No one can deny the Winter of 2011-12 has been off to a very slow start with generally mild weather and just spurts of light snow, hardly worth mentioning. The main culprit has been the one thing mentioned in the Outlook that is elusive and problematic in forecasting. I also briefly mentioned it in a recent article and I would discuss the phenomena that can make or break a forecast, the NAO/AO. And, more to the point this culprit, the NAO/AO, has been almost exclusively positive for several weeks which has resulted in a very mild beginning to the Winter of 2011-12.
From the Outlook:
By the recent data, the positive trace of the AO and NAO is easy to see on this graph. The Arctic Oscillation has been pretty much exclusively positive the past three months, while the broader NAO has been mainly positive to occasional neutral during the same time period. Basically, the bitter cold Arctic air has been held further north than it's usual position as the jet stream has been contracted closer to the Pole. I like to use the analogy the freezer door has been basically slammed shut with just the occasional brief opening to let out a bit of frosty cold. This type of pattern results in a nicely charged bitter cold Arctic air mass up further north that only gets fully re-charged as the winter drags on.
It is also discernible by the ensemble model graph projections, that the oscillations are projected to slowly fall toward a weakly positive or even neutral stance in the next week or two. The problems interpreted here are two fold, one: are the projections correct and two: what will be the outcome? The first is the most important as far as a possible change in the offing. The ensemble members have been gradually intimating a gradual breakdown in the stubbornly positive stance of the NAO/AO for several days now, but is it believable? At this point, I feel a gradual weakening is likely, at least enough to bring a more active pattern across the country. I've noted many times in the past when the NAO/AO drifted toward a neutral stance after being in a substantially positive or negative mode, the storm track became active. Simply put, this change indicates the dominant air masses across the northern hemisphere are about to get more "mixed or stirred up". In the case of going from positive to a neutral or negative phase, the Arctic intrusions dig more substantially south and thus, create more storm development.
In our present case, the mild temperate weather has held sway for much of December with only weak intrusions of Polar air masses. With this type of change projected, albeit it may be somewhat subtle, it should stir up the atmosphere up enough to at least get more action. Where and what the main action will be will be followed closely to relate in my blog. Stay Tuned.
Coming in January 2012:
Looking more in-depth into the "Wet Year of 2011"
Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian