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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This afternoon’s NAM and GFS model runs continue the trend mentioned last night and this morning that the northward movement of the low pressure system will be blocked by high pressure, resulting in a delay for the snow to reach us.

The current models show snow starting during the evening Friday and getting heavy late Friday night through Saturday.  Some of the model trends suggest a more southern movement of the low, so heaviest snow will be south and west of Philadelphia.

QPF values from the GFS are about 1.25 inches water, falling as snow.  The GFS snow algorithm suggests 16-20 inches of snow possible by Saturday evening.

In addition to the delay in the snow onset, the other model trend has been for the snow to end earlier on Saturday night instead of lasting into Sunday, as the coastal low is expected to spin into two low pressure centers, transferring the energy to the further eastward low.  This trend may result in lower storm snow totals if that transfer occurs earlier.  Right now, the models say 16-20 inches, but that double barrel low pressure system is known to be handled poorly by the models.  So the actual totals may be lower.   Either way, have that snowblower ready.  

I’ll update later tonight if thing change significantly.  Otherwise check back Thursday morning.

10 PM -Tonight’s NAM model output just available shows a QPF of 2.54 inches water. That would translate into well over 30 inches of snow!  The NAM almost always overstates the precipitation in this time frame, so I wouldn’t take this too seriously, If nothing more, it supports the likelihood of a big storm.  Still starts early evening.