Things are progressing nicely as the radar from the SouthEast is blossoming. As this point, there is little doubt that it will snow late in the day on Tuesday into Wednesday morning. The big question is just how much snow will fall. When it gets within the magic 48 hour window of the start time of a storm, the two “go to” weather computer models are the NAM (North American Mesoscale Model) and the GFS (Global Forecast System). One would think that with the technological advances and increased meteorological knowledge, it should be easy to provide an accurate weather forecast certainly within two days of an upcoming storm. Unfortunately that is not always the case (i.e. the snowstorm that recently hit on 12/26 and 12/27).
Currently, the NAM and GFS are differing on their respective outcomes for the upcoming storm. The NAM is showing a much stronger closed upper level low (which pulls the surface low pressure closer to the coast and ultimately provides much higher snowfall totals for the area) as opposed to the weaker (and farther East solution) currently being shown by the GFS.
As previously mentioned, the key to the final outcome of this storm will be the interaction between the northern energy and the southern energy and exactly when and where these two pieces of energy phase. A quicker, more southern phase, would provide greater snowfall totals (as per the latest NAM run). Conversely, a slower, more northern phase, would provide lesser snowfall totals (as per the latest GFS run).
As far as model biases, the NAM does have a tendency to overdue the QPF (Quantitative Precipitation Forecast), also know as the total precipitation forecast. The GFS does have a tendency to underdue the QPF totals. With this said, it is probably safe to provide an initial snowfall forecast based on a compromise between the two models. The NAM is showing about 1 inch of QPF while the GFS is showing about 0.4 inches of QPF. Based on a standard 10:1 snowfall to liquid ratio, the NAM is currently forecasting 8 – 12 inches of snow, while the GFS is currently forecasting 3 – 6 inches of snow.
As of late Sunday night, 5 – 10 inches of snow for Philadelphia seems to be good “first” forecast.
Below are the surface depictions of the coastal lower pressure system depicted by both the NAM and GFS solutions (the images are showing the storm’s placement, intensity and QPF at 7 AM Wednesday morning):
NAM : NAM Model
GFS: GFS Model