Not All NAOs Are Created Equal

Even if you have a passing interest in weather you’ve probably heard about the North Atlantic Oscillation. During the winter if you want a big snowstorm you need to watch the NAO. The NAO is technically defined as the difference in pressure between the Icelandic low and Azores high.

500 mb Height Anomaly Composite for a +NAO / Courtesy: NCEP

In general, below normal 500mb height anomalies over Greenland correlate to a positive NAO while above normal heights correlate to a negative NAO.

While a -NAO in the winter typically means colder than normal weather in the northeast and a storm track that’s favorable for snow not all -NAOs are equal. For example the location of strongest ridging in the north Atlantic means the difference between a pattern that favors storms tracking to our west (cutters) or storms that travel near or to our south.

Today’s model runs are a great example of the importance of the NAO ridge’s location. The 12z op Euro and Euro Ensembles show a pretty intriguing pattern. Let’s start with the Euro Ensemble 500 mb mean height forecast for 12z Wednesday 11/28 (216 hour forecast).

12z 500mb Euro Ensemble mean 216 hour forecast

The first thing to look at is off to our northwest. You can see strong ridging from the Aleutians to the Bering Strait which effectively dislodges the cold/polar vortex from the North Pole. Looking chilly! But the storm track would likely set up to our west. The reason why is the NAO is very east based. Strong ridging over Iceland doesn’t do much to help us get a good storm track from snow – we want the ridging over Greenland.

The 12z op Euro is very close to the ensemble mean. So this is a relatively high confidence forecast (for one that’s 200+ hours out).

12z Op Euro 216 Hour Forecast

The op Euro shows an impressive omega block/-NAO over the north Atlantic but don’t expect early season snow with that setup. In fact, you can see the storm tracking through Chicago on the 216 hour op Euro forecast.

I do think that there is some threat for wintry weather during the middle of next week. If we’re able to change the location of the NAO block we may see a storm track a bit farther south and east. Mix? Snow? Ice? Possible.

Regardless of the storm’s track there’s no question that we’re going to get colder as we dislodge the cold from the Pole. The northern tier of the U.S. looks chilly past this weekend for sure.


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