The model is showing quite a bit of snow for Saturday morning.  The RPM model has a bad record this far out, but it did do well with last system's surprise snow in NE PA all the way down into Lancaster and VA.  With more cold air expected to be in place and the lack of a strong WAA signal makes me question a few things with the other models. 

The lack of WAA signal is in regards to the beginning of the event.  As we go into the early PM hours the 850mb low will close off well west of the sfc low resulting in plenty of warming in the lower levels and sfc.

The reason why the 850mb low forms to the west in this situation is a little complex:

1) The boundary will have an ample amount of convection on it from 6z Saturday until 18z Saturday....this much latent heat being released along the boundary will allow the LLJ to accelerate to speeds from 35-40Kts at 6z Saturday to 60-70Kts by 18z Sat.  This allows very little in the way of thermal advection from west to east along this boundary hence resulting in the low level low forming to our west.

2) Is this possible?  Yes it is very common to see the 850mb low take away our snow in Philly.  When a low is developing and strengthening the low is not vertically stacked (aka not occluded or not on top of one another in the atmosphere).

3) What can make this low push east and give us more snow than modeled like the RPM?  Well this is where it gets interesting.  In a situation like this models can get caught off guard from underestimating the low level cold air.  If the boundary does not extend all the way up into because the cold holds on longer it will allow the orientation of the convection will not be as north to south and force a farther south development of the 850mb low that pushes more east.  This is something to watch as this will probably happen to some extent, but enough for Philly?  Theme of the winter makes this a no, but others in E PA could be on a more wintry side of this storm than some models suggest.  

Back to the point of Saturday Morning...NAM has this nut very little precip showing up:

This would suggst more in the way of precip like the RPM suggest while it is colder and supportive of more wintry weather.

The NAM does show a lot of sinking air in response to the convection which could be correct...or an error. This is something that will need to be nowcasted as we head into Friday.