The El Niño developing over the Pacific Ocean could be "the largest in living memory," according to the Met Office, the U.K.'s national weather service.
The climate phenomenon, which the Met Office is calling "unprecedented", can boost global temperatures and increase the likelihood of extreme weather. Forecasters said it's likely to become the largest of its kind since the 19th century, adding that 2027 is very likely to replace 2024 as the warmest year on record globally.
El Niño is a periodic warming of sea surface temperatures in the eastern Pacific caused by weakening trade winds. It occurs naturally every two to seven years and tends to last between nine and 12 months.
The Met Office's latest forecasts show a rise of more than 5.4°F (3°C) in equatorial Pacific sea-surface temperatures later this year. A typical El Niño brings warming of 1.8°F to 3.6°F (1°C to 2°C), with 3.6°F (2°C) considered a major event.
"I have never seen a forecast that looks like this," Adam Scaife, a professor at the Met Office, told ITN News. "The strength of El Niño in our forecasts is already at very high levels, even at this early stage.
"It peaks in November, December, and by that stage, it looks very much like it's going to be a record event, the biggest in recent history, the biggest in living memory, and perhaps even the biggest for over a century."
The forecast is consistent with warnings from U.S. forecasters that a historically significant El Niño may be looming. Berkeley Earth, a nonprofit climate research group, found in July that there is a 90% chance tropical Pacific temperatures will exceed all records over about 150 years of observations. The National Oceanic and Atmospheric Administration's Climate Prediction Center told Yahoo News it has classified the 2026 El Niño as ranking "among the largest El Niño events."
NOAA's outlook for December through February already shows a clear pattern taking shape, with the impacts of El Niño expected to become more pronounced as winter progresses.
"For the U.S., the impacts of El Niño are most clearly evident from fall through spring," Michelle L'Heureux, a physical scientist at NOAA's Climate Prediction Center, told Yahoo News.
"Generally speaking, El Niño tends to increase the storm track over the southern tier of the United States, increasing the chances of above-average precipitation," she added. "At the same time, because the storm track is shifted southward over the U.S., regions across the northern U.S tend to be drier. The northern U.S. has the highest chances for above-average temperatures, too."
As it strengthens, El Niño's effects are expected to become increasingly apparent around the world — including in the U.S. Forecasters have predicted that warm summer temperatures, including the hottest July on record, may persist for longer than usual.
Parts of the Northeast were still experiencing hot and humid conditions this week as severe weather swept through the region. On Thursday, storms tore through a beach on Long Island, stranded dozens of people in cars in New Jersey and knocked out power in parts of Delaware's capital, the Associated Press reported.
Mike Lichniak, chief meteorologist at Delaware-area TV station WBOC, warned earlier that morning that rising humidity would push temperatures into the 80s and 90s, with a heat index above 100°F in some areas. Severe thunderstorms and an isolated tornado followed that afternoon and evening.
In NOAA's Climate Prediction Center seasonal temperature outlook for fall 2026, released on Aug. 20, the agency favors above-normal temperatures across nearly the entire country from September through November. The Pacific Northwest, the northern Rockies and parts of coastal Alaska have the highest odds, at 50% to 70%, of a warmer-than-normal season.
Much of the rest of the U.S., including the South, Midwest, Northeast and Florida, also has elevated odds of above-normal temperatures, at 33% to 50%.
The lone exception is a pocket of the Ohio Valley, covering parts of Illinois, Indiana, Kentucky and Missouri, where forecasters see "equal chances" of above-, near- or below-normal temperatures. No part of the country is expected to see a cooler-than-average fall.