Deep low flinging Antarctic air toward NZ, with downpours, gales sticking around until Sunday

Wednesday, 4 December 2019

Water levels for both Lake Wanaka and Wakatipu are above alert level and heading towards historical flood amounts with more rain on the way.

The deep low deep in the Southern Ocean that is cursing many southern and central areas with huge amounts of rain and ferocious winds is set to hang around until Sunday.

A combination of the low to the southwest and a blocking ridge of high pressure to the east has caused a recurring pattern of heavy rain to the west and north of the South Island, and gales or severe gales in many eastern areas and Wellington.

'It's a combination of big high pressure to the east of us and the deep low to the southwest. We get this really quite steep pressure gradient… That's driving northwesterlies onto the country, between the ridge and the low,' MetService meteorologist Rob Kerr said.

Heavy rain and severe gales have caused disruption in many parts of the country.

Around noon Wednesday the low was almost directly south of Sydney with a central pressure of 952 hectopascals. It was not uncommon to have such low central pressures so far south. As lows moved north the pressure tended to rise, Kerr said.

* Wild winds wobble Air NZ plane on its way into Wellington

* Ute, cars crushed by trees as storm sweeps through Waikato**

'As the low has developed and become mature it's become quite broad. It's not really going anywhere. It can't get past the blocking ridge.'

Air from close to Antarctica was wrapping around the low then being constantly directed onto New Zealand.

Little change was expected in the situation through Friday, but by Sunday the low was expected to be moving over the country or skipping past to the south. By then it would have weakened, Kerr said.

The northwest flow over the country would still be around on Saturday. Things should start to change on Sunday, and a change to the southwest was expected by the early part of next week.

A long burst of weather from the northwest was not uncommon, although it wasn't something that happened every year, Kerr said.

A blocking ridge normally turned up at some point during the year. It's impact on weather conditions could be quite different, depending whether it was to the east, the west, or over the country.

In its monthly climate outlook for December, MetService predicted a ridge of high pressure would try to hang on over Northland through the month, although it would vary in strength.

That was expected to keep things relatively dry for Northland, Auckland and Coromandel Peninsula during December.

Prevailing westerlies across the lower half of the North Island and the South Island were expected to mean wetter than usual conditions would continue through the month for western areas, while it would be drier than normal in the east.