As I’ve been saying for several days, the final forecast can’t be made until Wednesday night’s data comes in.  And latest NAM model data has just become available.

In conjunction with my previous post this evening regarding warm surface temperatures as an impediment to high snow accumulation, the latest NAM data suggests that much of the QPF will occur before surface temperatures in the immediate PHL area drop to freezing.

The NAM has 0.59 inches water falling before 7AM, but much of that will be rain,  snow mixed with rain,  and finally snow about 7AM.    The NAM has reduced the QPF to only 0.20 inches water after 7AM.

So I’m going to go against the TV forecasts and predict only 3-4 inches of heavy wet snow for the immediate PHL area, ending about 10 AM.

I might be wrong about this forecast, but you’re reading this blog to get the real scoop without the hype.  While everyone gets all excited about a big storm, I simply can’t ignore the warm surface temperatures with this one.

10:30 PM Update:  Latest GFS supports the forecast of only 3, maybe 4, inches of heavy snow accumulation in the immediate Philadelphia  area.  The GFS has only 0.18 inches QPF falling after 7AM.  It’s looking like most of the precipitation occurs before the cold air moves in.

I don’t think the snow will be very heavy after daybreak.

Wednesday 6PM Update: The afternoon model runs (1PM) usually show a reduction in QPF and that’s the case this afternoon.  Latest estimate- the transition to snow occurs about 4 AM and snow continues heavy until 10 AM, then light snow tapers off after noon.  Latest estimate for precipitation is 6-8 inches of snow in PHL.

Next Update about 9:15 PM

Wednesday 1 PM Update: Latest GFS has become available and it has  a significant amount of precipitation falling as snow between 4 AM and 10 AM on Thursday. Total QPF is over 1 inch water which could translate into 8-12 inches in the immediate PHL area, more north and west.  That means that both models have reverted back to yesterday morning model’s rather impressive snow potential.   All the factors cited of warm surface temperatures make exact accumulations difficult to predict, but this storm will likely be one for a snow blower, not a shovel.

The GFS and NAM are remarkably similar, which puts high confidence in this forecast. More interesting is that the GFS has higher QPF values than the NAM, something that is usually the other way around.

Will wait for the next model runs this evening, after 9:15 PM.

Quick Update- Latest NAM from this morning just in.  It has reverted back to yesterday’s scenario.   Precipitation continues into the morning hours due to greater intensification and it’s showing about 4 inches during the daytime hours alone.  Total accumulation for the immediate PHL area looks to be over 6 inches!  More north and west.

An interesting storm with many factors coming into play.  Won’t have a final handle until this evening’s data which becomes available after 9 PM