Temperatures hover just above freezing, but precipitation rates have allowed accumulations to occur and the changeover occurred about 4 AM.
The HRRR (High Resolution Rapid Refresh) model shows about 2-3 inches total snow in the immediate PHL area with the snow ending in PHL about 9, maybe 10 AM or so, from west to east. (Areas in central NJ may have more accumulation.).
Latest NAM FOUS data just became available.
Precipitation starts at midnight as rain, gradually mixes with and changes to snow towards daybreak. Snow ends between 7-9 AM. Total QPF is now 0.46 inches water. Most of this will fall as rain or rain mixed with snow, becoming all snow by 5-7 AM. Still impossible to predict accumulations, because of warm ground temperatures, melting due to mixed precipitation and varying amounts of cold air at the surface. (Temps expected to fall to 32 by 7 AM). More QPF at the coast, but warmer temps. Precipitation drops off far north and west.
While impossible to predict accumulations with any confidence, if you’re looking to hang your hat on a number, I’d say between 0.5 to 2 inches, especially on grassy surfaces outside of Center City Philadelphia and near suburbs. This is a low confidence forecast.
Tomorrow’s precipitation is a perfect example of how poorly the models have been handling the extended range forecasts (more than 4-5 days out) this winter. Just 3 days ago, this precipitation had been expected to stay to our east, just brushing the coastline. Things clearly have changed.
With short range forecasts so changeable, the “long range” period- Monday through next Wednesday- presents a large forecasting challenge, as there are several potential coastal low pressure systems forecast to develop, with low confidence in each scenario.
A quick update. This morning’s NAM cranks out 0.46 water as the QPF late tonight and joins the NAVGEM with the western extent of the precipitation well west of PHL.
However, this will fall as a mix of rain changing to wet snow that may only stick to grassy surfaces in the city, maybe some accumulation north and west. This set up will be impossible to predict snow accumulations. Too many variables and too many unknowns.
Will update tonight.
Thursday 7 PM: This afternoon’s NAM increased the QPF to 0.60 inches water(!). It’s a good thing it won’t all fall as snow. The NAM also increased the cold flow, so some accumulation may occur, even in Philadelphia. But how much falls as rain, a mix, and wet snow is still impossible to calculate. Add to the uncertainty the ground temperature (which has warmed over the past several days) and we only know that some snow will fall, but accumulations will only be a guess. The NAM consistently ends the snow by 7 AM.
I’ll update when the next NAM data comes available about 9:50 PM