Well this has certainly been a difficult winter to initially forecast; then observe. Some of the main issues I stated in the Outlook and subsequent blog troublesome winter have unfortunately materialized.
From the Outlook:
The first and utmost problem with this winter is one of the main issues every winter, the North Atlantic/Arctic Oscillation phase. Most forecasts this winter were counting on a negative or at least neutral average of the NAO like the past several winters but this surely has not been the case. The NAO/AO has been in positive mode (sometimes strongly) since last fall. Of course when the NAO/AO is in a positive mode or phase, the real Arctic cold air remains bottled up around the polar regions and other far northern latitudes; one of the reasons why Alaska has been so cold and stormy this winter. In the strongly positive NAO/AO phase, basically the Arctic Jet stream retracts northward somewhat as it circumnavigates around the North Pole. It has been only recently where the NAO/AO has been transitioning toward a neutral or occasionally, a negative phase. It is interesting to note that the projected phase of NAO/AO is mainly neutral-negative early, then becomes quite variable again but with a subtle trend downward as of 1/21/2012. One positive note in this recent transition is the fact it does reflect the Winter Outlook's analogue's call that the second half of the winter would contain the worst of the weather: being the coldest part of the winter and back-end loaded with snowfall. Of course the obvious question that arises; will the NAO/AO trend continue?
The second reason why much of the lower 48 and Southeast Lower Michigan have been mild is also the reason Alaska has been so stormy: the strong northern Pacific Jet Stream discussed in the Outlook and later in the Troublesome Winter. This part of our Winter Outlook equation is there, in spades. The jet has been so strong, occasionally it established a strong zonal flow into the US, ironically not unlike the jet seen in moderate to strong El Ninos; just further north. The analogue projected Pacific Jet "did it's part" to verify our expected winter jet pattern but it's partner, the Arctic Jet in which phasing or pardon the expression, "mating" was routinely expected but has occurred, infrequently. Therefore, much of the country has actually experienced in my opinion a "pseudo El Nino" winter with the strong Pacific jet bringing modified air into the country. In addition, many times this strong jet off the Pacific dug into the West and actually creating ridging into the center and eastern part of the country in the absence of the Arctic Jet. This in turn; brought periods of above to well above normal temperatures. So much so, that up until recently Southeast Lower Michigan had experienced perpetually late fall-like temperatures with readings the first half of the winter averaging a good 5 degrees above normal!
Visualizing this further:
The Pacific Jet of the analogue's winter (Fig -1) upper wind pattern has been fairly accurate; while the Arctic Jet pattern has been focused further north across Alaska east into north and central Canada. It's interesting to note that between these patterns there was a notable absence of wind or "lighter wind path" west to east, across the center part of the country just north of sub-tropical jet. In the absence of the Arctic Jet, which has retreated north, one can visualize how the intense Pacific Jet would easily flood across the country in this basic analysis of the pattern (fig-2) (and remember to take into consideration the Arctic Jet being further north).
Fig - 2
Ok, this is the past and present; what about the future?
While we have definitely seen a pattern change of late, the NAO/AO is still in a transitioning or "unstable" pattern which has been even more erratic than usual, which makes forecasting the second half of the winter just as troublesome (or difficult) as the first. But, if you know me by my years with NWS and my Southeast Lower Michigan Outlooks (and now Weather Historian Blog), I'm not going to let a "little" adversity get in the way. If I fall on my face, so be it...it wouldn't be the first time nor the last. And, using the famous pep-talk line; "Get up and put one foot in front of the other and get on with it!" ;-)
Well, obviously taking into consideration the first half of the winter, my original call of 1 to 3 degrees below normal was much too cold. Therefore, adjusting for the exceptional warmth first half of the winter (with an average departure through mid January of +5.0) , I now expect the entire winter average temperature to range 2 to 4 degrees above normal. With these departures, one would expect the second half of the winter to be colder than the first half but I still look for it to average around 1 to 3 degrees above normal.
"If the storms ride further north than expected, this could also botch up the temperature forecast too with the upper level ridge holding stronger in the Southeast."
Bill Deedler -SEMI_WeatherHistorian