Winter weather: The big trends forecasters are picking this season

The first half of 2024 brought with it droughts, deluges, endless westerlies and a terribly cold end to autumn. So what’s ahead? Jamie Morton looks at three big questions facing forecasters regarding the months to
come.

Was May’s chill a prelude to a freezing winter?

Yes, last month was unusually cold – and once the final numbers are crunched, it might go down as New Zealand’s chilliest May in more than a decade.

But that wasn’t a sign of things to come for the season ahead, with the National Institute of Water and Atmospheric Research’s (Niwa) just-issued climate outlook instead picking near or above-average temperatures for winter.

“On social media, we’ve heard a lot of people saying, ‘Wow, it’s been pretty chilly relative to recent autumns, and is that going to mean winter will be really bad?’” Niwa forecaster Ben Noll said.

“That’s a myth that can be dispelled, because those southerly winds that have been common over autumn are going to give way to more westerlies and northwesterlies, with air masses coming more from Australia than the Southern Ocean.”

That change was coming with the demise of El Nino – the climate driver that’s been shaping our weather since spring, and which has been helping to bring up bitterly cold southwesterly flows from below us.

Out of the gate, the season was forecast to begin with a warmer spell ahead of a return to more typical conditions over the second half of June.

Noll said while there’d be cold snaps over the season, they’d likely be brief.

What about ski season?

“We get asked this question a lot, and while we don’t provide a direct snowfall outlook, you obviously need two key ingredients: temperature and precipitation,” he said.

“With temperatures unlikely to be colder than average, and precipitation also unlikely to be higher than normal in those key ski areas, that’s two strikes against a really active snow season.”

Still, Noll said one or two good dumps of snow could change things quickly for ski field operators – and these could still happen, despite that overall picture.

Why is it still so dry?

Even at the doorstep of winter, the New Zealand Drought Index shows large swathes of our eastern coast are still running dry.

Niwa’s outlook didn’t offer much in the way of a reprieve, picking near to below-average rainfall for nearly all regions – with rain-makers expected only irregularly.

“For many farmers, particularly in the northern and eastern South Island, this is a bit of a depressing outlook, because those areas are in desperate need of rain,” Noll said.

“In these places, we are looking at soil moisture deficits running to 90-110mm – that’s very severe, and it’s been that way for many months.

“So, they could use above-normal rainfall, but that’s the least likely outcome for the lower North Island, as well as the northern and eastern South Island.”

That eastern dryness had come as a result of El Nino’s persistent westerly winds and constant ridges of high-pressure keeping rain away.

While we’d now moved into an “Enso-neutral” regime – meaning no major climate driver was at the steering wheel – Noll said climate-forcing patterns on the other side of the planet were helping to keep the tap off.

But he added the horizon wasn’t completely clear, with models pointing to a potential rainfall event for the North Island and northern South Island in the second week of June.

What lies beyond winter?

El Nino’s sibling La Nina – which delivered the North Island constant warm and wet weather for the first three years of the decade – is likely set to return, with Niwa giving a 60 to 70 per cent chance of the pattern forming up over spring.

Noll stressed this system was still developing – and we shouldn’t assume next summer would be as extreme as 2023′s.

That summer came on the back of years of ocean warmth which had piled up in the West Pacific and featured standout weather events which were simply freakish.

“I’d add we still need to consider this La Nina is likely to happen on the backdrop of a very warm globe – and, much like we saw with the impacts of this El Nino, it could well behave in some unusual ways,” Noll said.

“So, there may be a couple of surprises up its sleeve, but we’re not suddenly expecting to revert back to 2023.”

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.