My previous post included the link for the official snow totals. I always like to evaluate my forecast to improve on future forecasts.
For the immediate PHL area, snowfall was between 7-11 inches and areas somewhat further north and west, totals approached 15 inches.
With my call of 4-7 inches with an emphasis on “7 most likely”, my forecast fell short. So what happened?
Official QPF measured at the airport yesterday was 1.06 water, somewhat higher than the 0.87 predicted by the NAM and higher than the GFS. But overall the models did well on QPF. (The actual snow total for just Wednesday was 6.7 inches at PHL airport, according to the NWS.)
My emphasis on solar insolation through clouds lead my forecast astray. Had it been January, I would have predicted 8-10, which I mentioned in several posts. So I learned yesterday that heavy snowfall rates trump solar effects, even in late March.
While the models did well on QPF, they did less well on wind. That was lucky for our region. Temperatures were predicted well.
Mesoscale banding set up northwest of the city and snowfall was heavier in those areas.
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.