Weather: El Nino to deliver NZ parting shot with colder-than-average May

Forecasters are picking a cool, dry May for New Zealand – but not before a patch of wet weather later this week.

Sunny conditions that have marked the return to school are forecast to take a turn around mid-week, starting with a bout of rain for the western and lower South Island.

On Wednesday, the weather system is expected to roll over the eastern South Island and North Island over Wednesday and Thursday.

“This system moves through quickly and by the end of Thursday most of the country should be in the clear, apart from the occasional shower for southern and western coastal areas,” MetService forecaster Mmathapelo Makgabutlane said.

While the system was expected to take the rain with it, cooler temperatures were likely to remain.

“The switch to a chilly southwesterly flow will be the most noticeable thing this week once the rain clouds have cleared,” she said.

“Thursday morning may see inland parts of the South Island waking up to frost, while daytime temperatures will feel cooler all round during the second half of the week.”

Cool southwesterly winds have been a hallmark of an all-but-faded El Nino climate pattern, which last month delivered New Zealand’s coldest March in more than a decade.

While the event has been officially declared over, its influences were likely to linger through a colder-than-average May.

“Broadly speaking, it’s going to be a continuation of what we’ve experienced over autumn,” Niwa forecaster Ben Noll said.

“There’ll be some nights and mornings that feel pretty darn chilly, but given our recent pattern, I don’t think it’ll be anything shocking.”

Another major contributor to a colder May would be plenty of high pressure predicted to develop around New Zealand.

“Sometimes that high pressure will be over the country - and sometimes it’ll be to the south or the west – but it’ll bring cold mornings, drier days for the time of year, and maybe even a little reduction in wind,” Noll said.

For skifield operators, the cool weather didn’t necessarily make for good snowmaking ahead of the season.

“If there was more moisture in the mix, you might get a big dump of early snow; but given the proximity of that high pressure, I’m not seeing a whole lot of moisture available at this point,” Noll said.

Once winter arrived, and with El Nino no longer at the steering wheel, forecasters expected more variable weather patterns.

Beyond that, it appeared increasingly likely that La Nina would return for the fourth time this decade – reopening the door to easterly flows and more subtropical rainmakers later in the year.

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.