In order for the NAO to promote cold, there needs to be cold air around....-NAO is not a source. Its like a lid on the cap, so to speak...it prevents the cold from being eradicating and retards its departure once in place. It does help to bleed cold air in from the north, but primarily due to the AO component. A +EPO-/-NAO will keep the NE in the game for snow because you can get some conducive tracks, but its not a frigid pattern.
Bottom line is that if you want cold, keep the EPO negative.