What I will be tracking through the overnight hours is the intensity of the s/w seen over Canada extending down into the Northern Plains.  The deeper this s/w can get the more precip we will see stream northward.  The circle shows where precip is expected to develop in the beginning stages around 4AM.  The intensity and timing of the development of this precip will give us clues on how the event will unfold downstream.  The last thing is the cut off low off CA where it could aid in pumping up heights depending on strength and position. 

This is the sounding for Philadelphia when the precip is moving in.  One thing that stands out is the very cold temperatures throughout the profile with no threat of mixed precipitation.  The sfc temperatures are very cold below -5C with a very deep dendritic growth zone.  The reason I went slightly higher is because of this deep snow growth zone because it extends over 300mb! Notice as it falls from the cloud it moves through avery small layer of above -10C.  This means the dendrites will have very little interference with supercooled water droplets.  

This means we can expect big powdery flakes.  The ratios will exceed 20:1 in some locations given the favorable thermodynamic profile and the snow growth layer extending so close to the ground.  The main trigger for the precipitation will coincide with the lower levels as the WAA/isentropic lift move in:

Models and water vapor analysis have been showing the *potential* for a better looking coastal coming closer to the coast keeping in heavier precip longer.  Eventhough the latest data suggest this won't come close enough to be significant but something to nowcast.  The WV imagery looks more promising now with the s/w appearing better defined than models show, but differences are very minor right now.  

The snow map:  Will adjust if needed tomorrow morning!