Droughts and flooding are likely to follow in otherwise fairly stable regions as worldwide climate patterns are thrown into disarray for the first part of 2019.
The most recent El Nino event ended in 2016, and was associated with catastrophic coral bleaching on the Great Barrier Reef as global temperatures reached the highest levels ever recorded.
While the predicted event is not likely to be as severe, it could still bring dangerous weather to vulnerable areas around the world.
“The forecast El Nino is not expected to be as powerful as the event in 2015-2016, which was linked with droughts, flooding and coral bleaching in different parts of the world,” said Maxx Dilley, director of WMO’s Climate Prediction and Adaptation branch.
“Even so, it can still significantly affect rainfall and temperature patterns in many regions, with important consequences to agricultural and food security sectors, and for management of water resources and public health, and it may combine with long-term climate change to boost 2019 global temperatures.”
Scientists have been predicting the likelihood of such as event since May, with the chances gradually increasing as the year progresses.
The UN agency said that ocean temperatures have already reached weak El Nino levels in parts of the tropical Pacific, although the atmospheric patterns that accompany these changes have not yet materialised. Forecasters in the US and Australia have already warned of an approaching El Nino.
Chances of dry conditions and drought will increase in nations from the Caribbean to southern Africa, while heavy rainfall will likely strike parts of the US and Europe.
Under the most extreme conditions currently predicted by WMO, sea temperatures will rise up to 1.2C above average.
The Met Office has previously warned that El Nino could impact winter weather in the UK next year.
“An El Nino can create wetter and windier conditions in the first half of winter and it can bring a colder and drier second half, but El Nino is just one factor and others will vie to affect our winter,” said Met scientist Professor Adam Scaife.
“For example, the Quasi-Biennial Oscillation with its 14-month pattern of alternating easterly and westerly winds along the equator can weaken or strengthen the jet stream.”
He also noted that the phenomenon will have a marked effect on global temperature measurements in 2019.
The effects of this warm phase helped boost 2016 to be the hottest year on record. The following year was cooler, but the hottest year ever recorded in which an El Nino event has not taking place.
Though El Nino is part of a natural planetary cycle, previous research has suggested its effects are made worse by human-driven climate change.