Intense weather ahead: Tasman rain-maker due to douse New Zealand

A moisture-packed system is set to saturate much of the country in what a forecaster says could prove among the year’s “busiest” weeks for weather.

While the South Island’s southwest is expected to cop the biggest downpours, regions spread from Fiordland to target="_blank">Northland can all expect rainfall throughout the week.

MetService has already issued several heavy rain watches - likely to be upgraded to warnings - ahead of the front’s arrival from the Tasman Sea overnight Monday, with most of Westland potentially in line for periods of “significant heavy” rain from Tuesday to Thursday.

The system was also due to bring plenty of wind and MetService has flagged the potential for severe north to northwest gales in exposed parts of Fiordland on Tuesday.

By Wednesday, the strongest winds were likely about inland Otago and Canterbury, the Marlborough Sounds, Wellington and southern Wairarapa.

Later, on Thursday, the picture changed as a series of lows developed on the slow-moving front over the South Island, while strong northerlies and rain moved on to the North Island.

“On Thursday, we’re still expecting heavy rain on the West Coast of the South Island - that’s on top of whatever has already fallen,” MetService meteorologist Juliane Bergdolt said.

For that day, MetService currently had “moderate” confidence - meaning a roughly 40 per cent likelihood - that warning amounts of rain would fall in northern Tasman, Nelson and western Marlborough.

On Friday, there were similar odds of rain reaching warning amounts in central areas of the North Island, from Taranaki to Bay of Plenty.

Bergdolt said it was too early to say just how much rain was likely to fall - but noted that warning amounts generally included 50mm over six hours and 100mm over 24 hours.

“For those areas where we have higher confidence, like Westland and central areas of the North Island later in the week, that’s where you’d expect to see those sorts of numbers.”

Niwa meteorologist Ben Noll said the event could prove critical for delivering much-needed rain to the South Island’s hydro lakes ahead of winter - but also for alleviating dryness in places like drought-hit Marlborough.

“The risk, however, is that some of the dry, cracked ground in these regions simply won’t be able to take the level of rain that’s coming - and then you get surface flooding,” Noll said.

“So, we’re potentially looking at one of New Zealand’s busiest weather weeks since the start of the year: it’s been a while since we’ve had a high-impact event like this covering so many regions.”

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.