NZ's tornado activity: A look at what's causing our significant thunderstorms

Saturday, 27 June 2020

Victoria Dairy CCTV captures debris - including a trampoline - being thrown around by a tornado in Papatoetoe, south Auckland.

In less than 24-hours, there have been a number of reports of tornado activity and thunderstorms in the North Island.

On Friday, trees and signposts were ripped from the ground, and roofs were damaged after a tornado swept through Northland's Mangawhai.

Later that evening, there was a report of a weak tornado at Auckland's Dairy Flat, which brought down trees and mangled a trampoline.

On Saturday, East Tamaki residents spotted a 'short-lived' tornado swirling above the Auckland suburb, carrying debris along with it. Trees and fences were toppled, and a trampoline was photographed wrapped around a power pole.

**READ MORE:

A trampoline wraps itself around a power pole during a tornado on Victoria Rd, Papatoetoe, south Auckland.

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Eden Tudman was in the lounge when the tornado hit in Dairy Flat late on Friday evening.

But reports of stormy weather didn't stop there. People in the Bay of Plenty also said they saw a small tornado and damaged caused by it on Saturday.

Papamoa resident Christen Thompson told Stuff he saw 'trees ripped to pieces' in the area.

MetService meteorologist Lewis Ferris said the instability in the atmosphere is the key ingredient in severe thunderstorms forming.

A warm, humid airmass has moved down to New Zealand, and this, overlaid with cooler air above, is what leads to unstable atmospheric conditions in which thunderstorms can form in.

Tornado damage on Arwen Place in East Tamaki, Auckland.

During severe thunderstorms, there's also sometimes potential for small, localised tornados with strong wind gusts to form too.

MetService's website generally states that some tornadic systems such as funnel clouds, waterspouts and small land-based tornados were also possible with thunderstorms that might not be classified as severe.

More thunderstorms likely

Mangawhai locals have reported seeing debris being sucked into the sky by a tornado.

An unstable airmass with embedded troughs continues to bring a risk of thunderstorms and heavy rain to the upper North Island on Saturday.

On Saturday morning, MetService also warned that small, coastal tornados with strong wind gusts were possible for Auckland, due to the severe thunderstorms predicted for the city.

A severe thunderstorm watch was still in place for the Coromandel Peninsula, Waikato, Bay of Plenty, Rotorua, and Gisborne until 11pm on Saturday.

MetService's thunderstorm outlook stated that Rotorua and the far north of Gisborne had a moderate risk of severe thunderstorms throughout the evening.

Downpours with rainfall intensities of 25 to 35mm per hour and small tornadoes with strong wind gusts were possible.

Wind gusts of this strength can cause some structural damage, including trees and power lines, and may make driving hazardous. If any tornadoes occur, they will only affect very localised areas, the outlook states.

Rainfall of this intensity can also cause surface and flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.

Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.

As for tornados, they're very hard to predict due to their small size, short lifespan and tendency to form offshore.

Looking ahead

Towards the end of the weekend, Ferris said the severity of the thunderstorms was expected to diminish.

A trough, with an associated moderate risk of thunderstorms over the Bay of Plenty and Gisborne was expected to clear early Sunday morning. Localised heavy rain with intensities of 10 to 25 mm/h were possible with these thunderstorms.

Another trough over central New Zealand will bring an early morning low risk of thunderstorms to Taranaki, northwest Nelson and northern Buller.

A low with embedded troughs and cold air aloft was expected to sit to the west of New Zealand on Sunday evening, bringing a low risk of thunderstorms to Taranaki, northwest Nelson and Buller at night.

Next week, a complex trough will cross northern New Zealand on Monday and Tuesday, followed by a strong and cold southerly flow.

Strong to gale force winds were likely in exposed places, but were not expected to approach severe gale force.

Snow was also likely about parts of the South Island on Monday and Tuesday. Although the risk of heavy snow was considered minimal. Some higher roads and farms could be affected.