Negative NAO & AO: What does it mean for our weather?

It's not just the weather around us that drives our temperatures. We have to look at patterns at large scale or global level to fully understand our weather. Two of the products that we look at to assist us are the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) forecasts put out by the Climate Prediction Center (CPC) in the National Weather Service.

We'll first take a look at the North Atlantic Oscillation. The NAO is located in the North Atlantic Ocean near Greenland and we usually look at how it impacts our weather during the winter months.
The black line on this chart shows us observed data for the NAO. Currently, it is negative and has been that way for nearly a week. What this tells us is that we are in a blocking pattern thanks to an area of high pressure sitting near Greenland that isn't moving.  This causes a trough to form over the eastern half of the United States that can't move anywhere thanks to the blocking from the high pressure near Greenland.

For us it means cold temperatures, as this trough will pull in cold air from Canada. What's real impressive about this cold air sitting over the area is that it originated from around 60°N. To put that in perspective, the Artic Circle is located at 66°N. That is some really cold air.

Looking back at the NAO map, we'll look at the forecasts. These are the red lines on the right side of the chart, with each individual line being a different model run trying to project what the NAO will do. When you see all of the lines bunched together, we have a very high confidence in the forecast. Right now it looks like this blocking pattern should hang around for a few more days at least, keeping the cold air in our area. As we look farther out into the future, there is a lot of uncertainty whether we are going to warm up a little or stay cold. That comes from the wider spread in each model run.

When we look at the forecast put out by the CPC for the Arctic Oscillation, we apply basically the same concepts to understand it.

The big difference is the loaction of the AO, which originates just south of the Arctic Circle in Central Canada. The observed data for the AO is really negative which again will mean an area of high pressure. The difference this time is that it is helping to drive the trough over us to the south, which is sending that cold air right to us. The models do seem to agree that we will go less negative, and that the high might try to move off. This could help our temperatures warm up a little. As in the NAO though, there is some uncertainty when we get a few days out.

-Eddie Wildermuth
 WTVO Weather Intern