WASHINGTON — The El Niño building in the Pacific Ocean has an 83 percent chance of becoming the strongest on record, the US National Oceanic and Atmospheric Administration said, as it released a winter outlook that favours a wet southern United States and a mild north from December through February.

Weekly sea surface temperatures in the central Pacific region known as Niño 3.4 were 2.4C above average as of 15 October, according to NOAA's Climate Prediction Center, with waters off South America running as much as 4C warmer than normal in places.

The event's strength matters well beyond the United States. El Niño shifts rainfall and storm tracks across the tropics and mid-latitudes, pushes up global average temperatures and heats the seas around coral reefs. NOAA said in October there is an 83 percent chance that the strongest El Niño since its records began in 1950 is now unfolding, according to NASA.

Ken Graham, director of NOAA's National Weather Service, said on Thursday that "a very strong El Nino will likely bring a southward and eastward shift to the jet stream for the contiguous U.S." That shift tilts the odds toward stormier weather across the South, from California to Florida and up the East Coast, and toward milder, drier conditions from the Pacific Northwest to the Great Lakes.

The outlook expects drought to improve or end across the Southwest, the southern Plains, the Southeast and parts of the East Coast by late January. But Graham warned that "too much precipitation on dry soil can fuel destructive flash floods and mudslides," and urged communities to plan ahead.

Michelle L'Heureux, a physical scientist at the Climate Prediction Center, told ABC News that El Niño can set "preferred pathways for weather systems to repeat over certain locations." The forecast expresses seasonal odds rather than certainties, she said. "This is why our seasonal forecasts are expressed in terms of probabilities."

El Niño is the warm phase of a natural cycle in the tropical Pacific. Trade winds weaken, warm water that normally pools near Indonesia spreads eastward, and the atmosphere above responds. NASA satellites first picked up the signal in January, when a Kelvin wave, a pulse of warm water moving along the equator, appeared near Micronesia, the agency said in an account published on 15 October.

By early April those waves had reached the eastern Pacific, according to NASA. Sea levels have since risen by 30 to 40 centimetres in places from South America north to California and fallen 15 to 20 centimetres around Indonesia and northern Australia, measurements from the Sentinel-6 satellites show.

"Kelvin waves are like the heartbeat of El Niño," said Josh Willis, project scientist for sea level missions at NASA's Jet Propulsion Laboratory. "And our sea level satellites let us peer into the ocean and watch every beat."

Whether this El Niño is already a record depends on how it is measured. Daily readings using the older Oceanic Niño Index would make it an all-time record, weather.com reported, but NOAA's official classification uses three-month averages and a relative index that strips out the background warming of the oceans from climate change. By that measure the event had not reached record levels through August.

"Daily and weekly changes in sea-surface temperature are not tied to El Niño impacts over the United States," Erica Grow Cei, a NOAA public affairs meteorologist, told weather.com in an email.

Climate scientist Daniel Swain wrote on his Weather West blog on 12 October that the event was already at or near record intensity by almost any metric and would likely peak between late November and January, most probably in December. He put the odds of a wetter-than-average winter in California above 90 percent.

The event has already left a mark on the Atlantic hurricane season, where El Niño's wind shear tends to suppress storms. Through mid-October the basin had produced nine named storms but only one hurricane, Isaias, which reached that strength late on 7 October, the latest first hurricane of the satellite era, weather.com reported.

Scientists in Australia are watching the Great Barrier Reef. A study published this month in Weather and Climate Dynamics by researchers at Monash University and the University of Melbourne found that spells of weak winds known as doldrums allow the Coral Sea to heat up, and that a strong El Niño raises the background risk that a prolonged calm could tip the reef into a marine heatwave and mass bleaching.

"When doldrums hang around, the ocean remains in a heating phase for longer," said Lara Richards, who led the research as a doctoral student at Monash and is now a research fellow at the University of Reading.

NOAA's next monthly update of the three-month outlook is scheduled for 19 November, and the agency said El Niño's effects on US weather are expected to be most widespread from December through March.