Expert explains ‘atmospheric river’ set to soak parts of NZ

An ‘atmospheric river’ is set to bring heavy rain to parts of the country this week NIWA has warned. But what does that mean?

Heavy rain file image. (Source: istock.com)

Atmospheric rivers are a year-round occurrence in New Zealand – but NIWA say this one is unusual for the time of year, with one expert describing it as “pretty exceptional”.

“Atmospheric rivers are huge plumes of moisture that move from the tropics to the mid-latitudes,” NIWA Meteorologist and Forecaster Tristan Meyers said.

“New Zealand sees our peak atmospheric river activity during summer, and our lowest atmospheric river activity during winter.

“When these atmospheric rivers encounter other weather features or New Zealand’s mountainous terrain, the vast amount of water vapour within these atmospheric rivers can get ‘squeezed’ out, falling as heavy rain or snow.”

Meyers added that atmospheric rivers are important for New Zealand’s water supply but “also responsible for extreme rainfall events”.

“This current atmospheric river is pretty exceptional; it’s long lasting and has a very anomalously large moisture content. In terms of impacts, we are anticipating very heavy rain over a prolonged period with this atmospheric river.

“All up, over a month of rainfall could fall for large parts of the West Coast, Canterbury High Country, Tasman, Marlborough and Nelson, with the heaviest rain about the elevated ground.”

This could lead to slips and flooding, he warned.

Dr Daniel Kingston, senior lecturer in geography at the University of Otago, added that the atmospheric river’s impact “might also be more damaging than otherwise because it comes on top of an already exceptionally wet winter”.

He said this means “the ground is already very wet with limited capacity to absorb further rain”.

A number of weather warnings are in place around the country this week, according to MetService.

A heavy rain watch is in place for Northland from 6pm Wednesday to 12pm Friday; Auckland and Great Barrier Island from 10pm Wednesday to 12pm Friday; Waikato, Waitomo, Taumarunui, Taupō and Taihape from 12am Thursday to 12pm Friday; North Taranaki away from the mountain from 6pm Wednesday to 6am Friday; the Tasman District from Motueka eastwards, excluding the Nelson region, from 10am Tuesday to 6pm Thursday; and the headwaters of the Canterbury lakes and rivers from 2am Tuesday to 1pm Thursday.

A strong wind watch is in place for Northland from 8pm Tuesday to 12pm Friday; and Auckland and Great Barrier Island from 9pm Tuesday to 6pm Friday.

An orange heavy rain warning is in place for Mount Taranaki from 6pm Wednesday to 6am Friday; the Tasman District west of Motueka from 5am Tuesday to 6pm Thursday; the Marlborough Sounds, Richmond ranges, Rai Valley and Nelson region, including Bryant Range, from 1pm Tuesday to 9pm Thursday; Buller from 4am Tuesday to 4pm Thursday; and Westland from 1am Tuesday to 4pm Thursday.

Metservice advised on Tuesday that people keep up to date with the latest forecasts as the situation develops.

Information on the latest weather updates can be found here.

Niwa weather graphic. (Source: Niwa)