A quick update.  This morning’s NAM cranks out 0.46  water as the QPF  late tonight and joins the NAVGEM  with the western extent of the precipitation well west of PHL.

However, this will fall as a mix of rain changing to wet snow that may only stick to grassy surfaces in the city, maybe some accumulation north and west.  This set up will be impossible to predict snow accumulations. Too many variables and too many unknowns.

Will update tonight.

Thursday 7 PM:  This afternoon’s NAM increased the QPF to 0.60 inches water(!).  It’s a good thing it won’t all fall as snow.   The NAM also increased the cold flow, so some accumulation may occur, even in Philadelphia.   But how much falls as rain, a mix, and wet snow is still impossible to calculate.    Add to the uncertainty the ground temperature (which has warmed over the past several days) and we only know that some snow will fall, but accumulations will only be a guess.    The NAM consistently ends the snow by 7 AM.

I’ll update when the next NAM data comes available about 9:50 PM

The models had shown a developing wave along the front that just moved through yesterday missing us, but as mentioned last night,  over the past few model runs, the NAM and the NAVGEM have extended the precipitation from this wave of low pressure further westward as a trend, now reaching PHL.

They latest NAM has 0.17 inches water (QPF) falling as rain, then snow, between midnight Thursday night and daybreak Friday.  Ground temperatures are warm and the surface temperatures are expected to be a bit above freezing, so any snow that falls will have a challenge accumulating, especially in the city. 

This is a difficult forecast.  Further westward movement of the precipitation will interact with colder air in the western suburbs, allowing more snow accumulation.   Still looking at just a coating to an inch west of PHL, if anything.  (The NAVGEM is predicting even more for PHL, west and north, lasting into the mid morning.).  But clearly, the modeling of the intensity of this coastal wave has been less than stellar to this point.    Won’t really know details until later today or this evening.

While I’m looking at the NAVGEM, (previously called the NOGAPS model) it’s showing a very different movement of the coastal low for Monday.  (The other models have it moving out to sea.). It has the low moving up the coast, giving us possibly significant snowfall by Monday morning.

I’m not big on the NAVGEM model except for hurricane prediction, but if it predicts tomorrow’s snow correctly, we’ll need to consider its solution more closely for Monday.