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La Niña to Depart by April's End
La Ni
Credit: NOAA.

The La Niña climate pattern that re-emerged late last summer should finally be gone by the end of April, government climate scientists announced today (March 8).

When a La Niña pattern is in place, temperatures in the tropical Pacific Ocean are cooler than normal, which has far-reaching consequences for climate and weather patterns around the globe.

The most recent La Niña cycle first emerged in June 2010 and had a substantial impact on the extreme weather of 2010-2011 winter, as well as last spring's terrible tornado season. La Niña conditions may have helped bring about some of the massive snows that blanketed much of the northern United States last winter, but its waning may actually have been the culprit in tornado season.

A "normal" La Niña would have kept the jet stream, which guides the flow of storm systems across the country, to the north, pushing cold arctic air (one of the ingredients of severe weather) away from the lower United States. But this La Niña petered out quickly, and no El Niño (La Niña's warm-temperature counterpart) rose up to replace it. This left the jet stream free to roam about the atmosphere and allowed arctic air to mix with moist air from the Gulf of Mexico — a classic recipe for storms.

The La Niña pattern had fully petered out by May 2011, but re-emerged at the end of the summer and gathered strength as winter approached.

Temperatures in parts of the Pacific have begun to warm, and these and other patterns in the ocean and atmosphere suggest that La Niña is weakening, according to the National Oceanic and Atmospheric Administration's Climate Prediction Center (CPC). Neutral conditions (neither La Niña nor El Niño) are expected to emerge by the end of April.

But the impacts of La Niña can often lag behind the pattern's disappearance and the CPC has said they expect these effects to persist into the spring. Among the impacts they anticipate are: above-average temperatures across the south-central U.S.; below-average temperatures in the northwestern U.S.; above-average precipitation across western Washington, the Ohio Valley, and lower Great Lakes; and drier-than-average conditions are more likely across Florida, the Gulf Coast and the southwestern U.S.