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La Niña, the direct opposite of El Niño, occurs when sea surface temperatures in the central Pacific Ocean drop to lower-than-normal levels.
The cooling of this area of water near the equator, which typically unfolds during late fall into early winter, yields impacts around the globe.
In the United States, a La Niña winter means more rain in the Pacific Northwest, brief periods of below-average temperatures in the Northeast and generally dry and mild conditions for the southern tier.
While an El Niño can fuel streams of moisture into California, a typical La Niña winter prevents storms from delivering snow and rain to the region.
"During a moderate to strong La Niña, Southern California can run drier than normal throughout the wet season, leading to more drought conditions," AccuWeather Senior Meteorologist Paul Pastelok said.
La Niña puts more emphasis on the northern jet stream, weakening the southern jet stream. The change yields more frequent storms in the northern U.S.
Western Canada and the Northwest endure the stormiest conditions during a La Niña winter, but the systems generally weaken as they travel over the northern Rockies. If a storm is potent enough, it can reach eastward and deliver snow, but on average the systems lack enough moisture to produce a major storm.
Weak storms may be higher in frequency in the Midwest and Great Lakes, keeping snowfall totals at average to above-normal levels.
Winter in the Northeast is extremely variable during La Niña, ranging from colder and drier to mild and stormier.
With a weakened southern jet stream, mild conditions span from Texas to the Southeast and into the mid-Atlantic.
La Niña keeps hot, humid air concentrated over the south-central U.S., increasing the likelihood of tornadoes and damaging hailstorms.
Meanwhile, La Niña creates prime breeding grounds for tropical systems in the Atlantic Ocean.
When La Niña occurs, less wind shear is found in the developmental regions of the Atlantic, increasing the potential for a higher-than-normal amount of tropical systems.
La Niña does not always immediately develop after an El Niño. However, research shows that strong El Niños are more likely to transition to La Niñas.
"Historically, some hurricane seasons that have followed a transition from El Niño to La Niña have been very active," AccuWeather Hurricane Expert Dan Kottlowski said.
La Niña typically lasts between 10 and 12 months.
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