particular piece of the preseaoson outlook is the most disheartening. The pattern just appears to be so overwhelmed by the El Nino this year including dry, warm weather which has dominated the northern plains, wet weather across the south and particularly intense activity across the Pacific. Just in the last week, the jet has become viciously strong in the central Pacific Ocean thanks to the jet energy south of the Aleutian Islands coming over the top of strong El Nino induced ridge in the south Pacific. It appears structurally different than the EPO creatred "Evil Empire" which dominated the horrific winter of 2011-2012 but the effect is the same - limiting the amount of available cold across a broad area of North America.

Once this is all boiled down to an actual forecast it looks like this. Even the big snow cover number we saw in the NH this October/November will not be able to thwart the mild weather. Temperatures this winter will average 2-4 degrees above normal for the season. That said, I expect a stretch during the winter with some respectable cold though this will pale in comparison to the cold last winter. The snowfall result should be better than what we see on the temperature front. Expect some very crappy periods, such as the one staring us in the face for early December. When the pattern does align though, we should 1 or perhaps 2 amazing stretches of weather. I am going to guess that this all happens in the Jan/Feb period right now but it's tough to know for sure. Snowfall should come in around average.