Climate outlook: What's in store for winter?

Weather reports predict that next week will be stormy and wet for most of the country. Photo / Jose Antonio Maciel, Getty Images

Temperatures will be on the milder side during the slide into winter, according to a just-released long-range climate outlook.

The National Institute of Water and Atmosphere's outlook from May to July stated temperatures would be either average or above average for all regions of New Zealand, although cold snaps and frosts could be expected in some parts of the country as autumn marched into winter.

Coastal waters were also forecast to be in the near average to above average temperature range, while rainfall levels were likely to be either normal or below normal for the east of the South Island, and most likely near normal for all remaining regions of the country.

Across the top half of the country, covering Northland, Auckland, Waikato and the Bay of Plenty, temperatures are about equally likely - a 40-45 per cent chance - to be in the above average or near average range.

However, this month would likely see some "very changeable weather", MetService meteorologist Georgina Griffiths said this week.

This weekend was forecast to be settled and dry, but active lows over the Tasman Sea would soon bring in some "rough and tumble weather".

"Next week is signalled as stormy and wet for most of the country, with an elevated risk of extreme rainfalls in western parts of both Islands, as well as for Nelson."

By the middle of May, a pattern change to cooler southwesterlies heralded in a change to relatively dry conditions for regions in the north and east of both Islands.

Temperatures in the first half of May were likely to swing from one extreme to the other - and then settle slightly cooler for the second half of May, she said.

"After unseasonably mild northwesterlies next week, a rapid change to unusually cool conditions is expected by mid-month. Eastern regions will experience the largest extremes in temperature in May."

She noted April had been extremely wet between Waikato and Wellington, while Hamilton received more than twice the usual April rainfall, at more than 200 per cent of normal.

It was also the wettest April on record for Whanganui, Palmerston North and Paraparaumu, and was also a very wet May along the West Coast, with Milford Sound recording over 900mm of rain, and Greymouth and Hokitika well in excess of 300mm.

But over the next three months, soil moisture levels and river flows were also likely to be in the near normal or above normal range in the west of the North Island and the west of the South Island, and most likely near normal the east of the North Island and the north of the South Island, NIWA reported.

In the north of the North Island, these were equally likely to be near normal or below normal range, while in the east of the South Island, soil moisture levels were about equally likely to be in the below normal or near normal range, and river flows are most likely to be in the below normal range.

The calmed conditions over the period sat against a background of higher than normal mean sea level pressures for the New Zealand region, extending across the Tasman Sea.

Lower than normal mean sea level pressures were also expected well south of the country, and this circulation pattern was likely to produce more south-westerly quarter wind flows than normal.

Meanwhile, warming of the sea surface across the equatorial Pacific Ocean had continued last month, building upon the warmer than normal waters observed in previous months.

These patterns, in combination with weaker trade winds and increasing cloudiness near, and to the east of, the International Date Line were consistent with weak El Niño conditions.

International guidance suggested there was a close to 80 per cent chance of an El Nino system commencing over the next three months, although forecasts tended to be less reliable presently than at other times of the year, NIWA said.