The National Institute of Water and Atmospheric Research says there's more than a 90 percent chance that an official El Niño event will develop in the next three months.
It has released its forecast for August to October and says reduced trade winds in early August leading to warmer seas in the central equatorial Pacific are ripe conditions for El Niño.
NIWA said there was an 85 percent chance of the event continuing through summer this year.
So what does that mean for New Zealand?
NIWA said wind speeds are forecast to be stronger than normal, particularly across the South Island and lower North Island.
There could also be occasional heavy rainfall for the South Island, particularly in the west, as well the western North Island.
Temperatures were about equally likely to be near average or above average in the northern North Island and most likely to be above average in all other regions.
The institute said a colder than average three month period was unlikely, due in part to ongoing marine heatwave conditions near the South Island and lower North Island.
Increasing westerly winds, associated with El Niño, were forecast to transport warm air masses from Australia at times.
NIWA said July had warmer sea temperatures that ranged from 0.3˚C to 1.8˚C above average. It said this may reduce the intensity of episodic cold, southerly air masses that typically occur during late winter and spring.
