Models are picking up on a lead shortwave that will travel ahead of our main event after Christmas.  Some models are more impressive than others, but with the blocking in place the Euro and GGEM do show some possible outcomes:

The Canadian is clearly more impressive with this feature than the Euro but both show a system moving through Christmas morning with enough cold air around to produce wintry weather.

Euro stays south and weakens it as it moves east and becomes sheared.  This is something we will watch for with the blocking as it could shear this system apart before reaching us, but on the other hand models are catching onto this and becoming more impressed.

Canadian shows a nice swath of 1-4" of snow with this system!

NEW EURO AND GGEM: 3:45PM update

Summary:

Because of the models having a tough time with the blocking and the post Christmas storm, I am not sure what to make of this threat on Christmas just yet.  The models are becoming more impressed with it which is a red flag signaling they are now seeing something.  If it turns out to be something it should stay wintry for the Northern Middle Atlantic and not be rainy like the GFS shows.  The block is too impressive for this to gain that much latitude.  As I mentioned before this could get sheared out completely and be nothing.

We will know more after a few more model runs!