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):

Current upper level pattern:  Positive NAO index indicative of fast southwest to northeast flow.

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.

500 mb chart for February 13th: Neutral to Negative NAO indicative of Blocking over the North Atlantic