La Niña is officially here to shape U.S. winter weather, along with global climate

La Niña is back, ya'll. And it may have major implications for your winter weather, depending on where you live.

La Niña conditions were formally declared present in the tropical Pacific Ocean on Thursday, in a statement from the National Oceanic and Atmospheric Administration (NOAA). The agency's scientists found a large expanse of the equatorial tropical Pacific Ocean to feature cooler than average waters both at the surface and extending into deeper waters. The atmosphere is responding to this in ways that match expectations for a La Niña event.

In fact, the weather for much of the fall across parts of the U.S. has featured La Niña-like fingerprints, with milder than average conditions across the South and East, in particular.

La Niña is the lesser known sibling of El Niño, which features unusually mild sea surface temperatures across the tropical Pacific. Strong El Niño events can reverberate globally, altering prevailing weather patterns from California to Indonesia.

Together, El Niño and La Niña are part of a broader climate cycle, known as the El Niño-Southern Oscillation, or ENSO, that is a leading architect of weather patterns in North America, particularly during the winter. Therefore, La Niña is a main factor that NOAA forecasters examine in making their winter forecasts.

While La Niña also affects weather patterns spanning from the Pacific to the Atlantic Oceans — including generating less rain over the central Pacific, but dumping more on Indonesia — it's never garnered the same media attention as its sibling.

It was El Niño, after all, that got the Saturday Night Live treatment due to an intense 1998-99 event and the late comedian Chris Farley. Weather geeks young and old have that brief sketch memorized.

Despite the publicity gap, if you live in the U.S. or Canada, in particular, you're likely to feel the effects of the 2017-18 La Niña event. La Niña winters tend to feature cooler-than-average conditions from Alaska down across the provinces of northwest and western Canada, and into the northern Plains and Upper Midwest.

Milder-than-average conditions tend to be found across the southern tier of the country during such winters, often extending into the Mid-Atlantic. Wetter-than-average conditions during La Niña winters tend to be found across the Pacific Northwest, and in the Midwest.

For skiers, La Niña years can be blockbuster seasons for the Pacific Northwest ski areas as well as resorts in the Canadian Rockies, though there is a risk of rain events at lower-lying mountains.

However, there's a caveat. The temperature influence of La Niña is becoming somewhat muted due to global warming, as colder-than-average winter conditions become rarer even in the northern Plains.

From this, one might think that there will be less snow than average in the Mid-Atlantic and Northeast. But that's not a guarantee by any means, because other air and ocean cycles play a larger role in determining winter weather conditions in these areas, particularly the North Atlantic Oscillation, which can only be reliably predicted about two weeks ahead of time.

It's possible that cities from Portland, Maine, to Washington, D.C., will end up having quite a snowy winter, despite what the La Niña signals might suggest, but the odds of this are somewhat lower than for an average winter.

The La Niña news also provides a clue to how global temperatures may rank in 2018.

Whereas El Niño events tend to add heat to the global climate, making the odds of a record warm year even higher than they already are from human-caused climate change, La Niña pushes the climate in the other direction. Having a weak La Niña persist through early spring, which is what NOAA currently is predicting, would suggest that 2018 will end up being close to, or cooler than, 2017.

Keep in mind, though, that 2017 is likely to rank among the top 3 warmest years since reliable global instrument records began in 1880.

Given global warming, El Niño and La Niña years have been warming steadily. There's a decent chance that 2018 will set a milestone for the warmest La Niña year on record.

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