Hundreds of homes when a deluge caused heavy flooding in Westport last winter. Photo / George Heard
A spate of damaging extreme weather events - including three big deluges in recent months - has left even MetService meteorologists surprised.
Today marks three years since the national forecaster introduced its 'red warnings' - advisories that are reserved only for the most extreme weather - and it's since had to issue seven of them.
But six of those - including a Westport deluge that left around 450 homes damaged or uninhabitable, and Canterbury floods that put swathes of farmland underwater - have occurred in just the past 12 months.
"When we introduced the system, we had expected one or two events per year," said Lisa Murray, MetService's head of weather communications.
"It's fair to say we've been somewhat surprised by six in the past year, three of which have happened in the first five months of 2022."
Between 2010 and 2019, a total of 14 weather events led to local states of emergencies being declared.
Other events over the recent spate have included another West Coast deluge in early February that forced hundreds of evacuations, and storms in March and April that wrought hundreds of millions of dollars in damage on the North Island's East Coast.
"We work closely with regional councils ahead of issuing the red warnings and during the events to understand local concerns," MetService severe weather manager Elke Louw said.
"The more forewarning we can provide, the more time communities have to prepare and stay safe."
While each event was different, recent ones shared some common threads.
Most coastal regions around the country had been experiencing periods of severe or strong ongoing marine heatwaves - especially in the North Island, with waters in the Wairarapa reaching 3C above average for this time of year.
"Warmer waters around us means the air above it heats up, there is more moisture in the air and this feeds into any weather system, especially those that come from the tropics which means we are seeing more intense weather events," Louw said.
A big influence is La Niña, an ocean-driven climate system that's been meddling with our weather since late last year - and may do again later in 2022 if a rare "triple dip" of these events plays out.
For New Zealand, La Niña meant more northeast winds from the sub-tropics, leading to higher-than-usual rainfall in the northeast of the North Island, as seen in autumn's big East Coast storms.
Some of these big downpours had come in the form of snaking, moisture-packed systems called 'atmospheric rivers'.
"These conveyor belts can stall over a part of the country, delivering continuous rainfall for a number of days, or are a highway for deep tropical lows to come visiting," Murray said.
"A warmer atmosphere due to climate change will make more moisture available in these atmospheric rivers which can get dumped on our doorsteps."
On average, every degree of warming was adding an extra 7 per cent of moisture into the atmosphere, intensifying visiting weather systems.
"For example, a tropical cyclone is likely to have more energy now as opposed to 50 years ago - so can be more intense and more damaging."
Scientists have already estimated the extreme rainfall that caused Canterbury's flooding last year was 10 to 15 per cent more intense because of human-driven climate change - with a similar-sized influence also observed in last year's Westport disaster.
"Studies have shown us that we can expect an increase in the intensity of big rainfall events, especially in areas like the west coast of the South Island," Murray said.
"So, with more extreme events in the future, then it is likely that Aotearoa New Zealand will have more red warnings."
MetService was working towards continually improving weather models and tools, giving meteorologists more detailed information with longer lead times when issuing warnings.
"Hopefully, Aotearoa New Zealand will also have climate change mitigation measures in place which will enable our infrastructure to better cope with extreme weather events."
The Government recently put out for consultation its draft National Adaptation Plan, which aimed to determine how the country should meet climate-driven impacts, and fund the costs that came with them.
According to our most recent national risk assessment, some 675,500 Kiwis already live in areas prone to flooding - with a further 2,065 living in the firing line where some of the most dramatic effects of sea level rise could hit.
Some players in the insurance sector - which just saw two years of record weather-related claims, likely totalling more than a billion dollars - have responded to a rising flooding threat by moving toward new risk-pricing models.
Victoria University climate scientist Professor James Renwick expected the slew of recent events would've helped make the climate crisis more real to Kiwis.
"What we've seen happening here fits in exactly with what we can expect from climate change."
Fiordland flooding, February 3-4 2020
An extreme rainfall event that brought 1000mm over Milford in 50 hours and caused significant flooding and major damage to SH94, isolating Milford and trapping tourists. Local states of emergency were declared in Fiordland and Southland. Overall cost to insurers: $29.6 million.
Canterbury flooding, May 29-31 2021
A major flood caused by extreme rainfall that reached levels of 300mm to 400mm over just 48 hours. Totals of 200mm were observed along the Canterbury foothills, peaking at 540mm at Mt Somers in the headwaters of the Ashburton River. The event caused widespread damage to roads, bridges and farms. Local states of emergency were declared in Ashburton, Timaru and Selwyn, followed by Canterbury as a whole. Overall cost to insurers: $46.4 million.
West Coast flooding, July 15-17 2021
An extreme rainfall event for Buller and Westland with 330mm to 380mm of rain forecast. The heavy rain caused slips and rivers to break their banks, while Westport was cut off by widespread flooding, resulting in more than 2000 residents being evacuated. Overall cost to insurers: $97.2 million.
Canterbury winds, September 12-13 2021
Northwesterlies were forecast to gust up to 160km/h in exposed parts of the Canterbury high country but only reached as high as 130km/h. Impacts included downed powerlines, trees, fences and a few flipped vehicles, and many people being left without power. Overall cost to insurers: $36.5 million (across whole of South Island).
West Coast flooding, February 1-3, and Taranaki flooding, February 5-6 2022
An estimated 500 to 800mm fell in 72 hours over central and southern Westland, with 170 to 350mm for Grey and 300 to 550mm for Buller. Hundreds of people were evacuated on Friday night due to flooding and a local state of emergency was declared in Buller district. The rain moved its way up the country to the North Island throughout Waitangi weekend. A red warning was issued for Taranaki on the evening of February 5. A few weather stations exceeded 400mm, with Cape Egmont, at sea level, receiving a typical July's worth of rainfall in just four hours. Coastal and South Taranaki took the biggest hit, with roads washed out and flooding. Overall cost to insurers: not known.
East Coast flooding, March 23 2022
Gisborne Airport recorded a month's worth of rain in seven hours in one night with 86.9mm, while further inland, some places recorded totals of 200 to 300mm or more. The extreme rain event had major impacts with millions of dollars' worth of damage to property and infrastructure including the bridge at Tokomaru Bay being washed out. Overall cost to insurers: $79.6 million (across whole of North Island).
East Coast flooding, April 12-13 2022
Gisborne and Wairoa district were hit again by the remnants of Tropical Cyclone Fili. The region was still recovering from the recent events and was again lashed by an extreme rain event, with 150-200mm of rain falling in some places in 24 hours. Impacts included significant flooding, bridges washed away, falling of trees and power outages, while people were isolated after damage to several roads. Overall cost of flooding: not known.
Firefighters responded shortly before 9am on Friday.