Niwa’s summer outlook is out: What’s in it?

Niwa is picking a warm - yet often wet - summer for New Zealand, thanks to La Niña’s usual mixed bag and a major marine heatwave is already enveloping the country. Photo / Sarah Ivey

Niwa is picking a warm - yet often wet - summer for New Zealand, thanks to La Niña’s usual mixed bag and a major marine heatwave already enveloping the country.

The agency’s just-issued climate outlook for the season predicted above-average temperatures throughout most of the country, with more subtropical northeast winds likely to bring more scorching days for inland and western areas.

For eastern spots, however, fewer northwesterly foehn winds spells fewer hot days – a regional contrast typical of La Niña’s traditional influence over summer.

The ocean-driven system usually brought more north-easterly winds and rainy conditions to the North Island’s northeast, and drier conditions to the south and southwest of the South Island, although there were always some exceptions.

This time around, summer temperatures were equally likely to be near or above average in the east of both islands, very likely to be above average in the north of the North Island and the west of both islands, and most likely to be above average in the north of the South Island.

Rainfall, meanwhile, was equally likely to be near or above normal in the north of the North Island and the east of both islands, most likely to be near normal in the west of the North Island and north of the South Island, and most likely to be below normal in the west of the South Island.

A marine heatwave that’s bedded in for the season – coastal sea surface temperatures have been running between 1.1C to 1.7C above average – would help fuel more heavy downpours.

“In connection with La Niña, warmer than average seas in the Southwest Pacific are expected to fuel frequent low pressure systems, some of which will drop down into New Zealand, bringing the risk for occasional heavy rain and flooding, similar to what was experienced last summer,” the outlook said.

“The Madden-Julian Oscillation (MJO), a tropical pulse of rain and cloud, is another driver that will influence weather patterns around New Zealand this summer.”

But dry spells would still play out during periods of high pressure, especially in those western and inland spots, and one such stretch could occur from mid-December to early January.

There was also a higher risk of drought in western and lower parts of both islands, especially the South Island, inclusive of Otago, Southland, and the West Coast.

Elsewhere, the outlook found New Zealand also faced a slightly elevated chance of receiving a visit from an ex-tropical cyclone over the next few months.

Each season - usually around late summer - at least one ex-tropical cyclone veers within 550km of the country, packing gale-force winds and enough moisture to drive torrential rain.

Earlier, Niwa meteorologist Ben Noll warned Kiwis to expect a “disjointed” slide into summer.

“I don’t think it’s going to be a seamless transition, so I’d caution against the idea that December is going to be this beautifully sunny month.”

While some spots further south might have already started trending drier, for other places more exposed to easterly and north-easterly winds – the likes of Northland and Auckland – Noll said it was “going to take a little longer to get there”.

“Right now, as we go through December, the chances of getting drier than normal weather are higher the further west and south you are in both islands.”