Philadelphia Area Weather Forecasts

Philly Winter Weather Update

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The latest models continue the trend towards somewhat lower QPF values during the period when thermal profiles support snow.  Latest NAM is about 0.40 inches water for Philadelphia during the period from 7 AM to 7PM.   This afternoon’s GFS had very similar amounts. This translates into about 4-7 inches of snow.

Most of the heavier precipitation will occur with the actual frontal passage, however temperatures at the surface and aloft will not support snow during that earlier time frame, so much of the heavier precipitation will be sleet, then wet snow.

Still many factors that make this sort of precipitation event difficult to nail down accurately. It’s not really a “storm” or defined low pressure system in the usual sense.  Instead it’s moisture spilling over a stalled frontal boundary.  Aside from all the temperature unknowns, I don’t think the models are all that good at predicting the QPF in this situation.   However, they have been somewhat consistent about the timing and other aspects. I guess we’ll see.

Roadways may not accumulate as much snow as grassy surfaces during the day tomorrow.  Sleet and ice will comprise the bottom layer of this snowfall, making cleanup a bit of challenge later Thursday.

Tonight’s  NAM data will be available about 9:15 PM and the GFS data about 10:40 PM.  So I’ll update about 10: 50 PM.

Winter Storm Outlook

Philly Winter Weather Update

The 1 AM model runs continue with these trends.  The NAM, as it always does, has more QPF as snow than the GFS.  The GFS is looking  relatively meager, with less than 0.40 inches water falling as snow.  The NAM is a bit more robust with 0..55  falling as snow.   The snow ends mid to late afternoon Thursday.

(There is more QPF prior to 7AM Thursday, but some of the thermal parameters don’t support much snow in PHL, just sleet.)

So the trend right now is for maybe less QPF, less snowfall.   The temperatures are also looking a bit lower, suggesting the front may stall a bit futher south, putting us in the northern edge area of the precip max.  The advertised 4-8 inches still sounds quite reasonable, but this will be tough to get right  for a specific location and in the range of locations in the Philly suburbs.

Here are the unknown parameters that make this forecast complex:

Rate of frontal movement after passing through Philly.  Slower-> more QPF, not as cold initially, more sleet, less snow.
Final stalled  frontal position, Further south –> colder  –> greater QPF:snowfall ratio
Final stalled frontal position, Further south-> less QPF
Final stalled frontal positiion, Further north –> warmer, smaller QPF:snowfall ratio, more QPF

I’ll update early this evening.

 

Weather… and Other Things 'Up in the Air'