Atmospheric river to bring week of ‘significant heavy downpours’

An atmospheric river moving in from the Tasman Sea is expected to set off a week of “significant heavy downpours” for most of New Zealand, MetService says.

An atmospheric river is a long yet narrow passages of moisture in the sky which transport moisture from the tropics, Niwa meteorologist Chris Bandolino says.

These rivers cause downpours and strong winds and are responsible for much of the rain in New Zealand, and occur most frequently between October and April.

Bandolino said the weather event could cause some areas in the South Island to see a month’s worth of rain in the span of three days.

When these rivers meet with low pressure, water vapour is released like a “hand squeezing a sponge”, he says.

Fiordland was expected to cop the brunt of the downpours late yesterday, with the current heavy rain watch set for today expected to extend until Thursday, according to MetService meteorologist Oscar Shiviti.

A strong wind watch is in place for the region from midnight last night until 9pm today.

Another heavy rain watch is in place for Westland District (south of Otira) from midday today.

Meanwhile, the North Island will stay mainly fine in the beginning of the week, with showers expected in Gisborne and northwards of Taranaki and Taupō.

Isolated showers will stay in the central and northern North Island while downpours develop through the South Island today.

From tomorrow, the rain will spread northwards to Nelson and Buller, while strong winds are expected to be contained to the inland areas of Otago and Canterbury, as well as Wellington.

Bandolino said flooding could be possible as the week progresses, with warnings likely to arise between tomorrow and Friday.

By Thursday, the downpours will cover most of the North Island and the western area of the South Island. Hawke’s Bay and Gisborne are expected to stay dry.

Strong winds are expected to persist from Wellington to Taranaki.

These downpours will continue into Friday, expected to ease across the motu as the day progresses.

Despite the showers, Bandolino said the weather coming in from the sub-tropics means temperatures in a number of areas could stay above 20C.

Canterbury could reach the upper 20s, while Waikato and areas northward will stay in the mid-20s.