A rapidly intensifying El Niño in the Pacific is on track to become the strongest ever recorded.
El Niño is a weather pattern that causes hotter and drier conditions in most of Australia.
The warnings come from both the UN's World Meteorological Organization (WMO) and Australia's Bureau of Meteorology (BoM).
Here's what it means for Australia.
El Niño
El Niño is a natural warming of the central and eastern Pacific Ocean.
It happens every two to seven years.
In Australia, it typically means hotter, drier conditions across the south and east, and a higher risk of heatwaves, drought and bushfires.
The opposite pattern, La Niña, usually brings cooler, wetter weather instead.
A stronger El Niño doesn't automatically mean a bigger impact here, though this one stands out for how fast it's intensifying.
UN warning
On Thursday, the WMO said El Niño is "firmly established" in the tropical Pacific Ocean and will intensify into a "very strong event" in the coming months.
Forecasters now put the likelihood of El Niño lasting until February 2027 at almost 100%, the most certain the WMO has ever been in one of these updates.
WMO Secretary-General Celeste Saulo said: "If this trajectory continues, it may be stronger than anything since our monitoring began, so literally off the charts".
BoM warning
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The BoM's data shows El Niño is firmly underway in the Pacific, and it's already running hot.
The key patch of ocean scientists watch to track El Niño's strength was more than two degrees warmer than average in the last week of August, well past the point needed to call it an official El Niño.
The BoM says it could get hotter still.
Forecasts suggest it may reach levels beyond anything recorded since reliable records began in 1950, and the BoM expects the event to stick around until autumn 2027.
Here's how the BoM predicts El Niño will affect Australian rainfall for the month of September:
Climate change
A study published by the University of Michigan found El Niño events have grown nearly 40% stronger over the past 40 years when compared to the pre-industrial era.
Lead author Julia Cole said: "We don't see a time in the past where El Niños have been as strong as today, and we show that the strength of El Niño changes in parallel with the warming of global temperature".
The BoM's data backs this up locally.
Australia's winter was equal third-warmest on record, and Tasmania, Victoria and NSW all had their warmest winter ever.
What's next?
El Niño is forecast to peak in late spring or summer.
The BoM's outlook for spring points to below-average rainfall for southern and eastern Australia, and above-average temperatures south of the tropics.
Its effects on global temperatures could linger into 2027, potentially pushing that year past 2024's record as the hottest ever measured.
