Disruptions expected as southwesterlies sweep NZ this weekend

Rain and "very cold southwesterlies" are in store heading into the weekend for many parts of the country as a large active trough moves eastwards over New Zealand.

MetService said cold southwesterlies are expected to affect the lower South Island later Friday, with snow possible on the hills and ranges.

A heavy rain watch is in place for the Tararua Range from 1pm Friday to 1am on Saturday and the ranges of Westland from 6pm Thursday to 3am on Friday. Periods of heavy rain have been forecast, with amounts potentially approaching warning criteria.

A road snowfall warning was in place for Crown Range Rd from 6pm Friday to 6am Saturday, and Milford Rd (State Highway 94) from 10am Friday to 2am Saturday.

Crown Range Rd could have 3-6cm of snow accumulate above 600m. Milford Rd was expected to accumulate 10-15cm of snow above 800m, while 5-10cm was expected down to 400m from late Friday afternoon.

"There is low confidence of warning amounts of rain in Grey and Buller, the Tararua Range, Taranaki Maunga and the central North Island high-country," the meteorology service said in its severe weather outlook.

"Very cold southwesterlies" were expected to spread over the country on Saturday as the trough moved to the east, with snow over the hills and ranges of the South Island in the morning, MetService said.

A low was then forecast to approach central and northern New Zealand from the west late in the day.

MetService said there was low confidence that warning amounts of rain would continue about the central North Island high country and Taranaki Maunga in the morning. For Northland and Auckland, there was low confidence that southwesterlies would reach severe gales.

MetService said temperatures would be "especially cold" inland.

Mount Cook was forecast to reach a low of -11C on Saturday, while daytime highs were expected to remain between 5C and 7C in Twizel, Alexandra, Wānaka, Queenstown and Gore over the weekend.

"The central and southern parts of the South Island can expect severe frosts. Black ice is possible, where wet roads freeze overnight," it said.

On Sunday, a low and associated front should move eastwards across the North Island during the day, MetService said, while a ridge of high pressure builds over the South Island.

"There is minimal risk of severe weather."

A weak front embedded in a southwest flow should move northwards across southern and central New Zealand on Monday.

The weather is expected to bring disruptions for motorists in the South Island, the New Zealand Transport Agency says – especially on inland state highways.

Forecasts supplied to New Zealand Transport Agency Waka Kotahi suggested snow was expected to fall on Saturday on SH8 and SH80 in the Mackenzie Basin, and further north in alpine areas on SH7 and SH73. Accumulations could reach 5cm-plus for high parts of Porters Pass.

As the snow gives way to severe frosts from Sunday, ice was expected to pose a serious risk to motorists in many areas, especially inland and in sheltered areas. State Highways 1, 7, 73, 75 and 77 were likely to be affected through to the start of next week, while SH8 and 80 were also likely to have icy road surfaces, in addition to freezing fog in patches for the Mackenzie Basin, Waka Kotahi said.

"With potential for snow closures and delays, people are advised to carry warm clothes, water and food in case they get stuck or need to stop and wait for the weather to pass. Ensuring sufficient battery range or petrol in the tank is also advised in case there are detours and diversions required."

NZ Transport Agency contractors will be out laying grit or anti-icing agents to help keep people safe on icy highways. Drivers were urged to reduce their speeds, increase their following distances, and avoid sudden braking or turning movements on icy roads.

"Staying as visible as possible, with headlights on in foggy conditions, also helps to keep people safe."

Friday

For the North Island, rain developing in the north and west, with heavy falls and possible thunderstorms. Scattered rain spreading elsewhere later. In the South Island, periods of rain in the west, heavy and possibly thundery at times. Scattered rain elsewhere.

Saturday

For all of New Zealand, there will be showers, some heavy, gradually easing. Snow possibly lowering to 200m for a time about Fiordland, Southland, and Otago.

Sunday

Showers for the North Island, some heavy in the west, clearing later. Mainly fine for the South Island, but partly cloudy with isolated showers in the south.