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.