La Niña pattern shaping up for the fall and winter across the U.S.

Weather

COLUMBUS (WCMH) – There are strong signs that a La Niña pattern will return later in the year and likely linger through much of the upcoming winter, according to NOAA’s Climate Prediction Center.

The government agency reports there is a 70 percent chance of La Niña returning between November 2021 and January 2022. How does La Niña impact weather across the nation, and what does that mean for the weather across the U.S.?

La Niña occurs when colder-than-average sea surface temperatures in the central and eastern equatorial Pacific Ocean loom off the coast of Peru. Warm water is pushed westward, allowing deep colder currents to reach the ocean surface off the coast of South America.

This familiar climate regime (the opposite of El Niño) generally causes the jet stream to swing farther north across the northeastern Pacific and western Canada, then dip southward in the northern Plains and Great Lakes. A northwest flow over the lakes favors more frequent lake-effect snowfalls in northeastern Ohio.

In La Niña winters, there is a tendency for persistent dry weather from Southern California across the Southwest to the southern Plains. This would not be good news for areas in the Western U.S. experiencing severe drought.

Farther north across the Pacific Northwest, the jet stream is stronger, bringing Pacific moisture across the northern Rockies and Plains states, with cold air outbreaks mainly confined to the western High Plains and Upper Midwest, but with occasional surges into the Ohio Valley.

The Ohio Valley, Mid-Atlantic states and Southeast usually have relatively mild winters during a moderate La Niña, but with the potential for several mid- to late winter cold blasts when a blocking high in the North Atlantic forms and funnels cold air southward for periods of a few weeks.

La Niña winters often feature wetter-than-average winters in the Ohio Valley, which can be in the form of rain or snow, with snow generally falling 100 to 150 miles northwest of the storm track dictated by the jet stream position in the Eastern U.S..

Last winter was also a La Niña season. The winter was initially mild in Ohio, but with a Christmas Eve snowstorm and short cold snaps. A warmer-than-average January followed before we descended into a frigid and snowy February. Columbus ended up with slightly above average snowfall (28 inches), nearly half of it (12.1 inches) coming in February.

La Niña also lingered through two winters in the U.S. in 2016-17 and 2017-18. A deeper dip in the flow in the West promoted a persistent mild southwesterly flow east of the Rockies, leading to an exceptionally warm pattern from early fall through early spring, with only 9.3 inches of snow falling in Columbus. Arctic air was deflected north of the Ohio Valley due to the strength and persistence of the upper-level winds.

However, a weaker La Niña in 2017-18 brought a totally different result. Both December 2017 and January 2018 averaged several degrees below normal, with nearly 20 inches of snow falling in Columbus. February brought a complete reversal, with temperatures more than 6 degrees above normal. March and April were cold and snowy, raising the season snowfall to 30.7 inches.

There is considerable variability in the weather that is not solely driven by Pacific sea surface temperatures, but the cyclical Pacific climate systems play a significant role.

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