Jetstream blamed for torrential rain, late snow in South Island

Monday, 12 November 2018

The approach to the Goat Creek bridge on State Highway 73 has been washed out as heavy rain batters the West Coast.

A roaring ribbon of 300kmh winds about 10km up in the sky brought last week's extreme weather to the South Island.

While the low-pressure system that pounded the West Coast with torrential rain and dumped unusually late and low snow on parts of Canterbury looked insignificant on the weather map, it was a classic forecasting case of 'don't judge a book by its cover'.

Driving the storm was a spring jetstream about 10,000 metres above the South Island.

Heavy rain caused slips and damaged parts of State Highway 73 between Arthur's Pass and the West Coast last week.

Rainfall along western parts of the main divide exceeded 600 millimetres over two days, more rain than Christchurch usually receives in a year.

* Body found in West Coast river as spring storm causes flooding, closes major roads**

MetService severe weather forecaster Gerard Barrow said the 'mid-latitude northwesterly jet' and its core of super-strong winds had run from the coast of Queensland late last week, across the Tasman Sea and stretched southeast over the South Island.

The large temperature difference on either side of the jetstream fuelled the strong upper winds and explained how temperatures in Canterbury early Thursday afternoon were touching 30 degrees Celsius but were 25C colder at the same time on Friday.

'This was a classic spring conveyor-belt storm. We didn't actually get a storm like this last year. That jet was bringing a lot of moisture down and feeding that rain right into the West Coast.

'The other main mechanism was a strong ridge of high pressure to the east and northeast and across the North Island, which slowed down the progress of the front up the South Island. So consequently the West Coast got so much rain, as things were moving so slowly,' Barrow said.

The active jetstream had been a feature of the changeable and stormy last couple of weeks.

However, jetstream influence would be minor over the next few days, allowing for more settled conditions until later in the week.

Another, weaker, front, would probably bring more rain to the West Coast and warmer northwesterlies to the east of the South Island by Friday. Its effects on the North Island were expected, at this stage, to be minimal, he said.