For the second year in a row, a La Nina event — cooling of ocean surface temperatures off the western coast of South America — will influence winter weather patterns across the United States, while the less predictable Arctic Oscillation could produce “dramatic” swings in winter temperatures, according to the National Oceanic and Atmospheric Administration (NOAA).

Those weather events make it likely that the Southern Plains will continue to see drier- and warmer-than-average weather, while the Pacific Northwest will be colder and wetter than average during December, January and February, NOAA said. Colder-than-average temperatures are also expected in California, the Northern Plains and the Great Lakes.

The Northeast, Mid-Atlantic, Ohio and Tennessee valleys, Florida and the south Atlantic Coast, on the other hand, have equal chances for above-, near- or below-normal temperatures this winter, NOAA said. Winter weather for the Northeast and Mid-Atlantic regions is often driven not by La Nina but by the Arctic Oscillation, which is always present and fluctuates between positive and negative phases.

“The negative phase of the Arctic Oscillation pushes cold air into the U.S. from Canada,” NOAA said. “The Arctic Oscillation went strongly negative at times the last two winters, causing outbreaks of cold and snowy conditions in the U.S. such as the ‘Snowmaggedon’ storm of 2009. Strong Arctic Oscillation episodes typically last a few weeks and are difficult to predict more than one to two weeks in advance.”

NOAA expects the current La Nina, which formed in August, to gradually strengthen and continue through the upcoming winter.

Forecasters at AccuWeather.com have said they expect the La Nina event to prompt especially harsh temperatures and snowfall across the Midwest and Great Lakes region, while the Northeast can expect winter 2011-2012 to be somewhat less extreme than last year (see Daily GPI, Oct. 6). And Weather Services International (WSI) recently said it expects the northern and eastern United States to experience cooler-than-normal temperatures through December (see Daily GPI, Sept. 21). The newly emerging La Nina event and a trend toward North Atlantic atmospheric blocking indicate below-normal temperatures becoming more common in the eastern U.S. in November and across all of the northern U.S. by December, according to WSI.

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