December 7-8, 2011, Snowfall Lands Far Short of Expectations for Some - Cold Air Too Weak - NECN
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December 7-8, 2011, Snowfall Lands Far Short of Expectations for Some - Cold Air Too Weak



    The biggest concern I had going into this storm was the absence of deep cold air.  Often, a snowstorm presents the combination of a strengthening storm center, and the presence of a strong, cold high pressure (fair weather) dome over Eastern Canada.  The former was quite impressive.  The latter, was not.  It's one thing to look at a map, as I did yesterday, and worry about the lack of deep cold air, but the next step is always to look for a signal in the computer guidance that backs up your sneaking suspicion.  In the days leading up to this storm, I looked at every parameter I could think of, and with the exception of borderline temperatures in the lowest 1000 feet of the atmosphere, all other parameters were good for a change to snow in most of Northern New England.  Of course, 1000 feet is a fairly thick layer, but often in heavy precipitation events we see the freezing level lower as "dynamic cooling" occurs - essentially, the process of heavy precipitation allows for cooling of the atmosphere from the top down.  Every piece of computer guidance pointed to this factor - and it should be noted that is NOT to blame the computers...WE as forecasters are always responsible for what we present - and it was enough to convince me to abandon the fear of too much warm air in the lower levels.

    In the end, it was that warm air that was the culprit.  In fact, in hindsight my guess is that the rare setup of antecedent tropical air was a huge factor when combined with the lack of a strong Canadian high.  You see, often as a storm strengthens, we get an increasing wind from the north as air is sucked into the center of the storm - that wasn't the case here...the wind came on the BACK of the storm, as we saw with the 79 mph gust in Point Judith, RI, once the storm center passed.  Additionally, dynamic cooling events often require at least some temperature/dewpoint difference in the lower levels - that is, we must be able to lower the temperature as precipitation falls into it, and if you already have matching temperatures and dewpoints (saturated air) with no considerable north wind to lower either of those parameters, your goose is cooked for snow.  This was why we were so shy on any snow outside of higher terrain, but as it turns out, even many spots we thought would cool sufficiently did not.  Nonetheless, here is the verification, which I'm trying to do for each storm:

    Southern New England FORECAST:


    Southern New England ACTUAL:


    Vermont/New Hampshire FORECAST:


    Vermont/New Hampshire ACTUAL:


    Maine FORECAST:


    Maine ACTUAL: