Electrical Storm in Mangawhai. Video / Danielle Jones
A meteorologist says it’s unclear whether the damage in Mangawhai was the work of a tornado: but the ingredients were likely there for one.
The strongest gust amid the weather action was 122km/h, recorded at Tiritiri Lighthouse near Mangawhai.
Forecasters are now warning of more thunderstorm activity today around the Canterbury plains.
A meteorologist says it’s unclear if the weather system that blasted through Mangawhai last night was a tornado – but the damage wrought would likely have been the same regardless.
The seaside settlement bore the brunt of a front - packing strong winds and active thunderstorms that generated more than 800 lightning strikes - which rolled through overnight.
MetService meteorologist Katie Hillyer said the worst of the weather came at around 2am-3am, with a maximum gust of 66 knots - or 122km/h - recorded at Tiritiri Lighthouse near Mangawhai.
While local rainfall amounts were less impressive, at about 15mm/h, the fast-moving nature of the front meant the impacts were felt more through the powerful winds.
“That is definitely heavy rain – but it doesn’t do justice to how [much] punch that front was as it went through.”
Whether the system generated a tornado was difficult to confirm – the localised action occurred too far from MetService’s nearest rain radar - but the ingredients to form one were potentially present.
“What we do know that is, even on a broader scale with that front, we had 100km/h gusts as well as extremely active thunderstorms,” she said.
“So whether we call it a tornado, or more broad-scale and very strong winds, the damage was the same.”
Hillyer said the event owed to the alignment of a low-pressure system to the west and the associated fronts.
“It’s all about the ingredients and whether they line up – and we saw things line up very well here,” she said.
“The tight gradient around this low meant we’ve got very strong winds, and the upward motion meant we’ve got a really good recipe for thunderstorms.”
Niwa reported that Northland, Auckland, Coromandel, and Bay of Plenty have observed 10-30mm of rainfall with the event.
Another station on Aotea/Great Barrier Island recorded 14.5mm in just 10 minutes.
Looking ahead, Hillyer warned of more active weather further south, with a moderate risk of thunderstorms and possible downpours forecast for the Canterbury Plains.
“We’ll be sending out some messaging on that soon,” she said, encouraging residents to stay up to date.
Tornadoes are well known to wreak havoc, but they’re fortunately rare in New Zealand: on average, about seven to 10 moderate-to-strong tornado events are reported here each year.
They are typically 20-100m in diameter, track for just a couple of kilometres, and last only a few minutes.
Because they’re extremely localised, the damage they cause is very confined to the tornado itself, although the violent winds can fling debris hundreds of metres.
New Zealand’s tornadoes pack wind speeds of 115km/h to 180km/h - enough to tear roofs from buildings and tip vehicles.
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.
Most of the ours 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.
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.
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 aftermath of a tornado that tore through the Auckland suburb of Hobsonville in December 2012. Photo / Tammy Cowan
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.
Taranaki was another hotspot - but more so between Motonui and Ōākura.
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.
Tornadoes recorded here aren’t as common and destructive as those notorious twisters seen across the plains of the America’s “Tornado Alley”.
The Moore tornado that hit Oklahoma in 2013, killing 23 people, came with winds that reached 321km/h.
There appears to have been fewer tornadoes around New Zealand over recent years, although such events are probably under-reported as they tend to hit remote spots and cause little damage before fizzling out.
It was a big reason why 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 is expected to bring more severe storms to New Zealand over coming decades, it’s harder to tell whether that will also mean a higher likelihood of tornadoes.
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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