Think Snow, Tweet Snow !!!

Sunday, November 28, 2010

Big block over Greenland will ensure cold and some snow after Dec 1

A large upper air "block" has certainly established itself across Greenland over the past week. It has tag teamed with another high latitude blocking feature centered over the Bering Sea to allow arctic air to flood the country and will ensure a generally favorable early season pattern for the beginning of December. The benefits of the negative NAO will unfortunately not be felt until after the 2nd or 3rd of the month. The Bering Sea block has allowed a large trough to form across the inter-mountain west and this in turn will allow a large fetch of mild southerly to flow to establish itself across the east and this will flood the region with mild air very quickly on Tuesday. The rain will soon follow and rain it will to the tune of 1-2 inches. We might as well get this out of the way now because an event like this would have a devastating impact on skiing were it to come prior to the Christmas holiday.

So speaking from the standpoint of the MRG "base" we will be starting from scratch on December 2nd but a sustained period of cold weather will begin on that day. Temperatures will be seasonable through the upcoming weekend and by the looks of it, we will not have the instability for a big terrain induced snow event. There will be a lingering "baraclinic" area (you could say front) in the Gulf of Maine and this has the chance to stir up a bit of snow though the chances for accumulations will be best over coastal regions though much is left to be determined. The combination of better instability and even weak weather systems should result in some new snow early next week and hopefully by then we can start discussing a opening day party.

There is evidence that the block over Greenland will begin to break down around December 10th but our Bering Sea blocking feature will remain. The pattern will thus turn more variable but we should begin to churn out a few weather systems as we approach the middle of the month. There are a few additions or side features which should help to provide more of a numerical summary. Included will be seasonanal and 7-day snowfall totals at MRG and expected snowfall over the upcoming 7-day period. I will also include a favorability index. This will be a very generic, cumulative measure of the three teleconnection indices: the Arctic Oscillation (AO) the North Atlantic Oscillation (NAO) and Pacific-North American (PNA index. The AO and NAO index is favorable when negative while the PNA is favorable when positive. Thus for the purposes of the "favorability index", the signs of the AO and NAO will be flipped and added to the PNA index for a final total. This approach has weaknesses but it will be a simple non-subjective and numerical way of looking at the next two weeks since our newly devised "FI" will be calculated at 7 and 14 days.

Thursday, November 25, 2010

Winter 2010-2011: The revenge of La Nina !!!

It is the middle of November and the weather has been excruciatingly mild so it must be time to kick off the seventh season of Single Chair Weather Blogging. Preseason forecasts can be a mess to evaluate in hindsight for their inexactitudes, inaccuracies and unquantifiable subjectivities. It can generate some excitement an for me induce a sudden craving for Magic Hat's Single Chair ale, which seems to be in short supply during Vermont's stick season (someone will have to clue me in on where to find some out of season SCA).

The emails I have gotten, which can now be sent to (as opposed to yahoo) suggest that the SCWB reader continues is getting more and more weather literate asking the tough questions months ahead of the time when they usually get answered. We have an impressive audience of "in the closet" and "out of the closet" weather geeks and this helps to make running the SCWB a lot more fun. The talk this year has mostly revolved around La Nina and rightly so. It has blossomed quickly, decisively and promises to be a dominating force throughout the winter months. Its development also underscores how sharply the upcoming winter promises to deviate from the outcome last winter which featured a moderate El Nino, a vigorous and very moist southern branch of the jet stream and a season that I will now referred to as "the great inversion of 2009-2010" for its tendency to provide snow and cold to our friends down south and agony due to the lack of snow in Vermont. Preseason forecasts may be mostly famous for vagueness (I confess to share the guilt on this) but I can stress, even in mid November that the upcoming winter will be a striking contrast to the last and will quickly display a very different personality. If nothing else, it will keep the prognosticators on our toes, since frequent updates will be required to what should be a rapidly changing and volatile weather pattern. It will prove to be fun and challenging if nothing else.

La Nina's return
The ENSO conversation is typically the easiest part of the preseason discussion. Its associations and relationships are known and a ENSO event is easily quantifiable. This years La Nina has already received a ton of discussion and for good reason. As an aggregate measure, the current La Nina in September-October (-1.59 SST anomaly)is the strongest since 1955. I was somewhat surprised that we didn't blow right through the tropical storm alphabet since La Nina's of such ferocity usually are associated with very active hurricane seasons (this year's actually was but we did not see any major land falling storms in the U.S. and we didn't blow through the alphabet, at least not yet). Two other La Nina event that comes close developed in 1988 a winter we would just assume forget and 2007 a winter where I was pleasantly surprised. The winter of 1970-71 also featured a strong La Nina (although one that developed very late) and this winter turned out like a dream with incredible snowfall amounts across northern Vermont. This may seem like a pointless range of possibilities to discuss (since the range includes seemingly everything) but La Nina winters have a distinct personality that ties them together as opposed to specific snowfall amounts. Two features that seem most persistent and most influential tend to be a large and potent ridge across the Gulf of Alaska and a persistent ridge across the southeastern United States. This often dreaded southeast ridge can make winter's non-existent in our southern mid-latitudes below 38-40 north. Ski areas that received the high snowfall amounts last year like Seven Springs in PA or Snowshoe in West Virginia will not be so lucky this year. It is a different ball game up north however. Storm systems, and many of them, get deflected up through the Ohio or Mississippi Valleys toward the eastern Great Lakes and then often proceed to split New England into pieces in terms of the weather that results including heavy snow across northern areas, ice, ice and more ice across southern interior areas and rain along the coasts. This line of rain to ice to snow is by no means a constant and tends to fluctuate from north to south based on the storm track or readily available fresh supply of cold air. During some winters, MRG is on the favorable side of all the uncertainty (70-71), some they are not and most fall in between. This means some weeks turn out very snowy but there are several where ice and rain can greatly damage our skiing enjoyment. It is often unfortunate because there is not much even a little elevation can do to stop surges of mid-level warmth that often become so prevalent during these type of winters. Latitude does greatly help however and we do have that working for us (44 north thank you very much !!

2007-2008 winter forecast got suffocated much to our delight !!
The discussion about the current La Nina should also include a look at the 2007-2008 winter. That season was predicted to be a poor one since it was thought that the La Nina strength combined with the slow autumn buildup of northern hemisphere snow cover would be tough to overcome. That forecast was terrible but in retrospect revealing. For one, it is another in an invariable string of reminders of how inaccurate these forecasts can be. Secondly, it did provide a bit of inspiration for some preseason research. The results are interesting in a study that involved looking at strong ENSO events (strong La Nina's or strong El Nino's) and snowfall and temperatures across northern New England. The temperature results are not surprising. The strong El Nino events are dominated by milder temperatures and strong La Nina events feature above normal temperatures of a lesser degree. Snowfall in winters featuring significant ENSO events was a bit suprising. Both strong La Nina's and El Nino's featured above normal snow which to some might seem like a counter-intuituve result. It would be healthy to maintain a bit of cynicism regarding the results since the sample size which spanned 60 years and 21 qualifying winters is rather small. Still it is fair to suggest that the winter forecast made in 2007-2008 was a bit too pessimistic. The premises were right and many of those premises will remain this year.

The build up of snow and a further look at other strong La Nina winters
Looking at the buildup of snow across the northern hemisphere is our empirical method for assessing the hypothetical magnitude of cold airmasses through the upcoming winter. It is very inexact and can change over the course of the season. Snow cover area however often proves to be an important thermal feedback especially when trying to predict the sign of temperature. It helps cold air pool more efficiently especially early in the season when large expanses of high latitude land can either be covered in snow or snowless. The 10 most recent strong La Nina winters featured the following coverage of snow (in millions of square km) across the northern hemisphere during October and November. The end result of each winter is included on the right.

Year Snow Result
1970-71 28.6 Very cold, much above normal snow
1973-74 27.6 Slightly mild and above normal snow
1975-76 23.8 Slightly cold and above normal snow
1984-85 25.4 Slightly cold and above normal snow
1988-89 22.1 Slightly mild, way below normal snow
1998-99 26.6 Very mild, normal snow
1999-00 25.5 Very mild, below normal snow
2007-08 24.4 Mild, much above normal snow
2010-11 ~25.3 ??

The average snow cover area for the northern hemisphere during this period is 25.3. The period ending this November 30th will come in just at or slightly over that number. It is interesting that that the year with the highest autumn snow cover turned out to be the best and the year with the lowest (1988) turned into the worst as far as skiing goes. Somewhat circumstantial perhaps but probably not entirely so. This winter has seen a healthy buildup of snow and a rapid expansion in the last week or two across the North America but its not much better than average over a two month period so therefore difficult to draw any conclusions.

Our friend the PDO
The Pacific Decadal Oscillation (PDO) is another variable we have factored in to the preseason forecasts. The PDO refers to the configuration of temperatures across the mid-lattitude Pacific Ocean as opposed to the ENSO which refers to the equatorial Pacific. A positive or warm phase is often but not always teamed up with an El Nino and tends to feature a long-wave or jet stream pattern consisting of a trough in eastern North America and a ridge in western North America. A negative phase of the PDO is often prevalent during a La Nina and has the opposite effect. Data in recent years suggests that we have moved into a prevailing negative PNA and that last years strong El Nino was tag teamed with a very weak positive PDO. This year the PDO has swung back to negative and it appears it will remain there through the winter. This comes as no surprise and supports a pattern we are already expecting given the strength of the current La Nina.

The Quasi-Biennial Oscillation
Just like with the PDO, 10 years ago, there has been work and increased discussion over the last several years regarding the Quasi-Biennial Oscillation and its relationship to the behavior of weather patterns in the northern hemisphere winter. I hope to keep most of the in-season discussions focused on more benign terminology but I did learn enough from emails and other discussions to provide a brief blurb. The QBO refers to the oscillation of the equatorial zonal wind in the stratosphere and is measured as an westerly or easterly index with westerly having a positive index and easterly negative. Some work has been done to establish a relationship between the phase of the QBO and Sudden Stratospheric Warming Events (another lengthy term). The SSWE have their own lengthy description and definition but they are associated with mid latitude jet stream blocks that are often the cause of sustained periods of cold weather. The easterly phase of the QBO for instance last year was said to encourage the persistent negative NAO. It has since moved to westerly perhaps suggesting less frequent negative NAO events and a more zonal pattern. None of this really contradicts anything that we might expect in a strong La Nina year such as this.

A summary and a prediction
So in summary, La Nina will win the day and win it decisively. It will mean that temperatures will average above normal but not as much as they did last winter - lets call it 1-2 degrees above for the ski season. Many places south of 40 degrees north latitude will experience a winter dramatically milder than last and dramatically less snow. We will get outbreaks of some severe cold but they will be interrupted and very abruptly by intrusions of milder weather. The milder weather will in many cases be accompanied by rain or ice which will at times be damaging. On the flip side there will be many other storm systems that bring snow and we should see a few epic periods. The southern branch of the jet stream will not be as influential and we will instead see a consolidated west to east jet stream send storm systems in our direction and few should miss. This should be enough to allow snowfall to come in above average - lets say 290 inches. This is around 110 percent of average at the mountain. The winter will require our full attention since many of the best periods will be simply wet or icy along the coast and conditions will change very quickly. Enjoy the thanksgiving holiday and think snow.