Think Snow, Tweet Snow !!!

Monday, November 28, 2011

Arctic Oscillation crippling early season hopes

The Arctic Oscillation is the king of teleconnection indices. The ability to achieve a pattern of sustainable cold in areas south of 50 North with a minimal risk of rain is best done through a negative AO. A negative AO indicates that the upper air pattern at the poles is blocked and consequently consists of higher surface pressures at higher latitudes and lower pressures in the middle latitudes. The AO for the last several weeks has been positive and is largely responsible for the prevailing warmth in Vermont and the rest of the northeast. That warmth has reached a climax in the waning days of November. Yes, there are some signs of colder weather and snow particularly later next week but the ability for any cold weather to be a permanent fixture on the weather map will be compromised severely by the AO and NAO which show a lack of upper air blocking and encourages zonal flow. The SCWB has tried to quantify the teleconnection indices with the creation of the highly unscientific "favorability index". The index is a cumulative measure of the PNA, NAO and AO indices but we flip the signs of the AO and NAO so that positive numbers indicate "favorable" and negative "unfavorable". Negative AO and NAO indices indicate the blocking described above while positive PNA indices indicate upper ridging in western North America. For example, a NAO index of -0.7, an AO index of -1.5 and a PNA index of -0.1 produces a favorability index of +2.1. In this case the 14 day favorability consists of a NAO index of +0.7, an AO of +2.0 and a PNA of -0.2. This equates to a -2.9 which is an inauspicious number in this early part of the season.

The upcoming 5-7 days will remain very mild with only a brief break. A very moist storm system in the Midwest will track up through the eastern Great Lakes which is not good and lacks any access to cold air which is also not good. The front associated with this storm has had its eastward progression halted because of the prevailing upper air pattern and honestly I am just glad this hasn't happened later in the year. Get this nonsense out of the way now because such a scenario which includes the combination of rain and high dewpoints can obliterate snowpack. Since we don't have much of one anyway there is not much to lose.

Colder weather Thursday will also give way to another surge of warmer temperatures by the end of the weekend. More rain showers will mark the end of the warm weather on Monday the 5th or Tuesday the 6th of December. Much colder air will then grip the region for a period of around 3-5 days. Embedded in this stretch will likely be a clipper system capable of producing light accumulations. The question will be whether this cold air can persist through the middle of the month without another big warm-up. Right now I think that answer is no since too many of the above mentioned teleconnection indices are working against us.

Friday, November 25, 2011

La Nina 2011 has a sequel, lets hope its as good as the orginal

Like "The Empire Strikes Back" was to "Star Wars". Lets hope it is not what the latest "Wall Street - Money Never Sleeps" was to the original "Wall Street". The world continues to become more and more hyper-connected and the SCWB continues to struggle to keep up. This year will mark the blogs introduction to Twitter which I hope will prove a very useful piece of social media for the blog. The demands of my job and my time continue to increase and the ability to do any updates from work is long gone. The application at the top of the blog however will allow me to upload tweets all day and night if I choose and do so from my Iphone, something I really should have started doing last year. It will be used mostly to pass along information or updates between traditional blog posts. We will see how it works but feel free to interact via the Tweets and provide suggestions, comments or critique.

One quick shout out before we go to all the folks who have worked so hard to recover from the disastrous Hurricane Irene. I know it has taken a Herculean effort and many volunteering hours to prepare for this season after a weather event that none of us will soon forget. I am sure I am not alone but I wish everyone involved a speedy road to recovery and the best of luck this season and beyond. Hopefully the skies will produce a more constructive type of precipitation for the upcoming winter, the type we can make some turns in and certainly the type that stops at our doorsteps.

The Thanksgiving holiday brought with it a potent Noreaster and some heavy snow. It came in the midst of a rather unfavorable weather pattern with entrenched features that do no support sustainable cold or snow especially at this early juncture of the winter season. It is a winter that should continue to feature a La Nina but one that of this date is half the strength of the 2010-2011 version.

The link above illustrates SST trends over the last 12 months in the ENSO regions. There are a total of 4 regions, 1-4 with the first positioned close to South America and the fourth positioned close to the date line. The 3.4 region is a hybrid region stretching roughly from halfway across region 3 to west to halfway through region 4. In the link above, this region is the second down from the top and the difference between this year and last year is fairly evident. This year's ENSO will not be quantified as a strong event unless it strengthens considerably in the coming month. Theoretically, a weaker La Nina would allow traditional characteristics of a La Nina winter to be less frequent and easier to break down. These features include a persistent upper ridge in the southeastern United States deflecting storm systems to the Great Lakes region and an even stronger upper ridge across the Gulf of Alaska forcing much of the snow and cold in to the Pacific Northwest and northern Rockies. New England is often the battleground and can swing in either direction. The support of a very persistent and often very strong negative Arctic Oscillation and North Atlantic Oscillation forced the battleground southward and the region was on the snowy side of many systems. Amazingly, the best snowfall with many of last years big weather systems was again to the regions south which is very unusual in a La Nina event of that magnitude. Such a result is rationalized however through the support of the above-mentioned teleconnection indices.

The Pacific Decadal Oscillation or PDO has also been a traditional pre-season parameter worthy of discussion. It has in fact proven to have some skill in predicting the trough/ridge configuration across North America. To be fair however it has not done a good job of indicating the amount of powder days we will see at MRG. The last 5 years at MRG can accurately be described as ranging from slightly good to excellent good years and 4 of these years have featured a negative PDO when traditionally it has been the positive PDO which would seem to support a colder and snowier pattern. It is somewhat of a chicken and egg situation with the PDO since the configuration of temperatures in the mid-latitude Pacific (the PDO) are to some degree a function of sea surface temperatures in the equatorial regions of the Pacific. As of now, the index continues to run negative, more negative in fact than last year (1.34 as opposed to -1).

One of my favorite methods of glancing into the future is to actually look at what is happening with early season snow and ice expansion. For a while it looked ugly. With the exception of 2007, the ice had retreated to its lowest levels since this type of data has been recorded. And yes, unlike other phenomena such as a 20 inch snowfall in Washington D.C. the retreat of ice in the Arctic regions can be attributed to climate change. Much of the month of October continued to see a slow expansion of snow/ice and the month ranked 30th out of the last 44 October's in total northern hemisphere snow/ice cover. The last few weeks however has seen a remarkable turnaround. This can no doubt be partly attributed to a highly positive AO which has effectively bottled the cold near the polar regions. This cold however has pooled very effectively and allowed the ice to expand very rapidly. The actual ice has expanded to beyond 2006, 2007 and 2010 levels and now rivals 2009 and 2008 levels. The snow has also caught up to and surpassed the roughly 45-year average and is now running above average and should thus allow the month to chime in at above average. This is very important in my view since some of the warmest winters have featured a combined October/November Northern Hemisphere snow cover area that is way below average. It took a late inning rally, but we won't do that this year.

What we should see however is a sluggish start in spite of the foot of snow received Thanksgiving. Ironically, La Nina appears much more evident in the weather pattern than it ever did last year with strong upper ridging in the Gulf of Alaska region enhancing the jet energy across the west and making it difficult for Arctic cold to move out of the polar regions for any length of time. For much of November in fact, ridging in the mid-latitude Pacific was destroying the weather pattern much like it did in the early part of the 2006-2007 winter. After a relatively mild week we should succeed at getting some intermittent cold and snow after December 5th but overall pattern will remain anchored by a very positive AO and this will prevent cold from lingering for any real duration. More on this in a subsequent post but this sluggish start will ultimately be replaced by a winter which should not stray too far from normal. We may by the end of the season see above normal snowfall again but I will predict a relatively average 270 inch winter with temperatures slightly above average. For now however, be patient and don't get discouraged with what will likely be some early season frustration.