I agree that the block will initially shred some stuff as it strengthens... but the window really opens up for a major event as it decays
the pattern evolution for 18"+ events for NYC, as well as parts of SNE by extension, shows this quite well:
if the pattern depicted by the EPS / GEPS is correct, which I believe it is to a large extent, then our window for a major storm would be in the Dec 15-25 timeframe as the block matures and decays over central Canada