More low snow possible for parts of the South Island

Thursday, 8 August 2019

Christchurch will get rain on Friday night, while parts of the South Island are in line for snow this weekend but details are still vague because of a complex weather situation.

A large low-pressure system will cross the Tasman Sea on Saturday and Sunday ensuring wet, windy weather for many parts of the North Island, and cold, miserable conditions for much of the South Island too.

On Thursday evening, forecasters said the most likely scenario was for snow down to about 400 metres or so in parts of Canterbury and Otago on Saturday.

But it is where the small centres of energy within that larger system move to that will determine where the weather is worst.

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The other vital factor for this spell of snow will be how far an injection of very cold air from south of the South Island can sneak north across Otago and then Canterbury.

Where the edge of that cold pool stops will be the boundary between low snow to its south and rain and much higher snow to its north.

The set-up is quite different to that which brought deep drifts to South Otago and parts of eastern Southland on Sunday and Monday this week.

That was a true blast of Antarctic air from off the ice shelf about 70 degrees South. While this weekend's air mass has more moisture – meaning it could potentially bring more snow – it is not as cold.

Predicting snow and its location remains among the trickiest tasks for a forecaster.

If snow is to reach the surface, the air above needs to have the right moisture and temperature. But there remains the all-important question of where exactly it will actually fall, and that depends on the whims of the wind and where nearby high ground is.

This week's heavy snow in the far south was brought by what forecasters call a 'critical' southwesterly wind.

Just like a rock in a stream splits the flow of water around it, the South Island splits a critical sou'wester in two – as a result, Canterbury and further north is largely sheltered from the worst in a southwesterly while much of Otago and Southland bears the brunt.

For snow to make it to Canterbury and Marlborough, the wind has to turn more southerly, allowing the clouds to travel up the coast rather than out to sea. A 10 or 20 degree change of wind direction can make a huge difference, and can make cold-snap forecasting a nightmare.

This next spell of bad weather is expected to come from the east or southeast, which makes Canterbury, particularly higher parts of the plains and the foothills, much more exposed to low snow and the onshore wind.

A strong, very cold sou'wester is also possible next week.

MetService meteorologist Lewis Ferris told Stuff there was already a severe weather watch out for heavy rain in places as the low-pressure system developed.

In terms of snow, it was too early to say, although Wednesday's severe weather outlook said there was low confidence of heavy snow above 400m for much of the South Island's east coast, with flurries to low levels,

'There's still heaps of uncertainty about where the worst will happen. People need to be really aware of this.'

There was still time for snow watches to be issued once it became clearer how things would play out, he said.

Blue Skies Weather forecaster Tony Trewinnard said it would be 'a cold and miserable weekend' for Cantabrians.

Farmers above about 400-500m needed to be aware of the likelihood of snow on Saturday, not clearing until Sunday afternoon, particularly if they were involved with lambing.

It was also likely the passes would be hit hard by snow at the weekend.

North Islanders would experience strong winds at times and rainy spells, Trewinnard said.