Victoria Dairy came in for a close call after a trampoline flew into a nearby power pole. Photo / Emma Lunny
Victoria Dairy came in for a close call after a trampoline flew into a nearby power pole. Photo / Emma Lunny
The tornado that ripped through a South Auckland shopping centre this morning is a less than once-a-year occurrence for the city, and by New Zealand standards was of a moderate scale.
An East Tamaki twister has torn the roofs off houses and dismantled a double story shop front after a night of heavy storms and strong winds.
And while New Zealand only experiences seven to 10 genuine tornadoes each year, today's example is an even more surprising occurrence says, one meteorologist.
Metservice duty forecaster Heath Gullery said the drought conditions Auckland has experienced this year are not conducive for tornadoes to form.
"Auckland's been through quite a large drought so that implicitly means there hasn't been a lot of rain or thunderstorms to produce a tornado," Gullery said.
"But the last couple of days there's been quite an unstable air mass across the upper North Island that has been conducive to tornadoes. That's what we've seen in the last few days."
On the tornado-measuring Enhanced Fujita (EF) scale, New Zealand's tornadoes are almost always either EF0 and EF1 - that's wind speeds of 105-137km/h and 138-177km/h respectively - and far from the likes of EF5 tornadoes that rage harder than 322km/h.
A trampoline has bounced to a stop in front Victoria Dairy on Carruth Rd in Papatoetoe. Photo / Emma Lunny
This graphic shows the typical set-up of a New Zealand tornado. The updraft (UD), downdraft (RFD) and forward flank downdraft (FFD) are indicated. Image / NIWA
Gullery estimated this morning's twister would be in that EF0 to EF1 range, but they didn't have the measuring tools to say for sure.
"It's kind of hard to say from footage because it's difficult to get perspective of how large it is," Gullery said.
"Most tornadoes in New Zealand are usually F0 with a bit of a rare F1. It's more than likely it was an F0 or a very weak F1 tornado.
"It's such a localised feature that our weather stations around Auckland didn't pick up any of the gusts associated with it. It would really have to travel right on top of one of those weather stations, which it didn't."
Tornado damage at Elite Bathroomware in East Tamaki. Photo / Phillip Boughey
Because twisters are extremely localised, the damage they cause is very confined to the tornado itself, although the violent winds can fling debris hundreds of metres.
Most Kiwi twisters are associated with pre-frontal squall lines - bands of thunderstorms embedded in a strong, unstable pre-frontal northwesterly flow.
Thunderstorms have very strong updrafts and if they happen when winds rotate counterclockwise as the air rises, the updraft can start to spin and a mesocyclone can form.
A tornado ripped through an Albany shopping mall in 2011. Photo / File
Tornadoes are spawned from these mesocyclones, which can be as little as 1-2km across.
The main warning signs are hail or heavy rain followed by dead calm or a fast, intense wind shift - and a loud continuous roar or rumble, much like the sound of an approaching freight train.
Other signals are large, dark, low-lying clouds, or clouds filled with debris.
Auckland is hit by a tornado on average less than once a year, but there's much variation from year to year, with some years seeing no tornadoes at all.
Rahoutu in Taranaki today. Ripped-off roofs and downed trees and powerlines has served up another dramatic reminder that tornadoes can strike New Zealand. Photo / Mel Sorrensen
A Taranaki farmhouse devastated by a 2007 tornado. Photo / 1News/File
A tornado that ripped through Hobsonville in 2012 killed Keith Langford, Brendon Johnson and Tom Stowers when they were crushed beneath concrete slabs at a construction site.
The twister also damaged 400 homes and cost $8.7 million in insurance claims, $3.5m in damaged lines and $1.5m in council clean-up efforts.
Another lethal tornado struck at Frankton, Hamilton, in 1948, carving a 100m to 200m swathe through the suburb and killing three people.
It was ranked as an EF2 twister with winds between 150km/h and 200km/h.
Damage to Mt Maunganui's Baypark stadium from a tornado in 2015. Photo / File
Taranaki, where a tornado hit Opunake this year, was another hotspot - but more so between Motonui and Oakura.
Over two days in July 2007, a swarm of at least seven tornadoes hit the Taranaki coast, causing widespread damage in the region. Another that struck near Waitara in 2004 killed two people.
Still, the tornadoes recorded here weren't as common and destructive as those notorious twisters seen across the plains of the US' "Tornado Alley".
A tornado that ripped through Hobsonville in 2012 killed Keith Langford, Brendon Johnson and Tom Stowers when they were crushed beneath concrete slabs at a construction site. Photo / File
The aftermath of a 2013 tornado that blew through Devonport. Photo / File
The Moore Tornado that hit Oklahoma in 2013, killing 23 people, came with winds that reached 321km/h.
There appeared to have been fewer tornadoes around New Zealand over recent years, although such events were probably under-reported as they tended to hit remote spots and cause little damage before fizzling out.
It was a big reason the history of tornadoes here had not been studied in detail - and why it was harder to tie them to climate patterns such as La Nina or the Pacific Decadal Oscillation.
Although climate change was expected to bring more severe storms to New Zealand over the coming decades, it's harder to tell whether that will also mean a higher likelihood of tornadoes.
