Changeability a theme for NZ weather in coming months - NIWA

Changeability will be a theme in New Zealand's weather patterns over the next three months, according to NIWA's latest weather outlook.

It comes as La Niña — which brought heavy rains to the northeast of the North Island and dry conditions down south — transitioned to ENSO-neutral in late March.

El Niño and the Southern Oscillation (ENSO) refers to variations in the temperature across the surface of the equatorial Pacific Ocean, and influences rainfall, temperature and wind patterns around the world, according to NIWA.

NIWA's analysis indicates that ENSO-neutral conditions have a 80% chance of continuing through June, after which El Niño has a 65 to 70% chance of developing during winter and continuing through spring.

For the three-month period, rainfall is about equally likely to be near normal or above normal in the north and west of the South Island and most likely to be near normal in all other regions, NIWA said.

Should the climate system migrate toward El Niño in the months ahead, June could turn out less wet than normal, particularly if southerly quarter winds take hold.

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Seasonal temperatures are most likely to be above average in the east of the South Island and about equally likely to be near average or above average in all other regions.

While a colder than average season is unlikely due to the persistence of warmer than average sea temperatures (SSTs), more variable circulation and wind patterns will lead to more variable temperatures.

However, El Niño's potential arrival could result in cooler winter temperatures, particularly relative to recent record warm winters, including an increased risk for frosts.

According to NIWA's regional predictions for April to June 2023, temperatures in Northland, Auckland, Waikato, Bay of Plenty, the central North Island, Taranaki, Whanganui, Manawatū, Wellington, Gisborne, Hawke's Bay and Wairarapa are all equally likely to be near average or above average (45% chance each).

Rainfall totals are also most likely to be near normal (45% chance). However, rainfall in Northland, Auckland, Waikato and Bay of Plenty may tend lower than normal during early winter as southerly winds may become more common.

Further south, temperatures in Tasman, Nelson, Marlborough and Buller, West Coast, the alps and foothills, inland Otago and Southland are about equally likely to be near average (45% chance) or above average (50% chance).

Rainfall totals in Tasman, Nelson, Marlborough and Buller are also about equally likely to be near normal (40% chance) or above average (35% chance).

On the West Coast, the alps and foothills, inland Otago and Southland, rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance).

In Coastal Canterbury and east Otago, temperatures are most likely to be above average (55% chance). Continued marine heatwave conditions will likely prevent long-lasting below average air temperatures. Rainfall totals are most likely to be near normal (45% chance).

A period of northerly winds in mid-April may increase the chance for heavy rainfall.