Fox News Weather Center

Will an official La Nina be declared this year?

While AccuWeather forecasters are calling for impacts similar to what a weak La Niña would bring to the United States this winter, conditions might not shape up to meet official La Niña criteria.

La Niña is the cool counterpart to El Niño, characterized by unusually low ocean temperatures in the central and eastern equatorial Pacific.

A strong El Niño came to an end in June, putting forecasters on watch for a La Niña to develop over the fall and winter.

La Niña strengthens the northern jet stream and causes the southern jet stream to weaken, guiding moisture in the northern tier of the country.

But for an official La Niña to be declared, sea surface temperatures in the Pacific Ocean have to be within half a degree or more below normal for five consecutive three-month periods.

The water temperatures will fall just short of the needed criteria, AccuWeather Expert Lead Long-Range Forecaster Paul Pastelok said.

"Periodic warming throughout the summer in the East Pacific disrupted the expected cooling," he said. "However, the U.S. will still see impacts typical of a weak La Niña."

The cooler-than-average waters will still influence the late fall and winter season. Even if the waters miss the needed criteria, borderline conditions can still affect weather patterns and resemble a weak La Niña.

With a fast northern jet stream, there will be occasional cold shots in the eastern U.S.

"I think the Northeast is going to see more than just a few, maybe several, systems in the course of the season," Pastelok said.

Conditions will primarily be dry across the southern half of the country, leading to enhanced drought problems in the Southwest.

Warm and dry conditions will span much of the season for central and Southern California and the Southwest.

The northwestern U.S. will kick off winter with rain and winter storms. November and December will see the most action, before high pressure builds in and stormy weather eases back in late January and February.

Full winter impacts can be found in AccuWeather's U.S. 2016-2017 winter forecast.

However, there is still up to a 30 percent chance that conditions could evolve and lead to an official La Niña.

"From October to January, there could be enough cooling in the Pacific to meet La Niña standards," Pastelok said. "But past January, it'd miss the mark."

The National Oceanic and Atmospheric Association's Climate Prediction Center makes the official call in regard to La Niña.

Thumbnail image: (Flickr photo/Victor)