Think Snow, Tweet Snow !!!

Saturday, November 24, 2012

Winter-like temps for the upcoming week

There were some logistical changes and notes that were left unmentioned in the SCWB's preseason commencement. First, I think the new twitter timeline up on top is a little "noisy" and I prefer the old timeline that was used last year. Twitter seems to have cracked down down on the use of third party apps on Blogger sites leaving me little choice. We could eliminate the timeline entirely but I actually like the idea of it and will work to clean it up a bit if I can. The "favorability index" has been traditionally explained at the beginning of the year but I always get questions throughout the season regarding the index so I will leave the explanation on the right side of the blog. Plenty of readers have also suggested building a glossary. I have unintentionally ignored all those requests though it's a great idea and I may work to create one this year.

Winter has arrived in Vermont and snowflakes are flying up and down the spine of the Green Mountains. The pattern is being anchored by a powerful high latitude blocking structure near the Aleutian Islands along with a smaller one in the northern half of Greenland. This has allowed a broad area of cold to settle across Canada and finally make an entrance into the U.S. during the Thanksgiving holiday. The cold will provide the region with a healthy taste of winter and it should persist through the week. High terrain such as Mad River Glen should generally see temperatures remain below freezing for most of the upcoming week which might be normal by late December but is still a bit unusual in late November.

There are about 3 different disturbances throughout the upcoming week that are capable of providing at least flurries and at most a light accumulation of snow. A more organized storm system will exit the eastern Rockies and move east Sunday. This system will have a difficult time phasing with the various pieces of aforementioned and weaker polar disturbances and as a result, much of the moisture with this storm may stay to the regions south. These things do have a way of changing however and we will keep watching. Were it to happen, significant snowfall would occur this Tuesday the 27th, but the chances are low right now.

The chill will erode very slowly during the early part of December and the more medium range ensembles suggest a 2-4 day spell of warmer than average temperatures during the first full week of month. This time of the year, it could translate into a thaw and perhaps some rain before winter-like temperatures return. Throughout this period, the negative NAO-AO pairing will remain in tact and should promote a quick return to colder temperatures and additional chances for snow after December 5th. 

Wednesday, November 21, 2012

El Nino teases, but stays away from what should be a much colder 2012-2013 !!!

Welcome back!!! I doubt we can start too early this year. Last year's debauchery left many of us, me included, powder starved through the summer. Needless to say there has also been plenty of weather to talk about. Hurricane turned Superstorm Sandy was merciful on Vermont but historically destructive for the New Jersey and New York coastline. This was followed by an early nor'easter and accumulating snow in some areas that as of now are still recovering. For the tri-state area readers, hopefully some sense of normalcy has returned. I can tell you, Vermont can certainly feel your pain a little more than one year removed from Irene.  

The Big Picture

The broader weather picture has been interesting to follow as always. The excessive warmth and in some areas drought continued through a good part of the summer. Beginning in August however, the warmth across much of North American finally abated and some areas, particularly across the center of the country, have actually seen some significant periods of below normal temperatures. This of course is fairly typical for most years but the first half of 2012 was so historically warm that the contiguous United States saw the warmest consecutive 12-month period on record. Sustained cold of any kind was nearly extinct and the area around the Great Lakes saw 2 months where temperatures were 3-4 standard deviations above normal in a 4-month period. The end of the excessive and rampant warmth across North America marked a large scale behavioral shift in the weather across the mid-latitudes of the northern hemisphere. The change can be attributed to a variety of factors but the Arctic Oscillation would seem to be the prevailing force since it abruptly shifted from positive to negative during the middle to later part of the summer. In addition, the autumn months have featured more frequent high latitude blocking events, the biggest and most consequential was the block over lower Greenland which provided the steering mechanism that guided Superstorm Sandy into New Jersey as opposed to turning out to sea.

 ENSO - La Nina or El Nino or nothing

Regarding the upcoming winter, the early expectations largely hinged on what was expected to be a developing El Nino. Some forecasts had a significant El Nino developing by late autumn. As it turned out the upcoming El Nino was a lot of "huff and puff" and amounted to nothing more than a big tease. Yes, a few forecasts still have a weak ENSO event during the winter months and yes, sea surface temperature anomalies have crossed the "0" line, but they are not significant. As of now, the most noteworthy aspect of the ENSO situation would be the disappearance of the La Nina which presided over the previous two winters. I want to expand on this fact later in what I think further justifies some of my optimism about the coming months. Regarding the ENSO, the quick summary this winter is that we are expected to be close to neutral.

The bigger story - PDO  
The prevailing variable as opposed to ENSO appears to be the Pacific Decadal Oscillation or PDO. The index describes the configuration of sea surface temperatures across a broader swath of the Pacific Ocean as opposed to just the equator. A negative index is what I refer to as the "blue horseshoe" since it consists of horseshoe shaped ribbon of cold temperature anomalies enclosing a broad area of warm sea surface temperatures anomalies in the central Pacific. This autumn has featured one of the strongest negative indices going back 50 years and this sets up a very interesting dichotomy. A strongly negative PDO which lends support to a La Nina contradicts the trends in equatorial sea surface temperatures which have gradually slid toward neutral and is now levitating at just above zero. From here the task is to find a few winters that have jointly featured a relatively neutral ENSO and a very negative PDO in hopes of discerning a pattern. This proves to be a difficult chore since there a few winters with such a personality. 1994-1995 was essentially a 1-month winter (February) with the rest featuring mild weather and a lack of snow. A very negative PDO prevailed that winter along with a moderate El Nino. 1961-1962 was a relatively neutral ENSO winter with a very negative PDO and it was generally cold with average snowfall. Both winters are hardly a perfect match however since the 1994-95 ENSO was stronger than the current ENSO and the 1961-62 ENSO was tilted a bit more to the La Nino end of the pendulum though it was close to neutral. Other winters, particularly some good ones in the late 60’s have had some loose similarities but are far from good scientific matches. To say the least, the weak ENSO, strongly negative PDO dichotomy is quite inconclusive. Separately the strongly negative PDO favors troughiness and cold in the western United States while the weaker ENSO generally favors frequent blasts of rather intense cold throughout the United States. Many of the big negative PDO winters however have a significant La Nina as a companion thus clouding such cause and effect judgements. It could certainly be argued; however, that although the above favors colder weather relative to last year, the pattern will be fluid with favorable stretches for skiers in New England followed by favorable stretches for skiers out west.

Snow and ice make a recovery

If one is to thus reserve judgement on the basis of the above two factors, it is time to move on to a third - the snow and ice situation. I know many readers consider themselves closet weather enthusiasts and might have read about ice in the arctic regions decaying to the lowest total coverage ever recorded in September. By late summer, a majority of the Arctic Ocean was open water and could have been open to shipping if such a route was desired. The process of re-freezing water involves the release of latent heat and disrupts the normal process of "cold air pooling" in the arctic regions. The "cold air pooling" is the process where continental polar, or arctic air strengthens and it is the opinion of many that the strength of these airmasses has already been significantly impacted by the changes in the ice coverage over the Arctic. In recent weeks, the arctic has re-frozen and seems to have recovered a bulk of its summer loss (though not all of it). In addition, ice has been forming in the Hudson Bay and the process of freezing that key body of water will occur faster this year verses some of the previous few. All of this has been helped by a rapid expansion of snow across the high latitudes this autumn and has helped the "cold air pooling" process by making up for the loss of ice in the Arctic. The total amount of northern hemisphere snow, measured in millions of square kilometers, ranked 8th in the last 44 years of recorded data in October. This is an impressive recovery since the summer months featured the lowest total coverage of northern hemisphere snow and ice. It is very encouraging behavioral sign leading up to our winter since a fast freeze of the Hudson Bay and a broader area of snow cover should allow the region to have easier and more effective access to arctic chill.

Has Mother Nature tipped her hand ??

 Lastly, I wanted to allude again to the recent behavioral trends in actual weather during the past few months. Often times, the weather pattern in autumn can exhibit characteristics or closely foreshadow events or patterns that will occur in the upcoming winter. On a larger scale however, I am most interested in contrasting some of these bigger trends year over year. At the end of last year, I had noted how destructive the upper level ridge in the mid-latitude Pacific was on the winter as a whole. Last year, this ridge feature was fueled by the prevailing La Nina, a generally positive Arctic Oscillation (AO) and a Madden-Julian Oscillation that never “cycled”. This year there has been virtually no evidence that such a feature will be a persistent problem. When there have been indications of an upper ridge in the central Pacific Ocean, it quickly breaks down or never develops as advertised. The second such trend, and one which may have a hand in the behavior of the first, is the AO and frequent appearance of blocking mechanisms at high latitudes. Such a trend could quickly reverse itself of course, but if the high latitude blocking continues as it has over the past 2-3 months, the winter will behave more similar to 2010-2011 and very different verses last year.

Summarized into a forecast

A winter forecast stemming from the first two more traditional variables would be a very cloudy inconclusive picture. The rapid expansion of snow and ice in the high latitude regions this fall combined with the recent behavioral trends in the Pacific Ocean and high latitude regions has me thinking this winter will be on the colder side. In the past several pre-season prognostications, I have generally biased the forecast toward the warmer side of the 30-degree average since it has been statistically unlikely to get a cold winter relative to this average in recent years. I am going to forecast “average” this year which may seem rather tame but relative to other pre-season forecasts and relative to a more recent 10-year average this is actually rather bold and quite cold. Snowfall I think will be a shade below normal mainly because I think it will be drier than normal. The basis for this comes from looking back at ENSO-neutral winters and seeing the trend toward drier than normal conditions over New England. In addition to this is recent trend across the area of drier than normal weather. In a sense, I am forecast a poorer man’s version of 2008-2009 which was a generally good winter, full of cold, but also full of some drier stretches of weather and periodic instances where snow and cold is confined to western North America. Quantitatively this translates into a total snowfall of 225-250 inches which is slightly below the 250-260 inch normal but much improved over last winter.

Enjoy your Thanksgiving feast with a pre-season pass !! The first threat for real snow comes during the middle of next week with some accompanying cold weather. More on that in the days ahead !!