A continuing La Nina event off the west coast of South America will be the dominant factor influencing weather across most of the United States this winter, bringing colder- and wetter-than-usual conditions to the Pacific Northwest and northern Plains, while much of the Southeast, Gulf Coast, Southwest and California will experience decidedly non-winter weather, according to a pair of forecasts issued separately Thursday.

“La Nina is in place and will strengthen and persist through the winter months, giving us a better understanding of what to expect between December and February…Other climate factors will play a role in the winter weather at times across the country,” said Michael Halpert, deputy director of the National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center. “Some of these factors, such as the North Atlantic Oscillation [NAO], 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.”

NOAA said it sees equal chances for above-, near- or below-normal temperatures and precipitation in the Northeast and Mid-Atlantic over the next few months.

“Winter weather for these regions is often driven not by La Nina, but by weather patterns over the northern Atlantic Ocean and Arctic. These are often more short term and are generally predictable only a week or so in advance,” Halpert said. “If enough cold air and moisture are in place, areas north of the Ohio Valley and into the Northeast could see above average snow.”

AccuWeather.com meteorologist Joe Bastardi said he is predicting an early winter for the Northeast and harsh cold and snow across the nation’s northern tier.

“A quick start to winter in the East may have people concerned about another ‘snowmageddon’, but we expect that much of this winter’s snow will come relatively early in the season,” Bastardi said.

Temperatures in the East are expected to be near or below normal during most of November and December, and portions of New England and the upper Midwest will endure several cold waves with larger-than-normal temperature swings as winter progresses, Bastardi said. The first three months of 2011 will be especially “wicked” for people across the northern Rockies and northern Plains, he said.

But Bastardi is calling for a “non-winter” from the interior Southwest to the Gulf Coast and the Carolinas, with above-normal temperatures and below-normal precipitation. NOAA also expects much of the South to be warmer and drier than average. Both forecasters say the conditions are likely to exacerbate drought conditions across the nation’s southern tier.

The northern Plains and the Ohio and Tennessee valleys are likely to see increased storminess and flooding this winter, NOAA said. NOAA’s seasonal outlook does not project where and when snowstorms may hit or total seasonal snowfall accumulations.

WSI forecasters have said they expect temperatures in the Northeast and North Central portions of the United States to average warmer than normal through November, before turning colder than normal in December as the historically persistent negative phase of the NAO and the strength of the North Pacific climate overcome the effects of the La Nina event (see Daily GPI, Sept. 21). WSI is scheduled to issue its next seasonal outlook on Tuesday (Oct. 26).

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