The North Atlantic Oscillation (NAO) describes the changes in the strength of two pressure patterns in the atmosphere and can have major impacts on winter weather on the East Coast of the United States (US) and Europe. 

Why do I care about the NAO?

The NAO will affect your skiing and riding if you are on the East Coast or in Europe. If you are a skier and rider in the eastern US, you want a negative NAO, which means a higher chance for snow and cold temperatures. This can come from any type of storm but has the greatest impact on lake effect snow or Nor’easters.

If you are a skier and rider in Europe, the phase of the NAO will depend on whether you ski in northern Europe or southern Europe. A positive NAO means more chances of snow in northern Europe, and a negative NAO means more chances of snow in southern Europe. 

What are the phases of the NAO? 

The positive NAO phase means warmer temperatures and drier conditions are more likely for the eastern US. Northern Europe is more likely to have wetter conditions, but these storms can often be warmer. Southern Europe in the positive NAO is more likely to be cooler but also drier. 

The negative phase of the NAO reflects a weakening of the pressure anomalies. The negative NAO phase means cooler temperatures and wetter conditions are more likely for the eastern US. Northern Europe is more likely to be cold and dry, while southern Europe is more likely to be warm and wet. 

A neutral phase of the NAO reflects an equilibrium in conditions and usually does not last long. A neutral phase means the above conditions are a 50/50 shot of happening. 

The historical record of NAO since January 1950 averaged for each month. The red bars indicate a negative NAO, and the blue bars indicate a positive NAO. 

The key word for both positive and negative NAO phases is “more likely or higher chance”. The above conditions per NAO phases do not always happen. They are more likely to happen but that is never guaranteed and should be understood. 

Both phases of the NAO are associated with changes in the intensity and location of the North Atlantic jet stream and storm track. The stronger the phase of NAO, the more likely the above pattern effects will impact the eastern US and Europe weather.  

How is the NAO is forecasted? 

The NAO phase is able to be forecasted several weeks in advance thanks to long-range numerical models. The most common model to predict the NAO is the GFS model but other models such as the ECMWF (Euro) and CFS can predict it as well. 

The models are able to predict the phase of the NAO like they predict the weather in the extended forecast. The difference between NAO predictions is understanding both the pressure system's location and intensity. This adds more complexity to the prediction of the NAO due to the large area that a model needs to simulate accurately. 

It is easier for the model to predict the phase or transition of phases rather than the exact intensity of the NAO. This helps forecasters at OpenSnow understand if a phase of the NAO will interact with storm systems moving toward the East Coast or Europe, hopefully bringing snow. 

GFS Ensemble observation and forecast from March 1st, 2023 to March 15th, 2023. The red lines show several model predictions of the NAO index, and the black dotted line is the median of ensembles.

What is the NAO?

The NAO describes the changes in the strength of two pressure patterns in the atmosphere over the North Atlantic Ocean.  The pressure pattern is centered on a low pressure near Iceland and on a high pressure near the Azores Islands off of Portugal. These pressure patterns or oscillations affect winter weather along the East Coast of the US and Europe. 

There is a neutral, positive, or negative phase of the NAO, which is defined by the intensity of the low and high pressure systems, highlighted below. An NAO index is assigned based on the intensity of the phase. When the NAO is in a positive or negative phase, there are statistically likely impacts on East Coast and European winter weather. 

The average surface pressure locations and jet stream track per positive or negative phase of the NAO. The low (L), means lower pressures (storms), and the high (H), means higher pressures (dry). During a positive phase (right), the difference becomes even stronger than usual. The variation in pressure patterns influences the strength and location of the jet stream and the path of storms across the North Atlantic.  Schematic adapted from AIRMAP by Ned Gardiner and David Herring, NOAA.

How is the NAO different from ENSO?

The NAO is different than other climate oscillations such as ENSO. The NAO is strictly atmospheric pressure oriented while ENSO is based on sea-surface temperatures. NAO affects weather in the eastern US and Europe while ENSO can affect weather around the world. The impact of pressure differences (NAO) is much smaller rather than ocean temperature effects via ENSO. 

How often does the NAO change? 

The NAO is one of the fastest-changing oscillations and will usually switch between negative and positive phases every 2-5 weeks. There have been cases where a phase can last several months and will be dominant throughout the year. The NAO phase has impacts on weather year-round, but we most often care about the impacts on winter weather.  

Keep tabs on the Mid-Atlantic, New England, and Europe Daily Snow for mentions of the NAO and how it could lead to snow and cold temperatures. As always, the key to any good ski trip is planning several days in advance to make sure you and the forecaster know where the snow will fall.

Zach Butler, OpenSnow Meteorologist