A major shift is underway in the tropical Pacific Ocean, and its effects could soon be felt thousands of miles away. El Niño has already developed and is rapidly strengthening. According to the U.S. weather service’s August forecast, there is a greater than 90 percent chance that it will become a very strong event by the fall and winter of 2026. Even more striking, NOAA put the probability at 69 percent that it will reach historic levels between October and December, becoming stronger than any El Niño measured since 1950. Scientists are therefore preparing not simply for another El Niño, but for an event that could temporarily raise temperatures across the planet, rearrange patterns of rainfall and storms, and increase the risk of extraordinary winter rains, flooding and record-high coastal water levels in California. American Community Media recently held a press briefing on the subject featuring a single specialist in the field.
Climate scientist Daniel Swain has been tracking the development of this unusual El Niño for months. Swain is a researcher with the California Institute for Water Resources at the University of California Agriculture and Natural Resources and a research affiliate with the NSF National Center for Atmospheric Research. His work focuses on the dynamics and impacts of extreme weather and climate events, including droughts, floods, storms and wildfires, on a warming planet. Through his Weather West website and YouTube channel, he also regularly explains weather and climate developments directly to the public. In his assessment of the current El Niño, Swain makes clear that El Niño itself is a natural phenomenon. What makes this one extraordinary is its expected strength, its unusual geographic structure and the fact that it is developing in a world that has already warmed significantly.
To understand El Niño, it is first necessary to recognize that the Pacific Ocean and the atmosphere above it operate as a single interconnected system. Under normal conditions, trade winds blow from east to west along the equator, pushing warm surface water toward Indonesia and the western Pacific. Off the coast of South America, meanwhile, colder, nutrient-rich water rises from the depths. During El Niño, those trade winds weaken or partially reverse. Warm water spreads eastward, the surface of the central and eastern tropical Pacific becomes unusually warm, and the region where moist air rises over the warm ocean and produces thunderstorms shifts with it. That change ripples through the atmosphere, altering upper-level circulation and eventually redirecting storm tracks thousands of miles away. This is why an ocean anomaly near Peru and the equatorial Pacific can determine months later how much rain falls in Los Angeles or what kind of winter develops across the southern United States.
El Niño itself is not rare. It is the warm phase of a natural cycle known as the El Niño-Southern Oscillation, or ENSO, and occurs on average every two to seven years. What surprised researchers in 2026 was the speed and magnitude of the transition. La Niña still dominated the Pacific in March. In May, NOAA was only forecasting the imminent development of El Niño. By June, conditions had already met El Niño criteria, and by July, sea surface temperature anomalies in parts of the eastern equatorial Pacific exceeded 2 degrees Celsius. Conditions below the surface were far more extreme, with water in some areas as much as 10 degrees Celsius warmer than normal. The combined behavior of the ocean and atmosphere showed that the system was no longer simply warming. It was locking into an increasingly powerful El Niño state.
According to Swain, a strong El Niño was already in place by August, with substantial additional strengthening expected. He says there is a greater than 90 percent chance of a very strong El Niño and roughly a 70 percent chance that by the end of the year it will become the strongest event since at least 1950. The event is expected to peak between October and December. The strength of El Niño is not determined simply by how warm the ocean becomes at one particular location. Meteorologists examine temperature anomalies over specific regions of the tropical Pacific over several months and also monitor how the atmosphere responds. What makes the current event particularly unusual is that the warming is heavily concentrated in the eastern Pacific. According to Swain, very strong eastern Pacific El Niños are more likely to have powerful and widespread effects on California weather.
The danger, however, comes from more than the magnitude of El Niño itself. The phenomenon is unfolding on a planet that has already experienced more than a century of human-caused warming. During El Niño, the tropical Pacific releases enormous amounts of heat into the atmosphere. Warmer air can hold more water vapor, creating the potential for more intense precipitation when conditions are favorable. Swain says a potentially historic El Niño will therefore temporarily amplify the effects of long-term global warming. Depending on the region, the result could be extreme heat, drought or extreme precipitation. Swain expects that late 2026 and 2027 could bring an unprecedented likelihood of new global temperature and weather records. In early September, the World Meteorological Organization warned that this El Niño could become the strongest ever recorded and is highly likely to persist through February 2027.
For California, the first major impact may not come from the sky at all, but from the ocean. During El Niño, warm water accumulating in the eastern Pacific raises sea levels along the U.S. West Coast. In 2026, that process is being reinforced by a massive Kelvin wave. This is not a conventional wave that breaks onshore, but a slowly propagating displacement of ocean water. It has already raised sea levels by 9 to 14 inches in parts of Central America and is forecast to spread toward California beginning in late September. According to the San Francisco Chronicle, the effect could temporarily raise sea levels by roughly a foot along parts of the California coast.
That alone does not automatically mean disaster. The danger increases when elevated background sea levels coincide with winter king tides, large waves or a powerful storm. Swain says coastal risk researchers are seriously considering the possibility that California could experience the highest sea levels ever recorded along parts of its coast this winter. Under those conditions, waves begin from a higher baseline, penetrate farther inland, accelerate erosion of beaches and coastal bluffs, and more easily reach roads, buildings and other infrastructure. If heavy rainfall occurs at the same time, runoff from the land also has more difficulty draining into an elevated ocean. The two processes can therefore reinforce each other.
For most Californians, however, the biggest question is whether a historic El Niño means a lot of rain. The answer is still not a simple yes. Swain emphasizes that even an exceptionally strong El Niño does not guarantee a particular weather outcome in California. El Niño does not generate specific storms, and it cannot tell forecasters months in advance whether it will rain in Los Angeles on January 15. What it changes are the probabilities. Weak and moderate El Niños have an uncertain relationship with California precipitation, but very strong events much more clearly increase the odds of a wet winter, particularly in Central and Southern California. Because of the strength and eastern position of the current event, Swain says the probability of a wetter-than-average winter and of individual episodes of exceptionally heavy precipitation has increased significantly.
From a risk perspective, the total amount of precipitation over the entire winter may not even be the most important factor. California’s worst floods are often caused by a small number of exceptionally intense storms. Atmospheric rivers can transport enormous amounts of water vapor from the Pacific toward the continent through long, narrow corridors of moisture. If one of these systems remains over the same area for days, rivers can overflow, hillsides can give way and urban drainage systems can become overwhelmed. El Niño does not guarantee atmospheric rivers, and atmospheric rivers occur without El Niño, but in an exceptionally warm and moisture-rich atmosphere, the consequences of a major storm can become more severe. Swain says the current conditions therefore push the odds of major precipitation events substantially higher.
Timing also matters. Dramatic changes in the ocean will already be visible in the fall, but El Niño’s strongest atmospheric effects in California are expected to arrive later. Swain says the first third of the rainy season is not necessarily the most important period for precipitation. The probability of wetter-than-average conditions begins to increase more substantially around late December, then becomes greatest in January, February and March. Swain takes the possibility seriously enough that he told the Los Angeles Times he is clearing his calendar from January through March in case the weather gets “a little wild.”
Heavy rain, however, is not simply good news or bad news for California. A wet winter can replenish reservoirs, increase the Sierra Nevada snowpack and reduce drought risk. But the same amount of water arriving too quickly can produce floods and landslides. A warming climate adds another complication because more precipitation in the mountains can fall as rain instead of snow, while the snowline moves higher. The same storm can therefore send more water directly into rivers while storing less of it for months in the Sierra Nevada snowpack, which functions as one of California’s most important natural reservoirs. This is one reason why California cannot simply ask how much precipitation will fall. It also matters what form it takes, how quickly it arrives and exactly where it falls.
El Niño’s influence does not stop at California’s borders. Across the United States, one of its most important effects comes through changes in the path of the winter jet stream. During a strong El Niño, the Pacific jet stream typically shifts farther south and east, steering storms across the southern tier of the United States. As a result, the odds of a cooler and wetter winter increase from California across the southern states and into the Carolinas. By contrast, the Pacific Northwest, northern states and parts of Alaska often experience a milder winter. Snowfall can also be reduced in northern areas, while the chances of snow may increase at higher elevations in more southerly parts of the West. These are climate tendencies, however, not guaranteed forecasts for every state or every storm.
There is also one consequence that may initially appear beneficial. El Niño generally increases vertical wind shear over the tropical Atlantic, making it more difficult for tropical storms and hurricanes to form and intensify. NOAA therefore forecast a below-average Atlantic hurricane season for 2026, with the strengthening El Niño cited as one of the main reasons. According to NOAA’s August assessment, El Niño was already noticeably disrupting atmospheric conditions favorable for the formation of Atlantic tropical cyclones. In other words, the same global climate system that increases the risk of flooding and extreme winter precipitation in California can suppress hurricane activity in the Atlantic.
Swain’s most important warning is that a historic El Niño should not be interpreted as a single predetermined disaster scenario. It is impossible in September to say exactly which California river will overflow in February or how much rain Los Angeles will receive by the end of winter. What can already be identified is the change in the direction and magnitude of the risks. Far more heat has accumulated in the tropical Pacific, the ocean and atmosphere are strongly coupled in an El Niño state, the system continues to intensify, and all of this is occurring in a world already experiencing near-record warmth. Swain therefore says that in California, preparations that individuals and government agencies can reasonably undertake should be completed by November or December.
Along the coast, that preparation means accounting for the risk of high water, erosion and flooding. Inland, it means checking flood-prone areas, drainage systems and the surroundings of rivers and streams. Previous major El Niños, particularly the events of 1982-83 and 1997-98, demonstrated that much of California’s damage occurs when extreme wave action, elevated sea levels and powerful winter storms arrive at the same time. Based on measurements so far, the current event could become even stronger than those.
The 2026-27 El Niño will therefore also serve as an extraordinary natural experiment. An exceptionally powerful natural climate cycle is colliding with an atmosphere and ocean warmed by human activity. Swain says that combination makes it likely that at least some of the consequences will be unprecedented in the modern observational record. We do not yet know whether it will produce the wettest winter in California history, or which of the worst-case scenarios, if any, will materialize. But the possibility that an El Niño might develop is no longer hypothetical. It is here, it is strengthening rapidly, and over the coming months it will reveal what happens when one of the Pacific Ocean’s most powerful natural climate fluctuations encounters a planet warmer than it was during any previous El Niño of the modern meteorological era.
