El Niño in the central and eastern equatorial Pacific Ocean is expected to be a dominant climate factor that will influence the December through February winter weather in the United States, according to the 2009 Winter Outlook released by the National oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center.

Among NOAA’s predictions include a slight tilt in the odds toward wetter-than-average conditions over California.

“We expect El Niño to strengthen and persist through the winter months, providing clues as to what the weather will be like during the period,” said Mike Halpert, deputy director of the Climate Prediction Center, a division of the National Weather Service.

“Warmer ocean water in the equatorial Pacific shifts the patterns of tropical rainfall that in turn change the strength and position of the jetstream and storms over the Pacific Ocean and the U.S.,” Halper said.

“Other climate factors are also likely to play a role in the winter weather at times across the country,” Halpert added. “Some of these factors, such as the North Atlantic Oscillation (NOA), are difficult to predict more than one to two weeks in advance. The NAO adds uncertainty to the forecast in the Northeast and Mid-Atlantic portions of the country.”

Other highlights of the U.S. Winter Outlook (December through February) include:

• Warmer-than-average temperatures are favored across much of the western and central U.S., especially in the north-central states from Montana to Wisconsin. Though temperatures may average warmer than usual, periodic outbreaks of cold air are still possible.

• Below-average temperatures are expected across the Southeast and mid-Atlantic from southern and eastern Texas to southern Pennsylvania and south through Florida.

• Above-average precipitation is expected in the southern border states, especially Texas and Florida. Recent rainfall and the prospects of more should improve current drought conditions in central and southern Texas. However, tornado records suggest that there will also be an increased chance of organized tornado activity for the Gulf Coast region this winter.

• Drier-than-average conditions are expected in the Pacific Northwest and the Ohio and Tennessee River Valleys.

• Northeast: Equal chances for above-, near-, or below-normal temperatures and precipitation. Winter weather in this region is often driven not by El Niño but by weather patterns over the northern Atlantic Ocean and Arctic, such as the North Atlantic Oscillation. These patterns are often more short-term, and are generally predictable only a week or so in advance.

• Alaska: Milder-than-average temperatures except along the western coast. Equal chances for above-, near-, or below-median precipitation for most areas except above median for the northwest.

• Hawaii: Below-average temperatures and precipitation are favored for the entire state.

This seasonal outlook does not predict where and when snowstorms may hit or total seasonal snowfall accumulations. Snow forecasts are dependent upon winter storms, which are generally not predictable more than several days in advance.