Last night at 11:45, when the GFS data came in, I lowered the expected snow accumulation, based on a blend of the NAM and GFS QPF values. Current thinking is about 4-7 inches in Philadelphia and immediate surrounding areas. I may be under under-estimating the snowfall, but this approach has worked in the past.
The 2 am (06 utc) runs of both models were consistent with the previous runs — NAM QPF is about 0.85 water falling as snow while the GFS is in the 0.65 range . Taking into account March sun angle and a daytime event, I’m staying with 4-7 inches, with a sense that it might be closer to 7 than 4, but I’m not sure. Grassy surfaces is where this will be measured, it will be less on roadways.
One change- the NAM is colder now, so precipitation type “PTYPE”, will likely be all snow by 8 am, earlier north and west.
The storm appears less phased and less explosive and the models show it to be more sheared to the northeast; that’s the reason for the changes. Another change- something called PWAT, or precipitable water values, are not as high as I would think for a big storm.
Let’s see what happens.