Tornadoes: What You Need To Know

Monday, 27 January 2025, 8:55 pm
Article: RNZ

Samantha Gee, Nelson Marlborough / Te Tauihu reporter

Explainer: Mangawhai residents awoke early on Sunday morning to what appeared to be a tornado, that had ripped through the town and left a trail of destruction behind.

MetService initially couldn't confirm whether it was a tornado or simply a series of strong wind gusts but has since said it was indeed, being a violent rotating column of air from a thunderstorm that we know as a tornado.

What do we know about the storm that hit Mangawhai?

MetService meteorologist Surprise Mhlongo said a low pressure system moving across Northland on Sunday had brought with it thunderstorms, lightning and strong wind gusts.

"After reviewing on the ground observations and other evidence, it looks like it was a tornado."

Around 50 homes were damaged and some were destroyed in the seaside community, while trees were uprooted and power poles were snapped in half.

Two people were taken to hospital in a serious condition, one woman was flown to Auckland Hospital with "lacerations and broken bones" after the top storey of the house she was in was destroyed.

Many residents described feeling their homes shake and a loud roaring sound as the tornado bore down around 3am.

At least nine homes have since been yellow stickered.

NIWA meterologist Dr Richard Turner said it was "most likely a tornado" based on eyewitness events at the time, the path that it took and the damage it caused.

He estimated the wind speed to be over 150 kilometres per hour (roughly 45 metres per second) and said footage of the destruction looked similar to that caused by a tornado in Ōakura, Taranaki in 2007, where the wind speeds were assessed at the same speed.

"Given what I saw in the videos, the house with its second storey pretty much missing and the powerline down and the general amount of debris thrown around."

Turner said the speed varied along the path of a tornado, which is why some homes suffered more damage than others.

The lack of radar coverage in regional New Zealand made it harder to detect tornadoes.

"Here they are small scale, they are hard to see and they can often come out of relatively small systems that might not be picked up by radar and also might be under a heavy cloud shield on the satellite so it isn't easy to actually determine these things afterwards."

What is a tornado?

A tornado is a narrow, rotating column of air that extends down towards the ground from the base of a thunderstorm.

Warning signs include a long, continuous roar or rumble, or a fast approaching cloud of debris, which can be funnel shaped.

Turner said in New Zealand, most tornadoes were associated with bands of thunderstorms embedded in a strong, unstable pre-frontal northwesterly flow.

Local tornadoes were usually spawned from mesocyclones - spinning updrafts of air in such thunderstorms.

They were usually around 20 to 100 metres in diameter, lasted for a couple of minutes, and travellled two to five kilometres during that time with wind speeds of between 115km/h and 180 km/h.

Warning signs included hail or heavy rain followed by dead calm or a fast, intense wind shift; and large, dark, low-lying clouds

How is the intensity of a tornado measured?

The Enhanced Fujita Scale (EF Scale) is used to assign a tornado a rating based on its estimated wind speeds and related damage.

Most New Zealand tornadoes are relatively small and have ratings of EFO to EF1, reaching speeds of between 105 and 180 km/h.

On rare occasions EF2 (180 to 220 km/h) tornadoes have occurred in New Zealand.

The country's deadliest tornado was rated EF3 (220-265 km/h) and ocurred on 25 August, 1948 in the Hamilton suburb of Frankton. Three people were killed, 80 injured and about 160 buildings destroyed or badly damaged and property damage at the time was estimated to be as high as £1,000,000 ($60 million).

Turner said he estimated the Mangawhai event to be a very strong EF1 or low EF2, given the wind speed of around 150 km/h and the associated damage.

"Just judging by the damage and the fact power poles were knocked over, it takes a fair bit of wind to do that."

How often do tornadoes occur in New Zealand?

Around 10 moderate to strong tornado events are reported in New Zealand each year. They aren't as common or as destructive as those that occur over the plains of the United States, but even on a small scale as seen in Mangawhai, could cause damage, injury and even deaths.

Turner said they were likely to be more frequent than thought given many other tornadoes in remote rural areas went unreported.

He said they were most likely to occur in Northland, Tauranga, Waikato, Taranaki, Horowhenua and the West Coast.

A tornado in Auckland in December 2012 hit Hobsonville and Whenuapai, killing three builders and damaging almost 400 homes. It was estimated to have cost $8.7m in damage.

In July 2007, a swarm of 12 or more tornadoes hit the Taranaki coast and caused widespread damage in the region.

Ōakura, a town 12km southwest of New Plymouth was most affected. Roughly 50 houses suffered major damage, some of it irreparable, when two tornadoes ripped through the town.

In 2016, a tornado in the Auckland suburb of Albany killed one person, sent cars airborne and did about $10m in damage over a 15km path.

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