WHAT'S CAUSING THE COLD?
New York’s Central Park Sets Snowfall Record

March 14, 2005 � Since late February the mid-Atlantic and Northeast have been plagued with wind chills that have sent many back in the house for their hat and gloves. On the minds of many in the East, "Where is it all coming from?" Scientists at the NOAA Climate Prediction Center have isolated the negative phase of the North Atlantic Oscillation, or NAO, as a major contributing factor to this late-season winter weather. (Click NOAA satellite image for larger view of latest negative phase of the North Atlantic Oscillation, or NAO, as of March 14,2005. Click here to view latest image. Please credit “NOAA.”)

The North Atlantic jet stream patterns have a major influence on winter weather across the eastern United States. These jet stream patterns affect the frequency, severity and duration of cold-air outbreaks from Canada. They also help to determine when and where winter storms will form and what regions will be affected.

"Winter weather patterns are quite variable in part because the North Atlantic jet stream pattern can take on a variety of configurations from one day to the next, and from one week to the next," said Gerry Bell, meteorologist at the NOAA Climate Prediction Center. Scientists have long known there are certain preferred configurations of the jet stream over the North Atlantic Ocean, with the two dominant ones referred to as the "negative" and "positive" phase of the NAO.

"The so-called negative phase of the NAO has prevailed for the past three weeks," said Bell. In this phase, the North Atlantic jet stream is shifted south of normal over the eastern U.S., and the upstream polar jet stream coming southward from Canada is stronger than normal. These conditions favor more frequent and more intense cold-air outbreaks across the eastern U.S. Jet streams also determine where winter storms will form, and how intense they will be. A negative phase of the NAO and a southward shift of the North Atlantic jet stream sometimes means more frequent winter storms for the eastern United States, as has been observed in past weeks.

In contrast, the positive phase of the NAO features a northward shift of the North Atlantic jet stream pattern over the eastern U.S., and a reduced flow of cold air from Canada. In this phase, milder air overspreads much of the eastern U.S., and the potential for Nor'easters is reduced.

The jet stream patterns captured by the two phases of the NAO can be very persistent, sometimes dominating for 10-20 years. For example, the negative phase of the NAO controlled the circulation during the 1950s-1960s, and led to colder and more severe winters during that period. In contrast, the milder-than-normal winters between 1980 and 2000 were associated with frequent positive phases of the NAO.

"Week-to-week and even day-to-day variations in the NAO can have a major influence on winter storms along the East Coast," said Louis W. Uccellini, director of the NOAA National Centers for Environmental Prediction and the NOAA National Weather Service's winter weather expert. "The negative NAO is now a climate signal that many forecasters are looking for in their day-to-day predictions of winter weather along the East Coast. The negative NAO is a key factor in sustaining cold air in the northeast U.S., which allows precipitation to reach the ground as snow rather than rain, when a major storm is affecting the East Coast," he added.

The NOAA Climate Prediction Center and Hydrometeorological Prediction Center routinely monitor and predict the phases of the NAO out to one and two weeks. Bell notes, "It is currently not possible to predict changes in the NAO beyond two weeks. This adds to the challenge in predicting winter season scenarios for the East Coast."

NOAA is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and providing environmental stewardship of the nation�s coastal and marine resources. NOAA is part of the U.S. Department of Commerce.

Relevant Web Sites
NOAA North Atlantic Oscillation (NAO) — Current Conditions

NOAA North Atlantic Oscillation

NOAA Climate Prediction Center

Media Contact:
Carmeyia Gillis, NOAA Climate Prediction Center, (301) 763-8000 ext. 7163