2011-12 Winter Outlook
Weather forecasting is very tricky business. It is definitely frustrating, challenging and rewarding. The WBZ Weather Team concentrates on predicting the specific events of the more immediate future up to 7 days. To do this, we meteorologists rely upon a myriad of computer models based upon complex mathematical equations incorporating the variables of the atmosphere. The office of the National Weather Service that provides these worldwide forecast guidance products is known as NCEP. The chaotic nature of the fluid atmosphere is such that all short-range forecasters will probably always be limited in our ability to predict the weather beyond a theoretical threshold which is 7-10 days. Beyond that, the WBZ Weather Team has become increasingly intrigued with seasonal forecasting especially relating to winter. Over the past several years, we have learned much about the global climate and its variability by following the very highly respected brilliant meteorologist, Joe D'Aleo. His extensive research and knowledge of this complex field is nothing short of extraordinary. We thank Joe for his commitment and guidance. You can follow the work and findings of Joe D'Aleo and Joe Bastardi on WeatherBELL.
Beyond our curiosity about the future, the fascination of seasonal climate forecasting is naturally linked to a great variety of socio-economic applications. For seasonal outlooks especially pertaining to winter, many people over the past several decades have read with interest the prediction from the Old Farmer's Almanac. Today there exists real science behind seasonal weather prediction and the accuracy has improved significantly. Government forecast agencies such as the Climate Prediction Center run climate models that predict weather patterns months in advance. Seasonal forecasting utilizes dynamical models and statistical models. Dynamical models are coupled oceanic and atmospheric climate models that project future mean temperatures and rainfall based on warm and cold pools in the ocean which control the jet stream and the associated anomalies. Statistical models are empirical approaches that relate seasonal precipitations and temperature anomalies with persistent large-scale modes within the oceanic/atmospheric system and the sun. There is amazing connectivity between the oceans and the atmosphere and scientists have worked dilligently over the years to gain greater understanding of this link. The cataclysmic effects of the powerful El Nino of 1982/83 was the catalyst that ramped up this research. The goal was to determine statistical relationships between weather patterns and variables such as ocean temperatures, upper level winds and solar activity. Thus far, the findings of brilliant scientists in and out of the government sector reveal about a dozen prime factors that must be considered for global seasonal forecasting. The finesse is determining what factor(s) will be dominant. The factors include specific atmospheric and oceanic oscillations such as ENSO, the QBO, the PDO, the NAO, the AMO, the PNA, permutations in the phases of these oscillations, solar activity, geomagnetic activity, recent volcanism, fall temperature and precipitation patterns, storm track data, etc. It is an atmospheric Rubik's cube.
If you desire a more technical, detailed discussion, read this paragraph otherwise scroll down to the conclusion. Here is a look at the cast of characters with key roles in this upcoming chapter of winter. First of all, La Nina was very strong the first half of last winter before weakening commenced later in the season leading to neutrality in late spring and early summer. La Nina re-emerged in the last 2-3 months and global indicators point to it becoming moderate going forward into this winter before weakening sometime in the second half of the season. Next, the QBO is signaling a transition into an increasingly easterly phase. If this signal verifies quickly, it would lead to massive stratospheric warming which in turn would produce a weaker and warmer polar vortex which is incapable of containing arctic airmasses at high latitudes. Consequently, outbreaks of frigid weather into the lower latitudes is more likely. Add to that, the PDO which is entrenched in a cold phase that will likely remain for at least another 10 years. This supports a pattern delivered by La Nina. Remaining in the warm phase, the AMO was one of the culprits in creating a larger than predicted snowfall last year. La Ninas with a warm AMO lead to less ridging over the southeastern U.S. and more blocking in the North Atlantic which is exactly what happened last year. With that said, there are signals indicating that the AMO will transition into a cold or negative phase during the season. This has very important implications because that development would pump up the ridging leading to a warmer eastern and southeastern U.S. later in the season. The late winter regime is highly a function of the timing and the magnitude of the weakening La Nina and the cooling AMO. The cooling of the Atlantic is very noteworthy because it will be the first time in 20 years that the PDO and the AMO have been in the cold phase simultaneously. However, the genuine variability of our winter will ultimately be controlled by the wildcards known as the NAO/AO. The twist in the plot usually arises here because their abrupt phase changes cannot be accurately predicted well in advance for a full season. Their negative or blocking phase is enhanced by low solar activity and active high latitude volcanism which was the case over the past two years. More recently, however, here has been no high latitude volcanic eruptions and the solar flux is increasing but it remains relatively low.
In conclusion, based on the present and predicted indices of the various climate factors and considering analog years, confidence of a colder than average late November through December into January is high. The impending pattern favors above average precipitation which translates into above average snowfall through this period. This matches the outcome of the first part of last winter but there is an important difference. La Nina is definitely weaker and the blocking should be less pronounced. Consequently, the overall intensity will not match the magnitude of last year's multiple snowstorms. Nevertheless, the dominant factors should yield a snowfall above average across much of the region with 55-60" closer to Boston. This undercuts last year's 81 inches but is much above the average of 42". The main storm track is projected across the northern Rockies east-southeastward to the lower Midwest to the Ohio Valley to the Mid-Atlantic States then occasionally curving up to near or just south of Cape Cod. The frequency and magnitude of North Atlantic blocking will be key. The uncertainty lies in the second half of the winter. Assuming La Nina weakens and the Atlantic cools, it will turn milder than average in February and March. The storm track would then shift farther west but still produce some snowfalls farther inland and over the northern mountains. In fact, the second half of the winter looks to be stormier than the second half of last winter. Closely watching the variables in the next several weeks will be paramount in determining if a forecast revision is warranted for the second half of the winter.
Interestingly, despite warmth highlighted by a string of 80s over the Columbus Day Weekend, October ended up with an anomalous damaging snowstorm. While Boston received only one inch in that storm and despite our forecast above, records reveal that the three previous Octobers of measurable snow yielded seasons of below average snowfall. I am not expecting a repeat this time. Another rather short term study over the past 12 years shows that Novembers with the overall temperature more than 1 degree above average resulted in snowfall below the average of 42 inches. If November was more than 2.5 to 3 degrees above the average, subsequent seasonal snowfall was significantly below the average. With this week's warm weather, this November is more than 2 degrees above the average so far. After another batch of warm weather the beginning of next week, the temperature would have to go well below average the last 10 days of the month to correlate with the above short study and the projection for this season. We shall see.
With a very high degree of confidence, I can state that astronomical winter officially begins at 12:30am on Thursday, December 22.
