North Atlantic Oscillation (NAO)

The NAO is an index based on the pressure difference between the subtropical Azores high and the subpolar low. The polar pressure is taken near Iceland and the subtropical pressure is taken near the Azores.

Positive NAO: A positive value leads to more intense subpolar lows and a greater frequency of subpolar lows that track from the U.S. toward Europe. The tracks of the storm systems tend to be more northerly. The Eastern U.S. has relatively mild and moist conditions in winter. Europe tends to have more storm systems and warmer weather in winter. The subtropical high tends to be stronger and the hurricane season tends to be more active.

Negative NAO: A negative value leads to less intense subpolar lows and a slackened frequency of subpolar lows that track from the U.S. toward Europe. The Eastern U.S. tends to have colder and snowier winters. Europe tends to be colder in the northern portions and wetter in the southern portions. There tends to be more blocking due to a stronger Atlantic ridge.

The first half of December has been dominated be a negative NAO which helped to generate a blocking pattern for much of the Americas. The Upper air pattern was stuck in a long wave troughing pattern for the eastern half on the United States! This helped to keep the eastern states colder than average. The pattern was also responsible for developing fast moving low pressure systems called Alberta Clippers. Clippers typically move southeastward and because of their position, forming in Alberta, and speed they have little moisture to work with. What snow does fall is usually lake affect snow.

The long wave troughing was also causing the subtropical jet and the polar front jet to merge together for much of the United States. A Rex block type pattern over the eastern pacific was keeping a weak low off the southern coast of cailforna for a couple days before allowing it to move freely. This low then moved eastward as a shortwave trough splitting the two jets. Once this low moved within reach of gulf moisture it caused a few good rounds of severe weather.

NAO in its negative phase!