Here’s the very latest weather model data about the Philly snowstorm. The morning’s NAM data just became available. It shows a QPF of 1.35 inches water falling as snow between 7AM and 7PM.
Here’s the latest NAM 10:1 snow totals map. (Important- this map shows additional snow totals that need to be added to any snow that has already accumulated by 7 AM this morning.)
NAM (12Z model run) snow accumulation based on 10:1 ratio)
Temperatures are expected to fall to 32-33 as precipitation rates increase during the late morning and early afternoon. Expect heavy snow at times. Temperatures may rise at the end of the storm to 34!
In my neck of the woods, we’re already seeing the difficulties of predicting snow totals for a March snowstorm. Some surfaces and sidewalks have little accumulation, while our wood deck has about 2 inches right now.
[su_note note_color=”#ebf2d9″]With these issues with March snowstorms, we’re always predicting potential accumulations. Melting due to warm ground surfaces, radiant heat from solar insolation through clouds and snow compaction due to wetness can greatly affect final totals. Much depends on dynamic cooling to bring surface temperatures down.
Put another way, the precipitation rate is going to need to pick up considerably soon to meet the QPF values and the snow totals predicted by the NAM. [/su_note]
I tore myself away at work to look at the 1PM NAM model data that just became available. QPF values are back to 1.30 inches and higher north and west. Thermal profiles are COLDER! It looks like mostly snow even in Philadelphia! It does not look like rain will mix in after it starts.
Sorry for the vacillation on forecast amounts but I think we’re back to 8-10 inches in Philadelphia and 10-18 north and west of the city!
Weather forecasting is a tough business; glad I do other things when I’m not forecasting. 🙂
I’ll update with a final forecast about 9:45 PM or so.
I’ve had some unexpected free time this morning and was able to look at the GFS and NAM models.
QPF values for PHL have reduced to 0.85 GFS and 1.00 NAM. Higher QPF amounts west and north of Philadelphia. Thermal profiles are a bit warm around daybreak around the city, allowing a mix in with rain, reducing snow totals in the city and south/east.
Wet snow tonight may mix with rain around daybreak. Snow mixed with rain in the city until late morning, when it all changes to snow. In the city and areas near I-95, mix with rain will reduce accusations to about 4-6 inches, mostly accumulating late morning and in the afternoon.
Just outside of the city where it should stay as snow (Valley Forge, Main Line and north and west) 10-16 inches.
The real data you can hang your hat on will be available about 9:45 PM. I’ll update a little afterwards
