La Niña is coming. Here's what that means for winter weather in the U.S.

La Niña will most likely be joining us for the winter again, according to federal forecasters.

The National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center announced on Thursday that La Niña conditions have developed and are expected to continue, with an 87% chance that they will be in place from December to February.

La Niña (translated from Spanish as "little girl") is not a storm, but a climate pattern that occurs in the Pacific Ocean every few years and can impact weather around the world.

The U.S. is expected to feel its effect on temperature and precipitation, which could in turn have consequences for things like hurricanes, tornadoes and droughts.

Forecasters point out that this is actually the second La Niña winter in a row, a not-uncommon phenomenon that they call a "double-dip." The most recent period lasted from August 2020 to April 2021. (More below on what's happened since.)

"Our scientists have been tracking the potential development of a La Niña since this summer, and it was a factor in the above-normal hurricane season forecast, which we have seen unfold," Mike Halpert, deputy director of NOAA's Climate Prediction Center, said in a release. "La Niña also influences weather across the country during the winter, and it will influence our upcoming temperature and precipitation outlooks."

NOAA will release its official winter outlook on Oct. 21. In the meantime, here's a primer on how La Niña works and what it could mean for different parts of the country.

What exactly is La Niña?

Scientists stress that La Niña is not a storm that hits a specific area at a given time. Instead, it's a change in global atmospheric circulation that affects weather around the world.

"Think of how a big construction project across town can change the flow of traffic near your house, with people being re-routed, side roads taking more traffic, and normal exits and on-ramps closed," NOAA writes. "Different neighborhoods will be affected most at different times of the day. You would feel the effects of the construction project through its changes to normal patterns, but you wouldn't expect the construction project to 'hit' your house."

We'll start off with the technical explanation: It's one part of the El Niño Southern Oscillation cycle (ENSO), a natural climate pattern defined by opposing warm and cool phases of oceanic and atmospheric conditions in the Pacific.

La Niña and its counterpart, El Niño, alternatively cool and warm large areas of the tropical ocean, about every 2 to 7 years on average. (There's also a "neutral" state, which is where we've been since the last La Niña ended.)

As the agency explains (with a helpful flowchart), forecasters can officially declare a La Niña event when sea surface temperatures clock in below a certain level, are modeled to remain under that threshold and prompt a noticeable atmospheric response, like changes in winds.

Here's how that works.

"During normal conditions in the Pacific ocean, trade winds blow west along the equator, taking warm water from South America towards Asia. To replace that warm water, cold water rises from the depths — a process called upwelling," NOAA explains. "El Niño and La Niña are two opposing climate patterns that break these normal conditions."

Trade winds are stronger than usual under La Niña conditions, so they push more warm water toward Asia. Meanwhile, off the west coast of the Americas, an increase in upwelling sends cold water towards the surface. (The nutrient-rich water also draws more cold-water species, like squid and salmon, to places like the California coast.)

Cold waters cause the jet stream to move northward and then weaken over the eastern Pacific.

So what does that actually feel like on the ground?

The biggest impact of La Niña on North American rain, snow and temperatures tends to be felt during the winter, according to NOAA.

Generally speaking, La Niña winters tend to be drier and warmer than normal across the southern U.S., and cooler and wetter in the northern U.S. and Canada.

The Pacific Northwest, parts of the Midwest and the Tennessee and Ohio valleys can see more rain and snow than in a typical winter.

La Niña can also lead to a more severe Atlantic hurricane season, which we're already seeing this year.

The main things that forecasters stress is that while La Niña events are associated with certain climate patterns — namely, deviations in temperature and rainfall in different parts of the country — they're a matter of "probability, not certainty."

So if you're wondering whether La Niña will affect your home this winter, NOAA scientists offer this answer:

"Maybe. Probably. Probably not. The answer depends on many factors, including where you live, how strong the event continues to be, and other climate patterns that develop and influence the seasonal outcome."

What about weather events like snow, flooding and tornadoes?

Snow is hard to predict, but experts say La Niña could bring increased snowfall over the Northwest, northern Rockies and upper Midwest Great Lakes region. Parts of the Southwest, central-southern Plains, and mid-Atlantic are likely to see less than usual.

La Niña generally contributes to more Atlantic hurricanes, but fewer in the eastern and central Pacific (El Niño is the opposite). As NOAA explains, those Atlantic hurricanes form in the deep tropics from African easterly waves, so are more likely to become major hurricanes that could hit the Caribbean and U.S.

The position of the jet stream also appears to have an impact on tornadoes and which parts of the country are more likely to experience them. During La Niña winters, the jet stream and severe weather are likely to be farther north.

La Niña could also worsen California's ongoing drought and make its wildfire season even more of a threat. As Bloomberg explains, the state usually gets most of its annual water from rain and snow between November and April — the same period when La Niña is predicted to shift storm tracks north and away from the region that needs it.

While the southern U.S. could see prolonged drought, the northern part of the country — especially the Pacific Northwest — is likely to experience heavy rains and flooding.

How long will it last, and how often does it happen?

The National Ocean Service says that episodes of El Niño and La Niña typically last 9 to 12 months, but can sometimes stretch for years.

They both tend to develop during the spring, reach peak intensity during the late fall or winter and then weaken during the spring or summer.

In other words, La Niña's influence on the U.S. will be strongest between January and March, but could linger into the early spring.

Why is it called that?

As the backstory goes, South American fishermen had long observed warmer-than-normal coastal Pacific Ocean waters and dramatic decreases in fish catch happening around Christmastime. They nicknamed that phenomenon "El Niño" — Spanish for "little boy" — after the baby Jesus.

So when scientists discovered the opposite phase of El Niño in the 1980s, they decided to call it "La Niña." (Of course, language about gender identity and expression is a lot more nuanced these days.)

How does this relate to climate change?

It's too soon for scientists to say exactly how a warmer world would affect the ENSO cycle.

"But remember, just because we do not have high confidence on how ENSO might change in the future does not mean that it won't," NOAA wrote in a 2016 blog post. "It just means scientists have more work to do."

They are confident, however, that ENSO itself will continue into the future. And they say global warming will likely affect the impacts of La Niña, including extreme weather events. [Copyright 2021 NPR]