A small and relatively weak storm is currently moving out of the Great Lakes region. This system will slide to our south rather inconspicuously until reaching coast, transferring its energy (as these storms so often do) to a new, stronger storm center over the ocean.
Under normal circumstances, this coastal storms track would be too far east to really give SNE any significant precipitation. But a few ideal atmospheric conditions have set up for accumulating snowfall and even heavy snowfall bands in some areas.
1.) Cold air. Shortly after the Patriots waxing, some real cold air settled in over New England. Most areas are currently in the mid to low 20's, even in Cape Cod and the Islands. What this means is any moisture that does fall will be snow. It also means that the snow will be light and have a high "fluff" factor. Normally, the snow to rain ratio is 10 to 1 (1 inch of rain equals 10 inches of snow). The ratio will be closer to 20 to 1 tonight, meaning snow can pile up higher with less moisture.
2.) Atmosphere Instability. I will try to avoid too much technical stuff here. First, a physics review. Atmospheric instability is created when warm air at the surface rises uninhibited through a cold pool of air just above the surface. This phenomenon is commonly seen in those high and towering thunder clouds we see in the summer. The approaching storm will in a sense "kick-start" the comparatively warm air we have at the surface upwards, rising swiftly through the cold air and creating more clouds and enhanced precipitation. The areas of heavy precipitation can become localized along a trough (line) in a sort of inverted manner, like a tail hanging off the main storm. This phenomenon actual has a name, Norlun trough, named after the two meteorologists who first observed the pattern. With these troughs, its all about location. Where does the tail set up! The bands of heavy precipitation are quite narrow and don't move, resulting in high snowfall variability and a big bust potential in snowfall forecasts.
Alright, you hung with me through the boring stuff, so now to what everyone wants to know. How much? Since I can't make my own maps due to a lack of talent, I chose to borrow this map from NECN meteorologist Matt Noyes.
A couple of things to note. The snow won't really start to accumulate until around midnight. The heaviest snowfall will occur overnight, with most areas picking up a fluffy 2-4 inches. Not enough to cancel school I'm afraid. Some heavy snow bands will set up along the Norlun trough, which figures to be stationed over the outer Cape, Cape Ann and Portland, ME. It is here where some areas may hit the jackpot and a foot of snow. Unfortunately, we won't know where until the storm develops.
Its worth noting that this is a type of scenario where one town may get 2 inches of snow while the next town over gets 8 inches. Quite variable and hard to predict. Not that I would expect any sympathy from you good people. Some people will wake up tomorrow disappointed, while others might be pleasantly surprised.
Here are two interesting further readings on Norlun troughs. One from Matt Noyes concerning the last Norlun trough we had in 2011 and the other the summary of that last storm from the NWS.
Matt Noyes
NWS
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