El Niño episodes are associated with four prominent changes in the wintertime atmospheric flow across the eastern North Pacific and North America.
The first is an eastward extension and equatorward shift of the East Asian jet stream from the International Date Line to the southwestern United States.
The second is a more west-to-east flow of jet stream winds than normal across the United States.
The third is a southward shift of the storm track from the northern to the southern part of the United States.
The fourth is a southward and eastward shift of the main region of cyclone formation to just west of California.
This shift results in an exceptionally stormy winter and increased precipitation across California and the southern U.S, and less stormy conditions across the northern part of the country.
Also, there is an enhanced flow of marine air into western North America, along with a reduced northerly flow of cold air from Canada to the United States.
These conditions result in a milder than normal winter across the northern states and western Canada.