Strong El Niño points to a drier, warmer winter in the north and more rain in the south across North America

A developing El Niño that continues to grow stronger could have profound effects on weather across North America this autumn and winter. By some measures it is already among the strongest on record.

El Niño is a naturally occurring climate phenomenon in which the trade winds over the Pacific Ocean, which usually blow from east to west, weaken or reverse, and warm water spreads eastwards instead.

That warm water releases heat into the atmosphere, which can drive major shifts in weather patterns around the world.

Across the United States and Canada the impacts vary, with a greater chance of a warm and dry winter in northern areas and more risk of flooding rain further south.

There is also some evidence that El Niño can raise the likelihood of disruption to the stratospheric polar vortex, which can in turn bring snowfall later in winter.

No two El Niños are the same, however, and the impacts are not guaranteed; the odds are simply shifted in favour of certain outcomes.

The eastward shift of warm ocean water can push the Pacific jet stream south and extend it eastwards, directing rain-bearing systems across the southern United States. That is why El Niño winters are typically wetter there.

The latest three-month forecast from the US National Weather Service suggests that pattern is likely for the winter of 2026-2027.

South-eastern states, including Florida, Georgia and the Carolinas, have the greatest chance of above-average rainfall. The El Niño winter of 1997-1998 brought significant flooding in the south-east and spun up tornadoes in Florida.

California could also face a wet winter. Governor Gavin Newsom has already declared a state of emergency ahead of anticipated floods and mudslides. Areas scarred by recent wildfires are particularly susceptible, with climate change exacerbating the risks.

By contrast, the north-west United States, the Northern Plains and the Ohio Valley, along with western and central Canada, generally see warmer and milder winters. That lessens flood risk, but reduced 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.

In the south, above-average precipitation can also mean big snowfalls in the Californian mountains and the southern Rockies, which is good news for water stocks. It can raise the chance of snow at lower levels in the southern and eastern United States, but only if enough cold air is present.

The polar vortex is a ring of strong winds circling the North Pole in winter, about 10 to 30 miles (16 to 48 kilometres) up in the stratosphere, holding cold, dense air over the Arctic. When it weakens and is disrupted, cold air can spill across Canada and the United States, but this does not always occur and the links are far from clear.