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.
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
Latest GFS model data- during thermal conditions supporting snow, a QPF of about 0.40 inches water on Thursday. So if we average the NAM and GFS, we’re looking at about 0.50 or about 6- 8 inches of snow. Expect changes
So the rain today came as expected. I underestimated the the slower rise in temperatures and the ice accumulation . Temperatures are moving above freezing and will stay in the upper 30s and low 40s through much of Wednesday.
A cold front moves through Wednesday evening and temperatures will drop at the surface and aloft below freezing after midnight into Thursday morning . As advertized, low pressure will develop along this frontal boundary as it stalls out to our south. Tonight’s NAM shows a change from rain to sleet, then snow after midnight and towards morning, continuing through the day Thursday.
QPF values are over 0.60, suggesting 10 or more inches of snow possible by late Thursday afternoon or evening. This is a dynamic situation and further changes in the forecast are likely.
Tonight’s GFS data becomes available between 10:30 and 11pm