The 1AM run of the NAM further cut back the QPF for this morning and this afternoon,  The diminution in QPF during the ‘snow phase’ of this event has been the trend.   The High Resolution Rapid Refresh Model (which is based on the NAM) shows total snow totals between 1-2 inches before the the change to sleet and possible freezing rain between 5-7 PM.

With the exception of light flurries this morning, the light snow now starts about 1 PM.

As mentioned, the snow accumulation will not be the issue. The main issue is the possibility of period of icy conditions, as the precip changes to sleet and then briefly  freezing rain, around 5 PM in Philadelphia and later in the evening north and west of the city.

5:45 PM Update:  The transition to freezing rain has occurred.   Temperatures at PHL airport are, not surprisingly, a bit lower than predicted by the models.  The models almost always predict surface warming faster than actually occurs.  So the issue with freezing rain continues into the early evening.  Temperatures should be above freezing by midnight in the immediate PHL area.

The timing of the transition also contributes to the potential icy conditions, because the transition will be occurring at the time of sunset, so there will be little contribution from solar insolation through the clouds to warm ground temperatures.

Temperatures warm above to above freezing by midnight, when heavier precip in the form of rain is expected.

The latest NAM data just became available.   It shows a QPF of 0.06 before 1 pm and a QPF of 0.20 water between 1 pm and 7 pm.   Temperatures at mid levels of the atmosphere rise above freezing in the late afternoon, changing the snow to sleet.  So all of this QPF won’t fall as snow.

Based on this data and the latest HRRR and GFS LAMPS  forecast, it looks light snow flurries in the mid morning, then light to moderate snow early afternoon.  Approximately 2 maybe 3 inches of snow before mixing with sleet and then rain early evening. The NAM data still suggests a period of sleet and freezing rain in PHL late afternoon and evening with an extended period of the same north and west of the city.  A full transition to rain by midnight as temperatures rise.

Again, the major impact will not be snow accumulation, but rather the potential for icy conditions during the late afternoon and early evening, especially areas north and west of the city.