El Niño tilts North America toward wet south, warm north this winter

NWS three-month forecast favors above-average rain in the Southeast

The developing El Niño is already the strongest on record by some measures and continues to grow, and could have profound effects on the weather across North America this autumn and winter, BBC News reported on 13 November. Impacts in the US and Canada are varied: there is a greater chance of a warm and dry winter in northern areas, and more risk of flooding rain further south.

El Niño is a naturally occurring climate phenomenon in which trade winds over the Pacific Ocean, which usually blow from east to west, weaken or reverse, allowing warm water to spread eastwards. That movement releases heat into the atmosphere and can drive major shifts in global weather patterns. The eastward shift of warm ocean water can push the Pacific jet stream south and extend it eastwards, directing rain-bearing weather systems across the southern US — a pattern that typically produces wetter winters during El Niño years.

The US National Weather Service’s latest three-month forecast indicates that wetter outcome is likely for the winter of 2026-2027, according to BBC News. South-eastern states including Florida, Georgia and the Carolinas have the greatest chance of seeing above-average rainfall. The El Niño winter of 1997-1998 brought significant flooding to the south-east and spun up tornadoes for Florida.

California could find itself in the firing line as well, with forecasts suggesting a wet winter is likely. Governor Gavin Newsom has already declared a state of emergency ahead of anticipated floods and mudslides. Areas scarred by recent wildfires are particularly susceptible, and the effects of climate change are exacerbating the risks. Severe flooding hit Long Beach, California in February 2024. Past El Niños have been wet in the state — winter 2023-2024 brought relief from drought — but the pattern is not fixed: in the strong El Niño of 2015-2016, California recorded below-average rainfall.

Coastal erosion and large waves could also be worsened by a rise in overall sea level driven by Kelvin waves. This is ocean water that sloshes eastwards across the Pacific during El Niño and reaches the west coast of the Americas, where it can travel no further. The resulting large-scale wave spreads along the coast over weeks and months, raising sea levels by 15cm (6in) or more.

The same shift of the Pacific jet stream generally leads to warmer, milder winters in the north-west US, the Northern Plains and the Ohio Valley, as well as western and central Canada. That reduces flooding risk, but lower rainfall can leave groundwater and river levels low, fuelling drought and wildfires in spring and summer, and ski resorts can suffer from a lack of snow. Long-range predictions suggest a dry, warm winter is more likely than usual this year — though El Niño does not guarantee this: the winters of 1982-1983 and 2015-2016 were wetter than normal in the north-west US.

Across much of the northern US and large swathes of Canada, the greater chance of dry and warm weather makes snow less likely. Further south, above-average precipitation typically leads to big snowfalls in the Californian mountains and the southern Rockies, which is good news for water stocks. It can also raise the probability of snow at lower levels in the southern and eastern US, but only if enough cold air is in the mix.

The area around Washington DC often receives more snowfall than usual in an El Niño winter — but not always. The winter of 2009-2010 delivered record-breaking snowfall in February 2010 that brought the city to a standstill, while widespread blizzards affected much of the US east coast in 2016. However, some of the city’s least snowy winters have also occurred during El Niño events, including 1972-1973 and 1997-1998, which brought just 0.1in (0.25cm) of snow.

Higher winter temperatures in eastern Canada and the north-east US can turn precipitation that would normally fall as snow into rain, raising the chance of freezing rain events. The so-called Great Ice Storm of 1998 was blamed partly on the effects of El Niño; Montreal, Canada was badly hit by that storm. Snowfall amounts are also decreasing overall in most places because of human-induced climate change, which makes predicting the impact of El Niño even more complex.

An added complication is El Niño’s possible effect on the polar vortex, a ring of strong winds circling the North Pole in winter, around 10-30 miles (16-48km) up in the stratosphere. The vortex holds cold, dense air in place above the Arctic, but it can sometimes weaken and disrupt, and there is some evidence that the energy El Niño releases into the atmosphere makes this more likely. Such disruptions can ripple down through the atmosphere, weakening the polar jet stream and opening the door for cold air to spill across Canada and the US, and can bring snowfall later in winter — but the links are far from clear and the effect does not always occur.

The central caution is that no two El Niños are the same and the impacts are not guaranteed: the odds are simply shifted in favour of certain outcomes.