What Do Alaska and Greenland Have to Do With Cold and Snow in the Eastern U.S.?

The pattern consists of an area of high pressure near Greenland and another area of high pressure near Alaska. High pressure in both locations results in an amplified jet stream.

This particular setup with northward bulges of the jet stream near Alaska and again near Greenland results in a blocking of the upper-level pattern and forces the jet stream to dip southward over the eastern and/or central U.S.

This southward dip in the jet stream, or trough, allows arctic cold to surge southward into the U.S. during the winter.

This pattern can be persistent, which means a prolonged period of colder-than-average temperatures in the eastern U.S.

With cold air in place, any disturbances or low pressure systems that develop and track through the central and eastern U.S. can result in snow.

A blocking pattern can take shape when the polar vortex weakens and splits or becomes displaced.

Earlier this month, the polar vortex weakened and allowed this blocking pattern to develop. As a result, below-average temperatures returned to the central and eastern U.S. mid-month.

A negative AO is expected to last into at least early February, so colder-than-average temperatures are expected possibly into February.

Within this upper-level pattern there are variations in the exact location of the jet stream, which determines where the most anomalous cold will be. The orientation of the jet stream and the strength of low pressure systems determine whether the pattern could bring snow to the East.

Often when the Arctic Oscillation is in a negative phase, the North Atlantic Oscillation (NAO) is also negative. A negative NAO typically corresponds with a setup where there is a weakening area of low pressure near Iceland and an area of high pressure near the Azores.

This setup combined with a negative AO increases the chances for bitter cold to push southward into the U.S. and the chance for snow in the East.

In March 2018, the NAO was in a negative phase for several weeks. This produced four nor'easters that brought heavy snow, blizzard conditions and coastal flooding to parts of the mid-Atlantic and Northeast.

There is not a clear signal that the NAO will be in its negative phase now. However, this jet stream pattern is not necessary for arctic air to reach the U.S. or for big East Coast snowstorms.