Federal climate forecasters say the strengthening El Niño has a greater than 90 per cent chance of becoming very strong during the Northern Hemisphere autumn and winter, increasing risks of disruptive rainfall, drought and heat across several regions.

The US Climate Prediction Center said in its September assessment that El Niño had intensified during August, with sea-surface temperature anomalies exceeding 3C in the eastern equatorial Pacific. The closely watched Niño-3.4 index reached 1.8C above average, while unusually warm water also extended deep beneath the equatorial Pacific.

The World Meteorological Organization expects the event to peak towards the end of 2026 and persist through February 2027 with a probability close to 100 per cent. Its multi-model seasonal forecast put the September-November Niño-3.4 anomaly near 3.6C, a level consistent with an exceptionally powerful event.

Neither NOAA nor the WMO, however, has declared that 2026 will produce the strongest El Niño on record. NOAA describes the expected category as “very strong”, while the WMO does not use the popular label “Super El Niño”. Whether the event ultimately surpasses earlier episodes can be established only after observations covering its peak are available.

El Niño develops when unusually warm surface waters spread across the central and eastern tropical Pacific and interact with the atmosphere, weakening or reversing normal circulation patterns. The changes can shift jet streams, rainfall and temperatures thousands of kilometres from the Pacific, making the phenomenon one of the strongest natural drivers of year-to-year climate variability.

A powerful event does not guarantee extreme weather at any particular location. Forecasters stress that El Niño changes probabilities rather than dictating individual storms, and its effects can be strengthened, weakened or displaced by conditions in the Indian and Atlantic oceans and other atmospheric patterns.

For the United States, El Niño typically favours wetter conditions across parts of the southern tier during winter while tending towards warmer conditions across parts of the north. NOAA's September seasonal outlook also favours above-normal temperatures across much of the country, demonstrating that the developing event will interact with other climate influences rather than produce a uniform pattern.

South Carolina's catastrophic October 2015 floods illustrate that distinction. More than 20 inches of rain fell in some areas between October 1 and 5, causing major river flooding and 19 deaths. A National Weather Service assessment attributed the deluge to a slow-moving upper-level low, a persistent plume of tropical moisture associated with Hurricane Joaquin and already wet ground. The strong 2015-16 El Niño formed part of the broader climate setting, but was not the sole cause of the disaster.

Globally, the stakes extend well beyond the United States. El Niño is commonly associated with increased rainfall and flood risk in parts of South America, East Africa and the southern United States, while drought risks can rise in Australia, Indonesia, southern Africa and parts of South Asia. The precise pattern depends heavily on season and regional conditions.

The WMO has warned that the event will coincide with exceptionally warm oceans and a likely positive Indian Ocean Dipole. Its September-November outlook favours above-normal temperatures across almost all land areas, alongside substantial shifts in rainfall. In the Greater Horn of Africa, seasonal forecasters have identified heightened chances of wetter conditions in several areas, bringing both improved water availability and greater flood risk.

Southern parts of the South-West Indian Ocean region face a different threat. A regional climate outlook issued this month identified increased drought and water-stress risks for South Africa, southern and central Mozambique, Malawi and south-western Madagascar, while wetter northern areas face greater danger from floods and landslides.

Health agencies are also preparing for secondary effects. The WMO and World Health Organization have warned that extreme heat, flooding and drought can disrupt food production, water supplies and health services, while altered temperature and rainfall can change risks from mosquito-borne diseases.