AO/NAO Take a Sharp Dive to Start March

Both the NAO and AO are forecast to drop to their lowest levels yet this winter. The NAO has remained above zero since the beginning of December while the AO has fluctuated back and forth between slightly negative and slightly positive.  

NAO observed values (black) and forecast values (red). 

AO observed values (black) and forecast values (red).

As you can see in the graphs above, there is good consensus that both values drop sharply during the end of February to around the 1st of March. So what does this mean? In order to know this, you must first understand how the AO and NAO indices are calculated.

AO (Arctic Oscillation) Index

This is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is lower than normal in the polar region and higher than normal in the mid-latitudes. This causes westerlies to increase over Canada and keeps the cold air farther north.

Cold air is bottled up north when the Arctic Oscillation is in a positive phase.

When the AO is negative, surface pressure is higher than normal in the polar region and lower than normal in the mid-latitudes. This means the westerlies are not as strong since the pressure gradient force is weaker, and the cold air is able to move southward into the mid-latitudes. As the AO becomes sharply negative over the next week or so this will allow for shots of colder air to intrude upon eastern parts of the country. So even though many areas have been seeing spring-like weather recently, winter is not over. 

NAO (North American Oscillation) Index

The NAO is closely related to the AO. Essentially, the NAO index is based off the difference of the atmospheric pressure at sea level between the Icelandic low and the Azores high. The strengths and location of the the Icelandic low and the Azores high vary from year to year and this variation is known as the NAO.

Comparison of the negative and positive phases of the NAO. Source.

Much like the AO, if the pressure gradient between the Icelandic low and Azores high is strong, then the corresponding westerlies are strong. This is known as a positive NAO. Since the Azores high is stronger (and typically farther west), more of a southwesterly flow is present across the northeastern United States, meaning warmer than normal temperatures. When the high is weaker (and farther east), the Icelandic low tends to influence the northeastern US more than the high. This means more of a northerly component to the wind, thus allowing the colder air to intrude farther south.

So with the NAO expected to rapidly drop at the end of the month, this will likely mean another shot at winter for parts of the Great Lakes/Northeast US from late February into the first part of March. This doesn't mean there will be a big snowstorm, but at the very least cold temperatures should return for this time period.