So with all the talk about the positive, neutral and negative phases of the North Atlantic Oscillation (NAO index), I thought I would pull up the 500 mb chart (colorful images) to show why it’s pretty unlikely to get crippling blizzards (20″ +) in the DC Area when the NAO index is in its positive phase.
Positive NAO (Progressive Jet Stream pulls storms that form right out to sea):
Neutral to Negative Index of the North Atlantic Oscillation (NAO): High pressure (Blocking) in the upper levels of the atmosphere (jet stream level/500mb) will prevent storms that do form off of the Carolina coastline from moving quickly out to sea.
DC Storms 
I like that forward tilt trough! But then there are still the questions rain vs. snow, etc.