A subtropical moisture-packed atmospheric river is set to drench the upper north from this weekend. Image / Niwa Weather
A subtropical moisture-packed atmospheric river is set to drench the upper north from this weekend. Image / Niwa Weather
Another subtropical moisture-packed atmospheric river is set to soak the upper north from this weekend – along with “juicy” humidity and unusually warm overnight temperatures.
Niwa meteorologist Chris Brandolino said wet weather forecast for the upper North Island over the weekend and into next week would come with a tropical low forecast to develop near New Caledonia.
While the system itself wasn’t likely to be intense, it was expected to interact with a strong high that’d be centred near the Chatham Islands by the weekend.
“These two systems working together will create what’s called a pressure gradient – or a change in air pressure over distance – and excite the air so we get a good flow at low levels, or to about 1500m up,” he said.
“That means there’ll be a flow of air coming down from deep in the subtropics, which will serve as fuel for the rain we’re expecting.”
Just where and how hard that rain fell wasn’t yet fully clear.
MetService was forecasting the northeast flow to strengthen as it approached New Zealand, with an associated front likely to bring rain to northern areas from Saturday.
The agency was currently reporting low confidence – or a 20 per cent likelihood - that rainfall would reach warning criteria on Sunday, about eastern parts of Northland and northeastern parts of Auckland including Great Barrier Island and Coromandel Peninsula.
It gave the same odds of severe gale northeasterlies affecting exposed parts of Northland and Auckland on Sunday - and of “warnable amounts” of rain for Coromandel, the Bay of Plenty and northern Gisborne on Monday.
Brandolino said the centre of the low would likely pass to the country’s west, over the far-eastern Tasman Sea, while driving flows coming from the east and north-east.
Regions including Northland, and eastern areas of Auckland, Waikato and the Bay of Plenty, would have the greatest odds of heavy rainfall.
Source / MetService
“This might also be a prolonged event: that high pressure system won’t be in a hurry to move and may well hold its ground,” he said.
“This means that, with a low to our west, this flow may become what we call meridional, where the jet stream typically flows from a lower latitude to a higher one.
“That’s when flows can tap into moisture from the tropics, which can set up pockets of rain – and it might be that this persists for much of next week.”
Along with big downpours, those in the north might also be in for some uncomfortable mugginess.
“Under this kind of flow, we can expect very warm and humid air – certainly for this time of year, at least – and it’ll feel real juicy.”
Over the first half of next week, it was possible the north could record some of the warmest minimum temperatures ever logged in May.
“Other areas of the country might also get some heavier rain next week too, including upper north-west of the South Island, or west of Nelson, toward the West Coast region,” he added.
