La Nina Likely to Bring Weather Extremes to US This Winter

(Sept. 11) -- Weather extremes occur across the United States virtually every season, but the presence of a La Nina means that weather forecasters have a better idea of what those extremes might be during the upcoming winter.

La Nina winters often result in flooding and avalanche problems in the Pacific Northwest, drought in the Southwest, the start of drought conditions in the Southeast and bouts of unseasonable -- perhaps record-breaking -- warmth in the South and East.

The current La Nina, which is a cooling of sea-surface temperatures in the tropical Pacific, is expected to last through the winter and peak at a moderate or strong intensity, increasing the likelihood of those events.

During a La Nina winter, an active northern Pacific storm track is likely, with a less active southern Pacific storm track. Persistent storminess in the Northwest and Northern Rockies results in the threat of flooding in lower elevations and enough mountain snow, when combined with temperature fluctuations, to result in a deep and unstable snow pack.

The general lack of storminess in the southern tier of the country means less precipitation than during an average winter. The desert Southwest, which depends on winter rain for most of its yearly rainfall, is vulnerable to drought even after just one season of dry weather.

The dominant northern branch of the jet stream often floods much of the nation with mild air from the Pacific, resulting in unusual warmth in the South and East at times.

The overall expected conditions associated with a La Nina in the coming winter are much different from last winter's weather, which was influenced by a moderate El Nino, a warming of the sea-surface temperatures in the tropical Pacific that sets up a stronger-than-normal southern storm track.

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Last winter was wet across the southern part of the country and dry in the Northwest. Record-breaking amounts of snow fell in the Mid-Atlantic region, while the Southeast had bouts of extreme cold.

While extreme events are more likely to occur during a La Nina year, it's not as if La Nina gives long-range forecasters all of the answers to the winter forecast.

Although periods of warmth are likely in the Northeast and Mid-Atlantic, very cold air will also not be too far north of these regions, so occasional periods of extreme winter weather there are still possible. This is true particularly for the upper Midwest, which will be on the boundary between persistent cold to the west and persistent mild air to the south. The winter could go either way or fluctuate back and forth depending on the precise location of the dominant northern jet stream.