La Niña is coming, and here's how it could change the weather.

The climate pattern of cold Pacific conditions could have a cascading effect on extreme weather events in the region, in stark contrast to what a strong El Niño brought during its peak last winter. In the United States, it could bring drought in some places and heavy snowfall in others; elsewhere, its more severe effects could include drought in East Africa and flooding in Indonesia.

But there is some uncertainty about how this La Niña episode might develop because it comes amid a full year of record global average temperatures and unprecedented ocean surface warmth.

Climate scientists will be paying close attention to whether the typical global cooling effect of La Niña will continue as usual — and if not, what that might say about how humans have transformed Earth’s systems by burning fossil fuels and releasing greenhouse gases.

“It will be interesting to see how this La Niña intersects with the generally warm global oceans,” said Nathan Lenssen, a climate scientist at the University of Colorado. “We’re really in uncharted territory globally.”

Here are answers to some frequently asked questions about La Niña and its impact on the planet.

La Niña is a global weather pattern in which cold water from the depths of the eastern Pacific Ocean flows to the surface, creating a pool of cooler-than-normal water along the equator in the central and eastern Pacific. At the same time, stronger-than-normal trade winds blow across the Pacific from east to west, pushing warm surface water toward Asia and allowing that cooler water to rise in the east.

This pattern affects conditions around the globe as it shifts around the atmospheric forces that drive weather patterns across the mid- and high latitudes. The contrast between warm, stormy conditions in the western Pacific and cooler-than-normal conditions in the central and eastern Pacific helps drive changes in the natural flows of weather patterns such as heat waves and storm systems.

Some of the effects of La Niña may be imminent. This pattern is known to fuel tropical storm activity in the Atlantic. Among the changes it brings to weather patterns is a decrease in wind shear—a difference in wind speed and direction at different heights—over the Atlantic basin. This creates a more favorable environment for tropical systems to organize and strengthen.

The La Nina forecast has prompted forecasters to revise up their major hurricane season forecasts this week, now expecting 25 named storm systems, including 12 hurricanes and six “major” hurricanes, rated Category 3 or higher.

In the United States, La Niña is characterized by warm, dry conditions across the Southern Tier throughout the winter — including Southern California, the Southwest and the Gulf Coast — and wet, snowy conditions from the Pacific Northwest to the Northern Plains.

Elsewhere in the world, its effects could include flooding in northern South America and across Indonesia, and drought in East Africa – conditions that could exacerbate the hunger crisis amid Sudan’s civil war.

El Niño is associated with warmer than normal temperatures in the eastern and central Pacific Ocean. During an El Niño, the ocean's trade winds are weaker than normal, if not reversed and blow to the east, creating a circulation that allows warm surface water to pool and heat up significantly in the eastern Pacific.

El Niño often leads to La Niña conditions because it releases massive amounts of heat from the eastern Pacific Ocean, causing a rapid transition to the cooler conditions of La Niña.

Record-breaking temperatures have gripped many corners of the world’s oceans over the past year—including the western Pacific. That could exacerbate the natural contrast between warm water on one side of the ocean and cold water on the other, potentially intensifying what would otherwise be a relatively modest La Niña episode, says Nathaniel Johnson, a scientist at the National Oceanic and Atmospheric Administration who is involved in forecasting La Niña.

“This event may have had an impact greater than its size due to the temperature of the western Pacific,” said Johnson, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory.

Lensen said research is underway to determine whether climate change might alter the behavior of El Niño and La Niña. El Niño, known to warm the planet, helped push the planet into what scientists say were its hottest conditions in more than 100,000 years last July — and closer than ever to the dangerous threshold of warming, 1.5 degrees Celsius above pre-industrial temperatures.

Climate scientists will be watching closely to see if La Niña can counteract this acceleration in global warming, and to what degree.

La Niña typically lasts nine to 12 months, but can sometimes last as long as three years. It is too early to say how long it will last.

Currently, long-term climate models suggest that a period of so-called “neutral” conditions — the absence of an El Niño or La Niña — could begin after that, but those projections are far from settled, Lenssen said. A two-year La Niña “is certainly possible,” he added.

The stronger the previous El Niño, the longer the El Niño lasted, Lensen said. After the 2015-16 El Niño was one of the strongest ever observed, weak El Niño conditions persisted for two years.

But amid the relatively weak and short El Niño of 2018 and 2019, La Niña has continued for three years in what climate scientists call a “rare triple” of La Niña, from 2020 to 2023.

This time, the planet is coming off a historically strong El Niño — though not as intense as the strongest episodes on record, including 2015-16, 1997-98, and 1982-83.

The pattern's name comes from a legend associated with El Niño, which means baby Jesus in Spanish. Fishermen off the coast of Peru noticed periods of unusually warm water in the eastern Pacific that sometimes developed in the winter, changing fishing conditions around Christmas. El Niño is simply the opposite of El Niño.