Well, I said if it were January, I’d be predicting 8-10 inches. The March sun angle did little to reduce accumulations at this high precipitation rate.
So I may have underestimated totals.
In the far northwest suburbs, considerable snow accumulation has occurred. Nearer Philadelphia and adjacent NJ, there’s been quite a range, significantly lower in some cases.
Current radar is impressive, but upper air analysis shows dryer air moving in above 22,000 feet (in brown)
A few more inches of snow possible north and west, but the high precipitation rates should start diminishing around Philadelphia soon, as the dry air rotates in. Some snow still likely until after midnight.
The latest GFS has become available and QPF values are now similar to the NAM – about 0.85 inches water.
I’m inclined to stay with my earlier forecast—
With March sun angle, I’m still going with 4-7 inches of snow, measured on grassy surfaces, with higher part of that range more likely. (This means 7 inches.) This is for Philadelphia and immediate suburbs.
An additional 2 inches could be added to that range (meaning 6-9 in ) for far northwest suburbs.
This morning’s NAM data just coming in. QPF still high at 0.85 in water. If this were January, I’d be forecasting 8-10 inches with this thermal profile.
BUT With March sun angle, I’m still going with 4-7 inches of snow, measured on grassy surfaces, with higher part of that range more likely. This is for Philadelphia and immediate suburbs. I’m also assuming that the GFS (not available yet) will have a lower QPF as it almost always has.
Snow continues into the evening. Highest accumulation rate during the afternoon, according to the NAM
10:40 am update- NAM FOUS data available. The winds will not be as high as originally thought. 18mph plus higher gusts. A good thing. Areas far northwest may have snowfall 2 inches higher than the above range.
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.