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As Winter 2004-2005 approaches, NOAA scientists say the leading climate patterns expected to impact winter weather are: the El Niño/Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Pacific/North American pattern (PNA) and long-term climate trends. Scientists at NOAA's Climate Prediction Center (CPC) say these patterns are the physical basis for this season's winter outlook.

El Niño Southern Oscillation (ENSO)

Water temperatures in the central-equatorial Pacific Ocean are warmer than average, indicating the early stages of a warm (El Niño) episode. Current departures indicate that the intensity of this episode is weak. Most sea-surface temperature forecasts indicate that El Niño will continue through early 2005. Considering the sea surface temperature predictions, the time of year and the observed oceanic and atmospheric circulation patterns, the intensity of any warming is expected to be weak, or potentially moderate.

Weak ENSO years, scientists say, tend to be associated with the positive phase of the Pacific/North American pattern.

Pacific/North American (PNA) Pattern

The PNA pattern features changes in the strength and position of the jet stream and storminess over the eastern North Pacific and North America. The positive phase of the PNA is often associated with weak El Niño episodes.

During weak El Niño episodes the jet stream is stronger-than-average over the east-central North Pacific and over the Mid-Atlantic States, with greater-than-average storminess in the Aleutians/Gulf of Alaska and along the southeast coast of the United States. This results in warmer and drier than average conditions over western North America and cooler and wetter than average conditions over the Southeast United States.

North Atlantic Oscillation (NAO)

The NAO is a major source of variability over the United States, North Atlantic and Europe during winter. It modulates the circulation pattern over the North Atlantic and North America regions, thereby regulating the number and intensity of significant weather events affecting the U.S. During the winter, it can often flip flop between its positive phase and its negative phase.

The positive phase features a jet stream shifted to the north of its normal position along the U.S. East Coast. Associated with this phase is an increase in the number of extreme warm days over much of the contiguous United States.

The negative phase features a shift in the jet stream southward along the U.S. East Coast resulting in an increased number of extreme cold days, especially from the Great Plains to the Southeast.

Long-Term Climate Trends

In the absence of significant forcing by the leading patterns of natural climate variability (i.e. ENSO, NAO), estimates of decadal trends provide the basis for the forecast. One tool that is used at NOAA's Climate Prediction Center is the average conditions during the last 10 years compared to the long-term average for 1971-2000.

Implications for the U.S. in Winter 2004-2005

  • Weak ENSO episodes are often associated with the positive phase of the PNA pattern;

  • The PNA pattern favors a southward shift in the jet stream and storm track toward the southeastern United States, which leads to cooler and wetter than average conditions across large portions of the South;

  • The PNA also favors above normal temperatures over Alaska, and the western U. S.;

  • The unpredictability of the seasonal phase of the NAO introduces uncertainty in the seasonal outlook, especial in the Northeast U.S.;

  • The negative phase of the NAO is associated with relatively frequent Nor'easters and heavy lake effect snows in the Northeast and Midwest, and also cold air outbreaks into the deep South. Currently, researchers can only forecast the phase of the NAO out to 5 to 10 days, which results in some uncertainty in the seasonal outlook.

New on the weather prediction front

NOAA's Hydrometeorological Prediction Center (HPC) Winter Weather Desk products and services are being expanded this year. For the past three winters, the HPC and NOAA's National Weather Service (NWS) field offices conducted Winter Weather Experiments to examine new approaches to forecasting and collaboration on potential life-threatening winter weather across the U.S. As a result of these experiments, new winter weather products – involving the prediction of snowfall amounts and winter storm tracks – will be available to forecast offices and the public. Also, a refined collaborative forecast process has resulted that involves the HPC and the NWS forecast offices. The goal of the Winter Weather Desk is to combine the winter weather forecasting expertise – at the national and local levels – and bring greater geographic consistency and improved accuracy with NWS local forecasts, watches, and warnings to the public.

About the CPC and HPC

The Climate Prediction Center and the Hydrometeorological Prediction Center, located in Camp Springs, Maryland, are two of nine National Centers for Environmental Prediction. The Climate Prediction Center serves the public by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains. The Hydrometeorological Prediction Center provides nationwide analysis and forecast guidance products out through seven days.

October 2004


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  Page last modified: 11-Mar-2010 9:31 AM