Long-range weather outlook: Is NZ in for a rain-soaked Labour weekend?

There are signs of heavy downpours leading into the coming long weekend. And does another warm, wet La Nina still loom beyond? Jamie Morton explains.
It might feel pleasantly summerlike around New Zealand right now, but rainclouds hang over the long-range forecast for Labour weekend.
After more generally settled weather for the next two days, with maximum temperatures hitting the early 20s for many centres, a northwest flow is due to arrive in the mid to late week.
That brings a moderate chance of heavy rain over most of the western South Island and severe gales around central New Zealand on Friday.
MetService forecaster David Miller said it would again be regions like Westland that bore the brunt of the system: “And because it’s quite a prolonged event, there’s potential for this to be fairly significant.”
Further north, MetService is also forecasting rain and northerlies for multiple locations on Friday and Saturday, including Auckland, Hamilton and Tauranga.
There was potential for late-week downpours to get even heavier if the coming flows tapped into subtropical moisture from a globe-circling pulse of rain and thunderstorms, called the Madden Julian Oscillation, that happens to be passing through our wider region around that time.
“This is one of the bigger pulses that we’ve seen in a quite a while emerging to the north of New Zealand, and it’s going to have implications in both hemispheres,” the National Institute of Water and Atmospheric Resarch’s (Niwa) Ben Noll said.
He added there was the possibility of another large-scale rain event here, drawing comparisons with the subtropical components of this month’s Dunedin deluge.
Lying yet further beyond the horizon is the prospect of another warm, wet La Nina summer for northern New Zealand.
Forecasting agencies have been signposting the ocean and atmosphere-driven climate pattern’s return for most of 2024 – and with it the glum likelihood of wet and muggy weather for holidaymakers in places like Northland, Auckland, the Coromandel, the Bay of Plenty and Hawke’s Bay.
But such predictions have recently been growing somewhat fuzzier.
While Niwa’s most recent three-month outlook offered a 60-70% chance of La Nina officially forming by 2025, the US National Oceanic and Atmospheric Administration recently downgraded its odds to 60% – having earlier put them as high as 85%.
Australia’s Bureau of Meteorology, meanwhile, picked the climate state to remain “Enso-neutral” – neither El Nino or La Nina – through to February.
Any La Nina that did develop would likely be relatively weak and short-lived, the bureau said.
Regardless, Noll said we could expect to see “La Nina-like” conditions over coming months.
A classic feature of the last few La Nina summers have been moisture-packed “atmospheric rivers” snaking their way down here from the subtropics with humid northeasterly flows.
Noll said the eastern and northern sides of both islands should be “on an elevated alert level” for these potential rainmakers as summer approaches.
Another La Nina-like factor was unusually warm water engulfing the country.
“The emergence of a marine heatwave is definitely possible in November,” he said, adding the December-to-January period was the one to watch for sea temperatures peaking.
“As we’ve certainly seen, when these events do get going, they also enable low-pressure systems in the region to carry more water, and when that comes to land, it can mean potential for heavier rainfall,” he said.
“We certainly saw that marine heatwave set-up coincide with the likes of the Auckland Anniversary weekend flooding and Cyclone Gabrielle.”
Forecasters have been cautious not to give the impression another La Nina would mean a repeat of last year’s extreme summer.
That proved the climax of three back-to-back La Nina years that piled up ocean heat – essentially energy drink for big rain-makers – across a vast swathe of water called the West Pacific Warm Pool.
“Right now, the warm pool is not as advanced, in terms of its warming, as it was as we went into that that summer,” Noll said.
For the coming November-to-April season, Niwa has forecasted for New Zealand a normal to below-normal risk of tropical cyclone activity, meaning there’s potential for zero to one systems coming here.
But Noll added vigilance was always needed.
“For anyone who lives in the eastern or northern North Island, it’d pay to spend an extra minute thinking about that and having a plan in place,” he said.
“Because we know that La Nina events have brought cyclone activity here in the past.”
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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