The National Oceanic and Atmospheric Administration declared Thursday that a La Niña pattern had become established, having bearing on the remainder of the hurricane season and the upcoming winter. La Niña conditions are likely to continue through at least wintertime, potentially returning to a more relaxed “neutral” state by spring.

La Niña, which means “the girl” in Spanish, is the opposite of an El Niño. La Niña features unusually cool ocean waters in the equatorial tropical Pacific Ocean and can influence weather patterns beyond the Pacific.

The expectation of a La Niña pattern was a contributor in NOAA’s early August forecast of an “extremely active” hurricane season.

La Niñas and El Niños, which represent opposite phases of ENSO, or the El Niño Southern Oscillation, are major drivers of weather and climate trends in North America.

Much of late 2018 through early 2020 had skewed a bit more toward the weak El Niño side. Now, the pendulum is swinging in the other direction, which will have major implications in the months ahead.

What is a La Niña?

A La Niña pattern is characterized by anomalously cool sea surface temperatures in the eastern Pacific. That’s the opposite of an El Niño, during which the east tropical Pacific is atypically toasty.

With a significant change in ocean temperatures ongoing and further in the offing, a La Niña alters several key circulation patterns in the atmosphere and can influence weather across the globe.

Most La Niña events last for at least several months but can occasionally stretch for years. Strong La Niñas occurred between 1973-1976, 1988-1989 and from 1998 to early 2001.

Effect on hurricane season

During a La Niña, blocking high pressure in the eastern Pacific helps divert the jet stream north and reduce mid- and upper-level winds across the tropical Atlantic. That decrease in wind shear, or change of wind speed and/or direction with height, makes it easier for tropical storms and hurricanes to form.

A La Niña pattern also influences the Walker Circulation, or a large overturning circulation in the tropical atmosphere, in a way that can bolster Atlantic hurricane chances.

The presence of cool water over the eastern Pacific can induce a sinking motion, which in turn favors rising elsewhere — namely, square over the Atlantic, and also in the western Pacific.

That’s why La Niñas can bring increased rainfall in parts of the West Pacific, including Indonesia, Australia and New Zealand.

The increased lift in the Atlantic boosts the odds of hurricane formation, since hurricanes start as a collection of showers and thunderstorms that feed off upward-moving air.

La Niña is one factor at play this season, but with the dice already loaded, it’s probable that the hurricane season will rage on much more intensely than normal.

Winter weather

NOAA forecasters have stated there is a 75 percent chance that La Niña will stick around for the entirety of winter. Generally speaking, La Niña typically increases the odds of above-average snowfall in the Pacific Northwest, northern Plains, Great Lakes region and northern New England. However, every La Niña is different, and other weather patterns can overwhelm its effects.

In the Mid-Atlantic, South, southern/central Plains and Southwest, snow may be more scarce.

The impending La Niña is bad news for Central and Southern California, where potentially reduced snowpack at the higher elevations and fewer winter storms may prolong this year’s fire season, already a record, and set the stage for a challenging 2021.

Severe weather impacts

Because La Niña patterns shift the location of the jet stream farther north, there is generally less unsettled weather in the wintertime across the southern United States. This means drier, and subsequently warmer, conditions are likely for the Desert Southwest, South and Southeast.

It also limits the number of wintertime severe weather events in Florida and the South, commonly known as “Dixie Alley” by storm chasers. Disturbances in the jet stream can cause severe weather and tornado activity across the Interstate 10 corridor, particularly from Louisiana and Mississippi to Alabama, Florida and Georgia, between December and February. But with the jet stream retreating farther north, those chances are significantly diminished.

In fact, tornadoes in this region are only half as common during La Niña winters as compared with El Niño winters, welcome news for residents who live there.

Between March and May, however, a La Niña pattern can increase tornado and severe thunderstorm incidence for portions of the central and southern Plains, as well as into portions of Arkansas, northern Louisiana and the central Mississippi Valley.

It is unclear whether this La Niña will still be in place by then, though.