Well that's what I was about to say, lol. The 1,000,000 dollar question. I'm not sure exactly how to interpret it. Jack Sillin (I think a met grad student at Cornell) seemed to think it might be tied to the NAM being slower and more cut off, but the trend gif he is showing, also seems to be showing it trending south. I think we'd want to look for more confluence over New England (blue dots indicating 500mb heights are lower there) and the shortwave's trough being deeper than forecast (also blue dots).
That website also verifies 700 and 850 mb temps, as compared to how the GFS initializes.