The new World Meteorological Organization seasonal outlook predicted the El Nino event would become extremely strong, with temperatures in the central Pacific rising about 3.8 degrees Celsius above normal between November and January, peaking around December.
"We have not seen in our records 3.8. We have seen around three in some record El Ninos before, with very substantive impacts across the world. So we are beyond that limit," WMO Secretary-General Celeste Saulo said on Wednesday.
El Nino is the warm phase of the naturally occurring El Nino-Southern Oscillation cycle - a climate pattern that influences weather around the world.
It tends to cause higher global temperatures and disrupt rainfall patterns across large regions, meaning drought in some areas and heavy rains in others.
It is a natural climate pattern and is not caused by climate change. However, global warming can make El Ninos more intense.
El Nino-linked weather extremes could worsen pressure on housing, water, sanitation and health services, with flooding risks rising in southern Africa from November, the WMO said.
In Southeast Asia, warmer conditions and a lack of rainfall could impact agriculture and water supplies, while Latin America could see increased rainfall, according to the WMO.
The WMO and the UN Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator, Tom Fletcher, called for urgent action to get ready.
"This El Nino is the behemoth, it is a wrecking ball. It will tear through Latin America, Eastern and Southern Africa, Asia and the Pacific with extreme heat, droughts and flood," Fletcher said, warning of massive humanitarian consequences.
Some 50 million people could face increased hunger, he said.
