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Thursday, November 30, 2017

Big pattern shift poised to bring multiple weeks of cold and snow to Vermont ski country

Wish the blog could start this way every year ! Though the month of December will begin on the mild side, a very decisive drop in the Arctic Oscillation index over the next week to ten days teamed with a large upper ridge in western North America (+PNA) should bring a December weather weather extravaganza for at least two weeks. We haven't seen a set up this good in December since 2010 and though it will be difficult to pinpoint  the best days for "powder", we will get our share over what I expect to be a 2 week stretch of favorable weather beginning December 7th.

A series of rather "blah" days will kick off the month of December. Temperatures will remain mostly below freezing at night and get close to or slightly above freezing during the days. Sunshine will be limited and skiing in Vermont will likely be confined to the usual early season arteries. The early part of the first full week of December will feature a pattern poised to explode. Arctic air will enter the northern Plains and flood the Front Range corridor. As this happens milder air will make its way northward into interior New England. The entire state will likely see an extended almost 36 hour above-freezing stretch accompanied by a few hours of rain later Tuesday. When readings do finally fall back into the 20's late on Wednesday, it will begin what I expect to be a very long stretch of sub-freezing weather and a terrific opportunity to begin establishing what we hope to be a healthy 2017-2018 base.

Though we likely see some flurries late on Wednesday and more snow showers Thursday, significant snowfall might wait until at least at least the weekend before reaching the state. Relating to this upcoming cold pattern, the question relates whether or not we see much of a southern branch of the jet stream. The strength of the building La Nina suggests likely not. We will need a portion of the jet stream in the Pacific to undercut the large ridge in western North America and this kind of thing doesn't usually happen in La Nina years, at least not a lot. That said, we can still see plenty of fireworks along the Atlantic Coast and clipper systems galore. The unusually warm water situated in the Great Lakes and in Lake Champlain actually provide a rather exciting thermodynamic environment when the arctic air attacks. We might start slow, but we will see some terrain enhanced snow and substantial amounts of lake enhanced moisture. There are hints of something along the Atlantic coast during the weekend of the 9th or 10th. Though models were a bit more bearish as of late Thursday (Dec 30th), subsequent runs may very well show some activity and my guess is that our first few significant inches fall over that weekend and perhaps as early as Friday the 8th.

Re enforcing blasts of cold arrive in the December 10th-11th period and both cold and winter weather should dominate the ensuing week through what we hope to be opening weekend on the 16th/17th of the month.  A significant storm is possible during this period though I expect that a good chunk of snow falls as a result of a series of smaller systems, clippers and lake and terrain enhanced snow. I'll take it anyway we can get it, should be a great start to the year  !

Monday, November 27, 2017

Lots of analysis, optimism and excited anticipation for 2017-2018, but at the end of it all, temperatures and snow shouldn't be too far from normal

Hope everyone enjoyed their thanksgiving festivities and with those in the rearview mirror, our collective attention turns to ski season and the blog thus returns for another season of prognostications. Those of us following the meteorological world have been watching a few critical variables the last month or so such as a building La Nina, a seemingly non-committal Pacific Decadal Oscillation and an Arctic Oscillation that seems to be very resistant to "negative" behavior. The unrelenting blowtorch that consumed much of later September and all of October generated plenty of consternation but there are reasons for optimism, perhaps not the glowing variety, but enough to expect a fairly solid season in the months ahead.

The state of the El Nino - Southern Oscillation (ENSO) is always at the top of our radar going into a winter season. There are times when the ENSO retreats to relative neutrality, mitigating the impact it might have over a winter and elevating other variables to greater importance. The 2015-2016 situation presented the opposite scenario when a super strong El Nino, a "Super Nino", sucked all the air out of the room and in that particular case, the snow and the cold with it.

In the last several weeks, we have witnessed the ENSO take on a very La Nina look. This wasn't forecast a few months ago proving yet again how challenging it can be to forecast SST trends in the equatorial Pacific. As of early September, the ENSO had really yet to reveal itself but but in the last two months we have seen a decisive trend toward La Nina and in the last two weeks the Nino 3-4 which makes up a broad area of the equatorial Pacific from 120 to 170 west longitude has cooled to 1.1 C below average.

Seasonal SST Animations

A "-1.1 C" La Nina might not be the strongest La Nina, but it's nonetheless significant. If it holds at this level through the winter it will undoubtedly play a role on the weather in eastern North America. In Vermont, we have seen La Nina winter's go both ways. We have seen some huge snow years such as 1970-1971 and 2010-2011 and we have also witnessed some catestrophic seasons such as 1988-89 and 2011-2012. There are obviously several in between. La Nina's produce several obvious impacts such as wet and unsettled weather in the Pacific Northwest and dry and mild weather in the southeast United States. Every inch of latitude is critical, much like it was last winter, for a successful winter weather season. The Mid-Atlantic ski areas in Pennsylvania, New Jersey, Maryland and West Virginia will likely have another challenging year. Farther north however, the season could go either way and given how some of the other variables evolving as of late November, I am inclined to think that our 44 N latitude will serve us quite well, much like it did last year.

The Pacific-Decadal Oscillation (PDO) has a relationship though not a perfect one with the ENSO. The PDO describes the configuration of sea surface temperatures in the mid-latitude Pacific as opposed to the equatorial Pacific and is thus even broader than the ENSO. Rather than explain it further (which I have done in prior years), I'll just provide a link with showing the various positive and negative phases.

PDO Information and phases

I'll note that negative phases have an association with La Nina winters while positive phases often go with El Nino. Though the relationship exists, it is chaotic and a statistical nightmare to try and pin down. The positive PDO was close to a record during the winter of 2014-2015 yet the ENSO was neutral. During that winter, the nature of the PDO was a dominant driver of some of the repeated and particularly intense outbreaks of cold in February.

The PDO has been very non-committal during the autumn months of 2017. The last negative month was December 2013 and though we've gotten pretty close as of October, we haven't quite crossed the "zero" threshold. Whether or not we can turn the PDO negative, it is encouraging to see the PDO index neutral in a "relative" sense. The terrible winters of 1988-89, 1999-2000, and 2011-2012 all had decisively negative PDO's parlayed with significant La Nina's. 2010-2011 featured a positive PDO and was a terrific snow year. The verdict will come in the form of the EPO personality. In the common SWCB tongue, it refers to the prevalence or absence of the "evil empire".  

There is one more important element related to the PDO that I did not want to neglect. In the preseason outlook questionnaire that I did for Vermont Ski and Ride magazine I had expressed some concern about a strengthening blob of anomalous ocean warmth in the western Bering Sea and across the Sea of Okhotsk north of Japan. Though somewhat associated with a more positive PDO, the position of this large area of warm water was disconcerting since it was broad enough to potentially tighten the jet stream in the eastern Pacific and discourage ridging in western North America (positive PNA patterns). One could attribute, though not conclusively, the unrelenting blowtorch in October to this Pacific SST configuration. If this area of warmth had maintained its strength through November, it would have profoundly changed the mood of this outlook; fortunately, the area of warmth vanished in a matter of a few weeks. Be it by consequence or by coincidence, the blowtorch ended and November has actually felt like November and a large portion of the month was actually "below normal" in Vermont.

Moving on from our PDO discussion gets us to talk of snowcover expansion. I like to keep this analysis very global or at least hemispheric in nature and though I view snowcover as an important leading indicator, I understand that there are those that don't share that view. With Twitter weather personalities exploding, I have seen the debate over snowcover expansion and its relevance in seasonal forecasting play out visibly on social media. We've had healthy build ups of October/November snow in each of the last 6 years including the last two which preceded mild winters. This has fueled some of the skepticism but proponents of the data such as myself would never argue or attempt to over exaggerate its significance; in fact, I try and make a point to talk it down.

2017   21.17
2016   22.95
2015   21.40
2014   22.88
2013   21.01
2012   20.14
2011   17.34
Average: 18.40

Though ice cover is outpacing 2016 by a significant amount, it continues to run well below average. This is associated with the very discernible long-term trend of receding northern hemispheric arctic sea ice.  The interactive sea ice graph (linked below) is one of the most visible displays of the ice retreat. Click on years 40 years ago and compare them to recent years. This kind of trend is scary as its occurring over an incredibly short period of geological time. To say it's an accident or naturally occurring is simply not a reasonable assertion though I wish it were true.

As for the upcoming winter, the healthy expansion of snow in the Northern Hemisphere bodes well the pooling of arctic air masses. It is my belief that the greater the coverage of snow and ice, the greater the ability of arctic air to strengthen efficiently. Speaking slightly more locally, the Hudson Bay is 25-35 percent frozen as of late November which is normal historically speaking but pretty good relative to the last decade. Though the Great Lakes aggregate remains quite warm as a result of the warm fall, the recent stretch of cold has at least chilling down these bodies of water.

Last but not least is our annual examination of "atmospheric tells"  with "tells" being poker lingo.This really saved the outlook from going astray last year when there were several bullish indicators suggesting it might be pretty cold yet I suggested, correctly as it turned out, it would be mild. This is not meant to be self-promotional, I busted substantially on snowfall during the Super Nino and I am still trying to figure out how I could have been so blindly optimistic. In spite of some bullish data last autumn, the mild weather during the summer and fall was so persistent and below average temperatures of any significance were so hard to sustain or even achieve that forecasting cold was statistically maniacal. Don't weather patterns change or constantly evolve ? Absolutely. That said, the weather across Vermont and in many other places will exhibit similar patterns of behavior over the course of a season and multiple seasons and such patterns can be foreshadowed in October and November.  The back half of 2016 featured a persistently positive Arctic Oscillation (AO) that broke briefly in October (although with very little cold across the continental U.S.) but continued through the winter of 2016-2017. The mild weather ravaged ski seasons from Colorado to the Mid Atlantic during February of this past year and though Vermont also got a big thaw, we were fortunate enough to score big on March snow to save the season.

For much of the last two calendar years, mild air has dominated the U.S. and eastern Canada. 2015-2016 can certainly be attributed to the Super Nino while much of the last year can be associated with the persistently positive AO. The pattern of mild weather has in recent months been somewhat broken. We saw a widespread outbreak of cooler temperatures and a negative AO in August across a wide swatch of the continent. A ferocious blowtorch returned for late September and October but has been broken again in November and so has the positive AO. I'll discuss the shorter term outlook in more detail in a subsequent post but after some mild weather in early December, the northern latitudes look impressively blocked and quite favorable for both colder weather and snow. This is very different behavior relative to the last two years. It may not be enough for me to forecast a cold winter relative to "normal" but certainly colder than the last two seasons by a noticeable amount.

So to summarize, I am cautiously optimistic about the upcoming season. For the third consecutive season, I would have to say temperatures come out on the "above" side of average though not by much and I expect a noticeably colder difference relative to the past two seasons. The strengthening La Nina is a substantial concern as of this posting. If it gets much stronger, we will be dealing with a significant ENSO season and a ridge across the southeast United States could be an overwhelming feature in the weather pattern. Even if we see a relatively stable but moderate La Nina going forward, thaws are still going to be a problem and longer  6-8 stretches of favorable skiing will be unlikely. Our latitude plus what I assume to be the prevalence of arctic air at our latitude should help us immensely on snowfall. In short, I think we perform somewhat similarly to last year on snow, slightly above average, but both ice, periodic rain events and thaws should be expected. I would be surprised if see another thaw consume 3-4 weeks of the winter as it did last February into early March. I'll have an update on the shorter term within a few days. We have some milder weather to get through in early December but the blocking in the weather pattern discussed above should bring winter to Vermont, perhaps in a big way by December 10th making that December 16th potential opening a reality ! Think snow and as always, it's good to be back !