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.
This morning’s NAM data rolling in. The NAM is beginning to maintain a dry slot area over PHL for this snowfall with the same showing last night. QPF values have reduced to 0.09 inches for PHL with higher amounts far north and far west.
The snow is minimal before midnight, then the ‘bulk’ of the light snow falls. With temperatures at or above freezing and a similar QPF as the GFS model from last night, I’m revising the accumulations downward to a coating to 1 inch in Philadelphia and the immediate suburbs, starting late evening and ending early morning Wednesday.
After waking up this morning to very light snow, the question that begs is “what gives?” Last night’s 1 AM run of the NAM shows this snow this morning and suggests that very light snow may continue through the day around Philadelphia.
BUT, the GFS is in a different universe and has no snow here until later Tuesday night. The QPF values also differ greatly, with the NAM maintaining a total QPF of about 0.25 for the event, while the GFS is in the 0.09 territory. So big differences in timing and QPF values.
To add to the uncertainty, the NAM shows a tongue of warm air at the surface moving into Philadelphia and just south later in the day and the evening, keeping accumulations low despite the higher QPF.
Since the GFS is already wrong, we’ll hang our hats on the NAM.
So very light intermittent snow possible during the day, increasing in intensity this evening, but surface temperature increases in the immediate Philadelphia area may significantly reduce accumulations. Still a 1-3 inch snow best guess in Philadelphia, with a lean towards the lower range. North and west of the city may have 3-4 inches. This is a lower than usual confidence forecast.
The other change noted is a reversal from yesterday- the snow essentially ends earlier Wednesday morning.
Too busy at work to give detailed updates during the day today, but I’ll send out a quick update if I get some time.