Latest NAM and GFS data from the 1PM (18z) afternoon model run.  Both models now colder; little chance of changeover to sleet in PHL.  Both have the snow starting late evening on Friday in PHL and earlier south and west of the city.

The NAM cranks out 1.55 inches water (QPF) and the GFS cranks out 1.98 inches of water (QPF).    The snow algorithms suggest 20-30 inches of snow possible by late Saturday night.

The next model run is at 7PM (00z) and the data starts rolling out in raw form about 9:30 PM for the NAM and 10:45 PM GFS.

As we get closer to the actual ‘event’, confidence in the predicted QPF values increases.   But with both models having such high QPF values and similar thermal profiles,  the chance of a major snow storm is looking like a sure bet.

10 pm update:  tonight’s NAM has a QPF of over 1.80 inches water.  What’s new- starts between  7-10pm. Heaviest snow and highest winds Saturday morning. A lull late Saturday afternoon for the “dry slot” , then several more inches wrap-around Saturday night.  Snow totals 20-30 inches for PHL  by Sunday morning.

11pm update. Tonight’s GFS shows a 1.61 inches water. No dry slot and ending Saturday night. Taking average of NAM and GFS 20-24 inches snow total.

1 pm  Quick update   NAM and GFS models show 16-20 inches snow. Maybe more. Updates this evening. 

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).

Check back for further updates, or follow me on Twitter.      @theweatherguy

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