Two days before Christmas 2004, an arctic front pushed a deep layer of cold air across southeast Texas. Late Christmas Eve two upper-level low pressure systems combined to produce a narrow, 20-mile-wide slice of intense snowfall. Among the top snowfall totals:
The NAO or North Atlantic Oscillation is simply a "blocking" pattern that allows for cold air to slow down and allows LOW pressure to form off the coast and has time to explode into snowstorms.
It's basically the traffic cop of weather. Slow the pattern down and the odds of a juiced up system getting into the Eastern Part of the U.S. starts to increase. Negative NAO = More storm chances east of the Mississippi River.