Philly Winter Weather Update

Latest NAM data just rolling out now.  Surface temperatures below freezing by 7 am (upper temperatures still a bit warm).

QPF values for the rest of storm increased to 0.60 inches water.  So we’re back to 8 -10 inches snow possible, according to the NAM.

The HRRR model has 2-4 inches of snow by daybreak.  Seems high, but if correct, final totals could exceed 10 inches in PHL.

Curious about the GFS model which tends to run lower QPF values. Data available about 10:40pm

Get that snowblower out of cold storage.


Philly Winter Weather Update

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.

Weather… and other things "up in the air"