What Is La Niña Weather and What Does it Mean for Winter in the U.S.?

La Niña weather is expected to affect temperatures and precipitation across the U.S. in coming months, federal forecasters said Thursday.

La Niña conditions, which have developed over the past month, are projected to continue through the winter in the Northern Hemisphere, according to the Climate Prediction Center of the National Oceanic and Atmospheric Administration (NOAA).

The federal body said there is an 87 percent chance of La Niña happening from December 2021 to February 2022.

What Is La Niña?

La Niña is one of two opposing climate conditions in the Pacific Ocean (the other being El Niño) that break normal trade wind patterns. The two phenomena are collectively known as the El Niño-Southern Oscillation (ENSO) cycle.

Trade winds are prevailing winds that circle the Earth near the equator. Under normal conditions in the Pacific, trade winds blow west, pushing warm water from South America towards Asia. To replace that warm water, a process known as "upwelling" occurs, which sees cold water rising from the depths, the National Ocean Service of the NOAA explains.

"During La Niña events, trade winds are even stronger than usual, pushing more warm water toward Asia. Off the west coast of the Americas, upwelling increases, bringing cold, nutrient-rich water to the surface," the federal body says.

This nutrient-rich surface also attracts more cold water species, such as squid and salmon, to places like the California coast, the federal body notes.

El Niño and La Niña events take place every two to seven years, on average, but don't happen on a regular basis. El Niño occurs more frequently than La Niña and episodes of both typically last nine to 12 months. However, they can sometimes last for years, according to the National Ocean Service.

What Does La Niña Mean for Winter in the U.S.?

La Niña is marked by "unusually cold ocean temperatures" in the Equatorial Pacific region, the NOAA explains.

A La Niña year means winter temperatures will be warmer than usual in the South and cooler than normal in the North. La Niña can also result in a more severe hurricane season, according to the National Ocean Service.

Emily Becker, a climate prediction expert and an author of the NOAA's ENSO blog at Climate.gov explained Thursday: "We've already seen one likely effect of La Niña this year—a more active Atlantic hurricane season, with nearly twice as many storms as average so far this year.

"But the most substantial La Niña effect on North American rain, snow, and temperature happens during winter," Becker said.

La Niña winters are usually drier and warmer across the southern third of the country and cooler in the northern U.S. and Canada. The Pacific Northwest, the Tennessee and Ohio Valley areas as well as parts of the Midwest typically see more rain and snow than average, according to Becker.

The cold waters in the Pacific tend to cause drought in the southern U.S. region, while heavy rains and flooding usually take place in the Pacific Northwest as well as Canada, the National Ocean Service warns.