Unsettled, cold August weather may blast into September too

Friday, 16 August 2019

MetService's severe weather warning for Friday, August 16.

Whatever is gingering up August's weather has gingered it up so well it could stay unsettled into September.

The kind of major change to the weather New Zealanders have experienced this month is rarely the result of a single factor.

In this case it's a bothersome trio of jetstreams, the Southern Annular Mode (SAM) and a variant of El Niño which have brought frequent biting southerlies, snow and severe thunderstorms.

Stormy weather over the past two weeks is expected to continue for the rest of the month and into spring. Lightning seen from Te Awamatu this week.

This time last month, as winter barely made a ripple across New Zealand, who would have seen cold, stormy weather coming?

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Unusually shaped hail stones reflected severe turbulence in thunder clouds over the North Island earlier this week.

* Chaotic weather system behind weirdly-shaped hail

* 'Extreme weather event' in Auckland causes significant damage to buildings, marinas and boats

Squally thunderstorms slamming into the western coast of the North Island at 9am on Monday August 12 in a cold, unstable airstream spinning around a small low-pressure system.

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

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

Rangiora siblings Keertthana, 15, left, and Syardhana, 14, Rajinikanth spent a lot of time making their snowman on State Highway 73 between Springfield and Porters Pass last weekend,

* Polar vortex helping forecasters predict New Zealand weather**

Forecasters believe low-pressure systems south of the country, accompanied by a large area of cold air, will continue to threaten the country with volatile westerly and southerly weather for the rest of the month and into early spring.

Owhiro Bay, Wellington, last Saturday afternoon was still thawing out after an intense 45-minute thunderstorm dumped centimetres of hail.

SAM measures the strength of the 'polar vortex', the ring of westerly winds that encircle the Antarctic.

When SAM is positive, the westerly wind belt is in place south of New Zealand, containing the coldest Antarctic and stormiest southern ocean air behind it. But in its negative phase, that belt is weaker than normal, and storms and polar southerlies can break through the vortex and blast north on to the country.

Alternating layers of ice can be seen in these hailstones after their journey up and down through thunder clouds.

MetService meteorologist Lewis Ferris said the negative value of SAM was a player in the current energetic weather.

'It gives us a measure of what sort of storminess we can expect. It has been negative for the entirety of August so far.'

The position and intensity of the subtropical jetstream had enhanced the temperature gradient through the atmosphere, creating a large area of very cold temperatures well above the surface and the damaging thunderstorms in parts of the North Island this week.

'As that comes across the relatively warm Tasman Sea, it creates low-level instability and you get thunderstorms firing up.

'With the jetstream, the thunderstorms create a path for strong mid-level winds to get down to the surface, supporting tornado development and intense wind gusts.'

National Institute of Water and Atmospheric Science meteorologist Ben Noll said a likely third protagonist was an El Niño Modoki, a variant of El Niño in which unusually warm sea-surface temperatures were in the central equatorial Pacific Ocean instead of their typical location closer to South America.

Modoki was a Japanese word that meant 'same, but different'.

This kind of El Niño disrupted the Walker Circulation of airflow in the tropics in the lower atmosphere, which in turn affected airflow around the Pacific.

A similar Modoki in 2004 had brought a cool/cold and unsettled late winter-early spring, Noll said.

The long-term trend over the years was that SAM was becoming more positive, but with the exceptions of August and September.

SAM was forecast to remain negative for the next couple of weeks at least, he said.