The chances of an El Nino playing with our weather this summer have become more likely, according to a just-released report from the World Meteorological Organisation (WMO).

But while the probability of an El Nino becoming established between October to December have risen to 75 to 80 per cent, the system is expected to be weaker than earlier predicted.

In New Zealand, the ocean-driven system typically brings cooler, wetter conditions, bringing higher rainfall to regions that are normally wet, and often drought to areas that are usually dry.

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The report stated that while tropical Pacific Ocean surface temperatures had reached El Nino thresholds, and exceeded them in the far eastern portion of the basin, atmospheric indicators remained neutral, and hence an El Nino is not considered to have started.

As at early this month, model outlooks indicated a continued warming of the central and eastern tropical Pacific Ocean surface through the third quarter of 2014, with peak strength expected during the last part of the year, it said.

"Although a range of outcomes remain, models surveyed and expert opinion currently favour a moderate strength El Nino, while a strong event appears somewhat less likely than it had appeared earlier in the year."

Auckland climate scientist Dr Jim Salinger, who co-authored the WMO report, said El Nino systems traditionally brought more westerly winds to New Zealand.

"It tends to be a bit cooler in the west and south, and warmer in the north and east," he told the Herald.

"I guess the northern and eastern areas could be a bit droughtier than normal, and the west and south will be well watered and a bit cooler."

El Nino events have been devastating in the past - the last severe system in 1997 and 1998 caused a major drought that cost the country hundreds of millions of dollars.
While this year's emerging system would likely be much lighter, farmers still needed to consider how it might affect them, Dr Salinger said.

In El Ninos, farmers in the western, wetter parts of the country often faced significant damage to pastures from too much rainfall, and it was also harder for stock to thrive in the constant wet.

Those in the east, faced with dry conditions, needed to consider food availability for stock.

On the skifields, it remained to be seen whether the El Nino would bring the right conditions to extend ski seasons through into November.

Experts from the National Institute of Water and Atmosphere (NIWA) were reviewing the picture and are due to put out a long-range forecast next week.

NIWA's most recent report suggested a 60 per cent chance of El Nino developing this winter, and a 70 per cent chance of it being active in spring and summer.

Read the full WMO release here: