MetService: Conditions perfect for 100m-tall Canterbury twister

Monday, 19 November 2018

A family driving in Ashburton got a close up view of a tornado.

It looked like one. It moved like one. And the conditions on Sunday over parts of Canterbury were right for one.

But was it really a tornado?

Opinions have varied since the first clips of the swirling funnel cloud, estimated as 100m tall or more, near Ashburton were posted on social media in the late afternoon.

A photograph of the tornado on Sunday at 3.50pm, five minutes north of Ashburton.

The size of the funnel and its colour made some believe it might just have been a large dust devil.

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MetService's Rakaia radar image clearly shows the severe thunderstorm lying across Ashburton and out to sea at 3.37pm on Sunday. The green and pink area of the cloud over the highway are the most active parts, where the tornado originated.

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However, the official word from MetService is, yes indeed, it was a bona fide tornado.

Severe weather forecaster Andy Downs said without knowing the full extent of the damage it was difficult to say how strong the tornado's winds had been.

'What we really need is for someone to put up a drone and have a close look at the path it took.'

Conditions had been ideal across the Canterbury Plains for tornadic development, he said.

Warm air generated over the Plains by the intensity of the sun, and increasingly cold air aloft, allowed thunder clouds to bubble up to the top of the atmosphere above mid-Canterbury and South Canterbury.

Moist northeasterly winds ahead of the cold southwesterly change dragged in even damper air from offshore, fuelling the clouds' growth. The taller the clouds, the more intense the up and down-draughts inside them and the larger the hailstones could grow.

'It had all the classic ingredients.'

Fortunately, the severe thunderstorms remained south of Christchurch because of a dry northwesterly wind high up over parts of Canterbury north of the city.

Downs said the dust sucked into the tornado helped make it look more dramatic.

'That dust component helped it appear so much larger, like those full-on American ones. Clearly it was a tornado.'

MetService's Rakaia radar image, taken at 3.37pm, shows a line of thunderstorms extending offshore from inland of Ashburton.

The area in the image coloured green, to the west of Ashburton, appeared to have the 'classic hook' shape associated with tornadoes, Downs said.

The bright pink patch in the centre of the green was probably very large and heavy hail.

It was unclear how long the tornado lasted. It may have been 15 minutes or so, but the funnel could have 'come and gone'.

November was often one of Canterbury's most thundery months, he said.