First of several blasts of snow and cold air now heading north

Tuesday, 30 July 2019

Snow is forecast for large areas of the South Island this week.

A measure of storminess between New Zealand and Antarctica is poised to plummet to its most extreme level for decades as a major cold snap makes it way north.

The coldest blast of winter so far began moving on to the south of the country on Tuesday afternoon. Its heavy snow is expected to cause significant disruption across the east of the South Island and parts of the southern and central North Island by Wednesday afternoon.

Heavy snow warnings and watches have been issued for a significant part of Marlborough, Canterbury and Otago for that period, with road snowfall warnings also in place for alpine passes and highways across both islands.

Snow falls at Porters Pass. Heavy snow is predicted on the alpine passes on Tuesday night and Wednesday morning.

Over the next week, the Antarctic Oscillation or Southern Annular Mode (SAM) – which measures the strength of the 'polar vortex', the ring of westerly winds which encircle the Antarctic – is forecast to plunge to its lowest level since April 1990.

Some South Island ski fields are breathing a sigh of relief as heavy snow begins to fall on Tuesday night.

The week will be dominated by the biting south or southwesterly winds.

* Predicting how low snow will fall remains a difficult job for Kiwi forecasters

* Polar vortex helping forecasters predict New Zealand weather

* How do forecasters predict snowfall?**

When SAM flips negative, it indicates a weakness in the polar vortex, allowing storms and polar southerlies to break through and blast north on to New Zealand.

National Institute of Water and Atmospheric Research meteorologist Ben Noll told Stuff SAM was 'dipping into impressive territory'.

Snow in Springfield, Canterbury, in the 2017 winter, with siblings Blake and Brianna Taylor enjoying a snowball fight.

The predicted -4 value of SAM by August 8 would put it in the top 1 per cent of all negative SAM events.

'The record low SAM is -5 in April 1990.'

A very deep low pressure centre close to the Antarctic is churning freezing cold air clockwise around it and north on to New Zealand. This is the European Centre for Medium Range Weather Forecast prediction for Sunday evening, displayed on Windy.com.

By the end of next week, milder, spring-like weather was possible as a building anticyclone 'turns the cold tap off', Noll said.

However, until early next week, just about every part of the country is expected to have cold, squally showers at times, with further periods of low snow likely in the south and east of the South Island.

MetService meteorologist Georgina Griffiths talking about 'SAM' - the Southern Annular Mode or polar vortex, which plays an increasingly important role in determining our weather.

The Antarctic Oscillation or Southern Annular Mode (SAM) measures the strength of the 'polar vortex', the ring of westerly winds which encircle the Antarctic.When SAM is in positive mode, the westerly wind belt is in place south of New Zealand, trapping cold Antarctic air behind it. But when it flips negative - as is shown here over the next week or so - it allows those storms and polar southerlies to break through the vortex and blast north on to New Zealand.

MetService meteorologist Lewis Ferris said coastal Marlborough and much of inland Canterbury down to 200 metres would be affected by the period of snow, due to start on Tuesday evening and peter out by Wednesday afternoon.

It was unlikely snow would settle in central Christchurch. The freezing level over Banks Peninsula and the Port Hills would be higher than over the plains due to the strength of the southwesterly wind, meaning snow would not settle as low there as inland.

The Desert Rd highway in the central North Island closed by snow. (File photo)

MetService has issued heavy snow warnings for much of Canterbury and Marlborough, and parts of Central Otago, advising that snow down to 200m will cause problems for travellers and farmers.

Below 200m, sleet and heavy rain may also be a problem for Christchurch and parts of Banks Peninsula on Wednesday morning. Snow flurries are also predicted about the Dunedin hills.

The orange-level warning says snow is expected to settle down to 200m, with 15 centimetres to 20cm of snow to about 400m and 'even deeper snow' above that.

For the Canterbury high country and Canterbury Plains south of the Waimakariri River, and for parts of Central Otago, the most hazardous time is forecast to be from 6pm Tuesday to 8am Wednesday.

North of the Waimakariri, and about the Kaikōura ranges and Marlborough south of Ward, the heavy, low snow is expected to peak between 3am and 3pm on Wednesday.

The heavy rain watch for Christchurch and the plains below 200m is valid from 3am to 9am Wednesday.

MetService has also issued road snowfall warnings for the Lewis, Arthur's, Porters, Lindis and Haast passes, the Crown Range Rd and Milford Rd from Tuesday afternoon until Wednesday afternoon.

In the North Island, road snowfall warnings have been released for the Desert Rd and the Remutaka Hill Rd for later Wednesday afternoon and evening.