The phenomenon is caused by a shift in wind direction and water temperature in the equatorial Pacific. El Nino can bring about a change in typical weather conditions, sometimes with disastrous results.

"It is important to realize that interpreting the impact of any El Nino event is not always a straight forward matter since each one is somewhat unique," said David Arnold, a meteorology professor. "Considerable variability exists between El Nino years. Subtle differences between some El Nino winters may produce unexpected results."

"A seemingly minor change in the character of the mid-level water circulation pattern can bring about much different conditions at the surface than were anticipated," Arnold said. "For example, the El Nino event of 1976-1977 marked the first in a series of three of the coldest consecutive winter seasons in the Midwest."

An El Nino condition exists when an unusual warm pool of sea surface temperatures exists over the Pacific Ocean in the vicinity of the International Dateline. This ocean surface temperature anomaly has been shown to affect the larger scale atmospheric circulation pattern that dictates "spells" of weather over a good portion of the Northern Hemisphere during the cold season.

The appearance of El Nino will heavily influence this winter's weather, according to the National Oceanic and Atmospheric Administration (NOAA).

Arnold said how the warm and dry winter forecast issued by the NOAA works out is dependent on the performance of long-range forecasting methods.

"The accuracy of these methods is not as good as we are accustomed to with our daily weather forecasts," he said. "Atmospheric scientists would not be as confident in this coming winter's prediction as they would be for the weather this coming weekend."

Arnold said that even if the NOAA's winter forecast pans out, periods of cold and snowy weather might still occur during the coming cold season.

"We could easily experience a few very cold periods, mixed in with slightly longer periods of warmer and drier than average weather, and still meet the forecast of a warm and dry winter," he said.

(NOTE TO EDITORS: For more information, contact Arnold at darnold@bsu.eduor (765) 285-1768.)