Up until last night, there was good model consensus about the upcoming storm.  With last night’s 1AM runs of the models, there has been some divergence developing.   Let me cut to the chase:

Both models have the precipitation (as snow) beginning a bit later.   We may only have an inch or two by midnight Friday.  Snow becomes heavy during the night time and early parts of Saturday.

At that point, the NAM and GFS thermal structures begin to diverge.  The GFS wraps in some warm air aloft for a period, as the energy transfers to this secondary low center mentioned yesterday.  As a result, the GFS  has 8-10 inches of snow for the immediate PHL area when all is said and done Saturday evening.  (Most snow falls over West Virginia and central Maryland)

The NAM still spews out 20 inches of snow for our area with the main low pressure lingering and blocked right off the coast.  Too early to tell which is correct, but the NAM does have a tendency to over-estimate precipitation QPF in this time frame.

This sort of divergence always seems to happen.  The more divergence, the less reliable the forecast.

We may not know whether the snowstorm will be just bad (8-10 inches) or a whopper (20 inches) until model runs are done tomorrow morning (Friday).

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