The primary reason a -AO is number one when it comes to DC snow is it means the PV isn't a super wound up ball of doom, with all the cold locked up in the high latitudes. When there is a real deal -AO it reverses the "natural order", and allows anomalous cold to move into the midlatitudes with relative ease. As you said, we still want some NA blocking as it greatly increases chances for a favorable storm track underneath, and it is pretty much a requirement for an HECS as it inhibits a coastal storm from flying up the coast. Ofc 2009-2010 was the classic example of a negative AO/NAO working the magic.