The latest NAM model data has come in. Here are the trends—

The snow starts later in PHL, closer to midnight Tuesday.  QPF values are increasing, now 0.33 inches water, somewhat more north and west of the city. The snow continues into Wednesday morning. This translates into 3-4 inches.

So weak frontal passage seems to be evolving into more than a nuisance snowfall.  Expect further changes.

11 pm update:  The GFS has less QPF in the immediate PHL area, more north and west. For now, we’ll average the two models.  2-4  inches of snow is a best guess for now.