Forecasters expect La Nada winter instead of El Niño


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The Independent US

So much for having El Niño conditions this winter.

The National Oceanic and Atmospheric Administration has walked way from its forecast of meaningful warming of the tropical Pacific Ocean, which sometimes leads to snowy winters in the Mid-Atlantic and stormy conditions across the southern United States.

Since the summer, NOAA's Climate Prediction Center had been favoring the gradual development of El Niño conditions; it even issued an El Niño watch in recent months. But no more.

That watch "has been discontinued as the chance of El Niño has decreased," the CPC wrote in a monthly El Niño discussion released Thursday.

Instead of El Niño, NOAA predicts something sometimes called "La Nada," the murky middle ground between El Niño and its sister La Niña, which is associated with cool water in the equatorial Pacific.

Although the CPC says the chances of a full-fledged El Niño are remote, sea surface temperatures are forecast to remain above normal through the winter.

The tropical ocean and atmosphere "may resemble a weak El Niño at times," the CPC wrote.

What does all of this mean for winter weather across the United States, and the Mid-Atlantic in particular? That's anyone's guess.

Recognizing that El Niño might be muted, NOAA was decidedly undecided about how the winter would play out in its seasonal outlook released in mid-October. It called for equal chances of above- or below-average temperatures and precipitation over large parts of the United States.

"This is one of the most challenging outlooks we've produced in recent years because El Niño decided not to show up as expected," said CPC Deputy Director Michael Halpert.

When a full-fleged El Niño or La Niña is present, it can serve a useful guide to general temperature and precipitation patterns. Without one or the other, these patterns are much less predictable.

Instead, weather conditions become subject to the whims of the North Atlantic Oscillation, a measure of the relationship between pressure systems over the Atlantic Ocean, which can change every couple of weeks or get stuck in a certain phase.

When the NAO is negative, cold air plunges south over eastern North America, sometimes spawning storms along the East Coast.

But when the NAO is positive, mild air tends to flood much of the eastern half of the country.

In some past years, like the Snowmageddon winter of 2009-2010, the NAO was predominantly negative. During last year's unusually warm winter, it was mainly positive. In others years, it has varied wildly, leading to volatile conditions.

It's unclear how the NAO will behave this winter, but it's likely to be the key player in determining how the season unfolds.