The North Atlantic Oscillation (NAO) is an index that can help us calculate the risk of wintry weather impacting the United States. When there is strong high pressure over Greenland, the NAO is said to be negative, and the jet stream buckles south to send cold air into the East US. The jet stream also bends north along the East Coast to help out the prospects for coastal storms. In positive NAO scenarios, strong low pressure dominates Greenland, leading to a mainly west-to-east jet stream, not too wavy, lowering the risk of significant cold or warm weather intrusions.
Example of positive NAO and negative NAO.