Forecaster Niwa has this morning issued a stark warning, saying the incoming wintry blast is set to plunge the South Island to freezing levels, with wind chill making it feel anywhere from -5C to -15C.
It says this will threaten a "real hypothermia risk for people and livestock".
MetService says a sudden "Antarctic blast" is set to chill the country over the next few days, starting with a blast of cold polar air.
In Canterbury, the mercury is set to drop a staggering 15-20C within a matter of hours today.
MetService has issued a heavy snow watch for much of the South Island, leaving many high country farmers nervous about the impact the late wintry blast could have on lambing and calving.
The forecaster this morning updated its advisories to say that a watch applied to Marlborough south of the Clarence River, the Canterbury high country and foothills, North Otago, Central Otago from Alexandra northwards, and the Lakes District from Queenstown northwards, from 10pm today until midnight on Wednesday.
Further south, a watch was in place for Southland, Stewart Island, Clutha, Dunedin, Central Otago south of Alexandra, the Lakes District south of Queenstown, and Fiordland from Te Anau southwards, from 7pm today until 9am on Thursday.
MetService said for this more southern group, heavy snow above 600 metres was possible this evening. It said snow amounts may approach warning criteria above 600 metres about Central Otago south of Alexandra and the Southern Lakes District south of Queenstown.
From around 6am tomorrow until 9am on Thursday, snow showers were forecast to sea level, and snow amounts could reach warning criteria, in some places down to sea level.
MetService said parts of this watch could be upgraded to a warning.
There were road snow warnings in place for the South Island's alpine passes and for Dunedin's Northern Motorway, the Crown Range Road and the Milford Road (State Highway 94).
The forecaster said snow showers were likely to affect the Dunedin motorway tomorrow afternoon and evening, and 3 to 6cm of snow could settle.
Federated Farmers Otago president Mark Patterson said while most low-country farmers in Southland and Otago had now finished their lambing and calving seasons, high-country farmers were just beginning theirs.
"It’s more the high-country farmers that are just getting into their lambing now — they’ll be a bit nervous.
"An event like this, if it gets too bad, it will be concerning.
"They’ll be doing everything they can to make sure [stock are] well fed and in sheltered positions.
"It’s very much a wait-and-see-what-happens situation."
The storm had the potential to affect farm growth.
"Obviously, for the horticulture people, the spectre of late frosts will be making them nervous as well."
In the Dunedin area, rain is expected this afternoon and the wind will turn southwesterly, bringing snow down to 300m this evening.
Snow was expected to fall to 200m in Southland tonight, and down to 400m in inland Otago.
The MetService forecast for tomorrow was showers with possible thunderstorms, snow lowering to sea level in Dunedin and snow to low levels in inland Otago and Southland. Cold southwesterlies might become gale force around the regions from noon.
On Thursday, early snow showers were expected to turn to occasional rain from noon across the regions and strong southwesterlies were expected to ease in the evening.
Queenstown and Wanaka were not expected to be hard hit. Snow was only expected to fall to 1000m today and a few snow showers tomorrow, but it was expected to be clear at night.
Snow was likely to affect many roads and disruption to travel was likely across the South Island.
People were advised to proceed with care and keep up to date with the weather forecasts and travel advice.
MetService meteorologist John Law said temperatures could drop to freezing across the country as far as the top of the North Island, as air from the Southern Ocean swept northwards.
"We are all going to feel this very cold air when it arrives.
"While temperatures in the central parts of the South Island will be the coldest, even up in the winterless north we could find temperatures cold enough for a touch of ground frost.
"Daytime temperatures will be struggling to break into double digits for many on Thursday, and with strong southwesterly winds whipping around coastal parts of the county, the wind chill is going to make it feel colder still," he said.
- additional reporting NZ Herald/ODT Online
