We constantly talk about greenhouse gases and their influence on the world's climate. There are also several large-scale global atmospheric circulation patterns which influence short-term climate across large regions on the planet, and one of those patterns in the North Atlantic Oscillation or NAO.

The NAO is an upper level pressure anomaly pattern with one center over Greenland and the other center of opposite sign covering the central latitudes of the North Atlantic between 35 and 40 degrees north.

The positive phase of the NAO occurs when there is below-normal upper-level atmospheric pressures (heights) over the high latitudes of the North Atlantic (around Greenland), while above-normal upper-level pressures are present over the central North Atlantic, eastern U.S. and western Europe.

The negative phase of the NAO is the opposite pressure anomaly pattern of the positive phase.

The positive NAO is generally responsible for the following conditions........

--Above-normal temperatures in the eastern U.S. and northern Europe.
--Below-normal temperatures across Greenland, northeastern Canada, southern Europe and the Middle East.
--Above-normal precipitation in northern Europe and Scandinavia during the winter.
--Drier-than-normal conditions over south central Europe.

The negative NAO is usually associated with...........

--Colder than normal weather across the Midwestern U.S, Manitoba, Ontario, the Southeast U.S. and the interior Northeast.
--Above- normal temperatures over Greenland and northeastern Canada.
--Colder and drier conditions over a large part of Europe.
--Above normal precipitation and snowfall over the Northeast U.S., especially New England and then up into the coastal Maritimes.

The impact of the NAO on summertime weather is less pronounced and a little more variable. The strength of a particular NAO phase is another important factor to consider.

A particular phase of the NAO can last for several months, and even a few years, but the length is highly variable.

As of late, the NAO has been either neutral or slightly negative since mid-March. The latest ensemble forecast below predicts the NAO to remain negative through the third week of May before trending back to normal toward the end of the month.