Niwa’s summer forecast: Scorching days next week, but muggy months to follow

Summer is forecast to start with a sizzle, with more 30C days on the cards for some places over coming days – but a change to “La Nina-like” conditions is likely to spell muggy conditions later in the season.
“We’re expecting a very warm to hot week for a good chunk of the country next week,” Niwa forecaster Chris Brandolino said.
“And it looks like those eastern areas are going to see the best chances for the hot temperatures – we’re talking 30C-plus highs for multiple days next week.”
For the west of the country, however, Niwa’s just-issued seasonal climate outlook also carries some damper news, with a period of strong westerly winds over early to mid-December possibly bringing bouts of heavy rainfall.
“We’re talking the second week, maybe bleeding into the third week, of December, where western areas are going to be favoured to see some significant rainfall.”
Beyond that, though, the picture was predicted to change, with increased northeasterly winds likely to bring down rainmakers and humid conditions from around the New Year.
“Then, over late January and into February, our expectation is that we’ll see those northeast flows increase in frequency.”
While the outlook gave coin-toss odds of another La Nina officially forming over summer, Brandolino said it was those “La Nina-like” flavours that people in areas like Auckland and Coromandel could nevertheless expect.
“We’re looking at increased odds for rain events, possibly heavy ones, for the northern and eastern parts of the North Island – but also increased odds for dryness for areas like the west and southwest of the South Island.”
He added the muggy outlook for the northeast in late summer didn’t mean a repeat of the extreme La Nina summer of 2022-23 – its unprecedented deluges proved the climax of a three-year run of the climate pattern.
While air temperatures were likely to be above average right across New Zealand over summer, Brandolino said that heat would come in different flavours.
The mid-summer shift to north-easterly winds was likely to reduce the chances of westerly flows like next week’s, which brought maximums pushing into the 30s for places like Hawke’s Bay.
“What we may see in lieu of that is more persistent, low-to-mid-grade warmth, accompanied with higher humidity, particularly for the North Island,” he said.
“The one caveat to that is areas along the west of both islands, which may end up with some highly unusual warm days.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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