A quick update. Latest NAM and GFS have come closer together in QPF. The afternoon runs have 0.12 (NAM) and 0.16 (GFS) falling as snow, mostly before 7 AM Friday. This translates into about 1-2 inches of snow, less north and west.
The tv weather people’s phrase, “mostly on grassy surfaces” doesn’t apply here. The snow will be falling during the night time hours– insolation through the clouds won’t be a factor until after sunrise, when the snow will be essentially over. Once the sun angle increases (behind the clouds), most snow on roadways will quickly melt, leaving any accumulations on grassy surfaces.
But those hitting the roads for an early commute will likely see some snow on untreated surfaces, since temperatures will be below freezing during the snowfall, especially north and west.
Fri 7 AM: The northernmost cutoff of the precip was sharp and here in the Chestnut Hill area, we hardly had more than a coating. (In this scenario, “on the grassy surfaces applied”, but not for reasons I had considered- too light snowfall.). It appears that just south and east of PHL airport, snowfall has been more significant. )
There remains some uncertainty about the expected light snowfall late Thursday night into Friday morning.
The latest NAM continues with a very low QPF for Philadephia, only 0.07 inches water. The GFS has been consistently higher at 0.24 inches water.
My impression is that the low QPF of the NAM is more likely correct. That means about a dusting to an inch by 9 AM Friday morning. More south of the city and into Delaware and less north and west of the city. I’ll update if things change.
The model depiction of the small low pressure system expected to affect us on Thursday night into Friday continues to evolve. The NAM and GFS differ on timing and coastal development. The GFS has a more substantial coastal storm developing on Friday and coming very close to PHL, but the heasiest precipitation shield still misses us.
It should be noted that the degree of coastal development and the degree of upper air troughing continues to change, with the trends leaning to the possibility of more snow than the currently expected 1 to 2 inches.
Put another way, it wouldn’t take much change for the snow in our area to be more significant. It also wouldn’t take much change for it to be just a period of snow showers.
To give perspective on the model’s continuity, tonight’s rainfall, originally expected to be quite heavy, doesn’t appear that it will be be very significant. So confidence in the forecast for Friday continues to be lower than usual.
11 pm –tonight’s GFS has increased the QPF to 0.33 inches water. The NAM has reduced it to 0.07. Large model differences!
