Yet another forecast snow bites the dust.   The 1 AM runs of the GFS and NAM show the low pressure development Saturday night to be very unimpressive.   The track is further south, the intensification is less and the precipitation rate is lower.  Surface temperatures appear warmer.

With the current trends, I’m significantly scaling down the snow forecast.

In the immediate PHL area and immediate suburbs, little accumulation is expected on roads and paved surfaces and perhaps a wet coating on grassy surfaces.   It looks to be principally rain mixed with some wet snow here in PHL and the immediate suburbs.

Things may change again.  I’ll update this evening.

10 AM Friday Update- Latest NAM again has 0.60 inches QPF , starting about 6 PM.   Temperatures in Philadelphia appear just on the borderline too warm for snow at the start, but my concern about dynamic cooling might come into play here again. So I may have to backtrack to last night’s forecast  with a few inches of snow, mostly on grassy surfaces in the PHL immediate area!

Sorry for the flips back and forth on the forecast, but this forecast is plagued with boundary conditions that could go either way.   That said, this morning’s NAM was looking colder than its previous run.  We won’t know until tonight at the earliest, and it may need to wait until Saturday morning for the final handle on things.

The latest NAM model data shows a consistent scenario  of low pressure developing along a frontal boundary to our south and east Saturday evening.  QPF values still in the 0.60 inch range.

The usual rain-snow line appears to run through Philadelphia.  Here’s the difficulties with the snow forecast — temperatures at the surface may initially be above freezing, limiting accumulations, especially on paved surfaces.

With the usual “critical thicknesses”  for snow where they are, I would ordinarily be forecasting a mix of rain, sleet and  some snow.  BUT, with the precipitation rate as high as forecast (0.60 inches/6 hours), I am thinking that dynamic cooling may become a big factor with this event. As a result, I am leaning towards more wet, large flake snow than I would ordinarily forecast.  From Philadelphia north and west, 4-5 inches appears possible on grassy surfaces, less on paved surfaces.  Areas far north and west may have 6 -7 inches.

Again, I’m counting on the NAM high precipitation rate for dynamic cooling for this to be snow in Phladelphia; otherwise it will fall mostly as a wet  mix with significantly lower accumulations.

Precipitation starts about 5-6 pm Saturday and ends about 3 am in the morning Sunday.  Whatever falls starts melting Sunday.

Still 48 hours before this starts, so expect changes in the forecast.