This afternoon’s GFS and NAM have maintained a low QPF of less than 0.10 inches water in the immediate PHL area and south. Under ideal conditions, we would have about an inch or so of snow. HOWEVER, the surface temperatures have been above freezing today and won’t drop below 32 until 4 AM. While the heaviest QPF is expected between 4 and 7 AM, the warm surface temps will likely cut into accumulations.
So…still going with about 1 inch of snow in the immediate PHL area an adjacent suburbs. Area north and west (Doylestown, Allentown) may have closer to 3 inches. Again, a difficult forecast, since warmer surface temperatures will be reducing accumulations.
Whatever falls, there is agreement that temperatures drop below freezing for the morning rush hour. So icy conditions will be an issue.
Tonight’s NAM data first becomes available about 9 PM or so. I’ll update then.
This morning’s GFS model continues with the trend of low QPF of 0.09 inches water, similar to the NAM. This translates into about an inch of snow, assuming colder temperatures. But surface temperatures may be at or just above freezing for some of the time, reducing accumulations even further. So a coating to 1 inch of wet snow is still the best guess for PHL and surrounding suburbs, more north and west.
Still a lower than usual confidence forecast, since the higher resolution NAM shows greater QPF values and some of the other models suggest the same.
Whatever falls likely causes icy conditions, as temperatures are expected to drop early Wednesday morning.
The latest NAM model data has come in. Here are the trends—
The snow starts later in PHL, closer to midnight Tuesday. QPF values are increasing, now 0.33 inches water, somewhat more north and west of the city. The snow continues into Wednesday morning. This translates into 3-4 inches.
So weak frontal passage seems to be evolving into more than a nuisance snowfall. Expect further changes.
11 pm update: The GFS has less QPF in the immediate PHL area, more north and west. For now, we’ll average the two models. 2-4 inches of snow is a best guess for now.
How things change! A frontal passage with limited moisture and little development now appears to be somewhat more substantial. QPF values for the NAM and GFS have increased closer to 0.20 inches water for PHL with greater amounts far NW of Philadelphia.
The latest models are beginning to suggest that a low pressure system might form off the coast, close enough to enhance the snowfall amounts Tuesday afternoon thru Tuesday night. There’s been a wide range of model solutions with this frontal passage — most have either not had any development, or they had development far off the coast. The Navy NAVGEM and the Canadian model were showing more precipitation yesterday, but I discounted both models, since the GFS and NAM were showing no development.
The experimental statistical models have consistently shown a low chance of snow through the Tuesday-Wednesday time period.
So the minor snow for Tuesday into Wednesday is beginning to appear that it might be more substantial. Looking like 2-3 inches now, but too early to be that precise. Stay tuned!
7 PM Monday-Further Complications to the forecast. This afternoon’s models show surface temperatures at or above freezing during the evening hours of Tuesday, not going below freezing until after midnight. Some initial snow will melt and not add to accumulations in the immediate PHL area.