How La Niña can impact US winter weather: NPR

This graph from Climate.gov shows how La Niña generally affects weather conditions in the United States. Forecasters say there is almost a 90% chance that La Niña conditions will be in place from December 2021 to February 2022.

The National Oceanic and Atmospheric Administration


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The National Oceanic and Atmospheric Administration


This graph from Climate.gov shows how La Niña generally affects weather conditions in the United States. Forecasters say there is almost a 90% chance that La Niña conditions will be in place from December 2021 to February 2022.

The National Oceanic and Atmospheric Administration

La Niña will most likely join us for the winter, according to federal forecasters.

The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center said Thursday that La Niña conditions have developed and are expected to continue, with an 87% chance of being 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 all over the world.

The United States is expected to feel its effects on temperature and precipitation, which in turn could impact events such as hurricanes, tornadoes and droughts.

Forecasters point out that this is in fact the second consecutive La Niña winter, a not uncommon phenomenon they call a “double dip.” The most recent period was from August 2020 to April 2021. (More info below on what has happened since.)

“Our scientists have been following the potential development of a La Niña since this summer, and this has been a factor in the above-normal hurricane season forecast, which we have seen unfold,” said Mike Halpert, deputy director of the NOAA’s Climate Prediction Center, said in a press release. “La Niña also influences the weather across the country during the winter, and this will influence our forecast for future temperatures and precipitation.”

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

What exactly is La Niña?

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

“Think about how a large construction project across town can alter the flow of traffic near your house, with people being redirected, side roads taking more traffic and normal exits and ramps closed,” says one. NOAA web page. “Different neighborhoods will be affected the most at different times of the day. You would feel the effects of the construction project through its changes from normal models, but you would not expect the construction project to ‘hit’ your house. . “

We will start with the technical explanation: it is part of the El Niño-Southern Oscillation (ENSO) cycle, a natural climate model defined by the opposition of hot and cold phases of oceanic and atmospheric conditions in the Pacific.

La Niña and its counterpart, El Niño, alternate large cold and warm areas of the tropical ocean about every two to seven years on average. (There is also a “neutral” state, which is where we have been since the end of the last La Niña.)

As the agency explains (with a helpful flowchart), forecasters can officially declare a La Niña event when sea surface temperatures fall below a certain level, are modeled to stay below that threshold and cause a noticeable atmospheric response, such as changes in wind.

Here is how it works.

“Under normal conditions in the Pacific Ocean, the trade winds blow westward along the equator, carrying warm water from South America to Asia. To replace this warm water, ‘cold water rises from the depths – a process called upwelling, ”says NOAA. “El Niño and La Niña are two opposing weather patterns that break these normal conditions.”

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

The cold waters cause the jet stream to move north, then weaken over the eastern Pacific.

So what does it really feel on the pitch?

La Niña’s biggest impact on rain, snow and temperatures in North America tends to be felt during the winter, according to NOAA.

In general, La Niña winters tend to be drier and warmer than normal in the southern United States and cooler and wetter in the northern United States 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 are already seeing this year.

The main thing forecasters point out is that while La Niña events are associated with certain climate patterns – namely, variations in temperature and precipitation in different parts of the country – they are a matter of “probability, no certainty. “

So if you’re wondering if La Niña will affect your home this winter, scientists at NOAA come up with this answer:

“Maybe. Probably. Probably not. The answer depends on many factors, including where you live, the strength of the event, and other climate patterns that develop and influence the seasonal outcome.”

What about weather events such as snow, floods and tornadoes?

Snow is hard to predict, but experts say La Niña could lead to increased snowfall in the northwest, northern Rockies and the Great Lakes region of the upper Midwest. Parts of the southwest, south-central plains, and mid-Atlantic will likely see less than usual.

La Niña generally contributes more hurricanes in the Atlantic but less in the eastern and central Pacific (El Niño is the opposite). As NOAA explains, these Atlantic hurricanes form in the deep tropics from the East African waves, so they are more likely to become major hurricanes that could hit the Caribbean and the United States.

The position of the jet stream also appears to have an impact on tornadoes and the areas of the country that are most likely to experience them. During La Niña winters, the jet stream and inclement weather are probably further north.

La Niña could also worsen the ongoing drought in California and make its wildfire season even more threatening. As Bloomberg explains, the state typically gets most of its annual water from rain and snow between November and April – the same time that La Niña is expected to shift storms north. and far from the area that needs it.

While the southern United States could experience prolonged drought, the northern part of the country – particularly the Pacific Northwest – is likely to experience heavy rains and flooding.

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

The National Ocean Service says El Niño and La Niña episodes typically last nine to 12 months, but can sometimes span for years.

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

In other words, La Niña’s influence on the United States will be strongest between January and March, but could persist until early spring.

why is it called like that?

As the story goes, South American fishermen have long observed warmer-than-normal coastal Pacific Ocean waters and dramatic declines in fish catches around Christmas. They nicknamed this 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, the language about gender identity and expression is much more nuanced these days.)

What does this have to do with climate change?

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

“But remember, just because we don’t have much confidence in How? ‘Or’ What ENSO could change in the future doesn’t mean it won’t, ”NOAA said in a 2016 blog post.“ It just means scientists have more work to do.

They are, however, convinced that ENSO will continue to operate in the future. And they say global warming will likely affect the impacts of La Niña, including extreme weather events.

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