East Based v. West Based Negative NAO

The North Atlantic Oscillation (NAO) has two phases: Positive and Negative. It is known that in the winter, when the NAO is negative, the East US is at a favorable position for receiving snow and cold. But, a lesser-known fact is that the NAO has two 'sub-categories', if you will. The location of the NAO is key.

This image shows a typical negative NAO in the 'East-based' phase. When it says east-based, what it means is that the strongest part of the high pressure system is to the east of Greenland, hence east-based. In an east-based NAO, the Midwest and Ohio Valley, the jet stream shifts to focus the cold air in the Plains and Midwest, with the storm track giving the Midwest, Ohio Valley and similar areas the best storms. 

The counterpart of the east based NAO is the 'West Based' NAO. As you may imagine, west based means that the strongest part of the high pressure system is now to the west of Greenland, as annotated for above. In the west based negative NAO, one finds the cold and snow now focused in not only over the Plains and Midwest, but now over practically all of the East US. The storm track is now favoring the East Coast, and keeping areas to the west drier.

Images from http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html