The NAM and GFS model’s 1 AM runs have come together with a QPF of about 0.15 inches water for Philadelphia, or about 2, maybe 3 inches of snow (due to a higher snow:water ratio).
For Philadelphia, the snow starts about daybreak or so and the models have light snow continuing through about 4PM. For 8 hours of snowfall, 2-3 inches total snowfall is very light.
It still looks like a much more significant snowfall for southeastern NJ.
While the current radar trends are consistent with last night’s models, this is still a lower than usual confidence forecast. Any late change in intensification could throw the snow totals off in either direction.
The latest NAM data has become available. QPF values for Philadelphia is 0.28 inches of water or about 4 inches, with cold temperatures and a high snow:water ratio greater than the usual 10:1.
This forecast is below average confidence. This storm will involve late phasing of the northern and southern jet streams. The intensification will be occurring as it approaches and modeling often misses this scenario. The models show a very sharp cutoff north and west.
Sometimes the best forecast is one that emphasizes that just don’t know how much snow. It could be much more or much less.
4 inches for Philadelphia is a best guess. Less north and west, much more south and east.
As I mentioned last night, the NAM model showed a westward extent of the snow for Saturday from the coastal low.
Today’s NAM has continued with that trend, with a rapid decrease in QPF values west and north of PHL. Areas in New Jersey will have significantly more snow.
Currently, the QPF for Philadelphia Airport is 0.25 inches water which translates into 3 inches of snow. Areas such as Blue Bell, have a lower QPF, closer to 0.18 inches water.
The GFS has joined in with that westward extent of the precipitation shield.
The NAM data for tonight’s run becomes available about 9:40 PM and I’ll update after that time.
Additional info: the snow coverage and amounts with Saturday’s storm is unusually low confidence, especially here in Philadelphia. Slight differences in the low pressure’s intensification or track will greatly affect the snowfall amounts.
