Heavy rainfall yesterday caused surface flooding, power cuts and multiple school closures. Video / Supplied
New Zealand has likely just seen one of its soggiest Julys on record, with a "freight train of low pressure" delivering seemingly endless bouts of rain.
Niwa forecaster Ben Noll said more than three dozen locations, spread from Kaitaia to Balclutha, were tracking toward a record or near-record wet July.
It's already been the wettest month in 157 years for Christchurch, where residents were still cleaning up after a Tuesday deluge that delivered more rain than its entire monthly average.
In just 28 days, the city has received 266mm of rain – that's nearly half of its annual normal, and making for one of only 15 of 1739 months where totals have exceeded 200mm.
Normally, meteorologists wouldn't carry out a rainfall analysis for the country as a whole – rainfall figures in Niwa's monthly summaries tend to focus only on places that'd seen unusual trends – but July had been such an "extreme case" that one might be warranted.
"I wouldn't be shocked if this was one of the country's wettest Julys on record," Noll said.
"With a lot of places getting 300 to 400 per cent of their July monthly normal, this all suggests the rainfall we've had this month could be nationally significant.
The five wettest places in the country this month had been Mt Cook (716mm, or 270 per cent of average), Kerikeri (554mm), Arthurs Pass (551mm), Akaroa (410mm) and Te Puke (403mm).
Rounding out the top 10 soggiest spots were Farewell Spit (393mm), Whitianga (388mm), Motu, in the Gisborne District (373mm), Whangarei (363mm) and Warkworth (353mm).
"Kerikeri has seen its second highest total for July in records going back to 1935, while Farewell Spit, where we have recorded from 1874, has also had its wettest July," Noll said.
It's official: July 2022 is Christchurch's wettest month on record with 266 mm of rain & counting 📈
💧 Since records began in 1864, only 15 months out of a total of 1739 had more than 200 mm of rain.
For other spots, the departure from normal had been even more extreme.
The 204mm that's fallen on the Otago town of Windsor this month was a whopping 466 per cent of normal for July – while Timaru, Tara Hills, Christchurch and Dunedin (at Musselburgh) had similarly experienced four times their average amounts.
The five other places with the biggest recorded anomalies so far have been Farewell Spit (386 per cent of average), Oamaru Airport (344 per cent) Lincoln, at Broadfield (332 per cent), Akaroa (313 per cent) and Lauder, in Central Otago (298 per cent).
What's brought all of this rain?
"The main driving forces have been consistent low pressure systems approaching from the northwest, combined with tropical moisture from the Coral Sea," Noll said.
"And that's also come with a northerly quarter airflow anomaly – so, lots of moist air tracking down from the north, with what's been like a freight train of low pressure."
It hadn't helped that this low pressure had been effectively sandwiched between two blocking highs, which had slowed visiting moisture-packed systems down as they moved over the country.
Parts of Kaeo in Northland were under water late on Tuesday. SH10 north of the town had almost a metre of water across the road in places. Photo / David Fisher
Noll said this atmospheric blocking pattern had also been spurred on by a major pulse of what's called the Madden-Julian Oscillation – a tropical climate phenomenon that circulates rain and thunderstorms around the planet every 30 to 40 days.
Further in the background were the lingering remnants of a La Niña climate system - something that's helped leave our surrounding sea surfaces warm enough to energise incoming storms – along with the influence of another far-off phenomenon.
That was the positive phase of an Indian Ocean Dipole (IOD), which has contributed to a series of devastating floods across Australia in recent times.
While the handprint of global climate change was less clear cut, climate scientists point out the heating of our planet was adding more moisture to the atmosphere, while also contributing to warmer seas.
Scientists have already estimated that this warming made last year's disastrous deluge in Westport about 10 per cent more intense – with a similar influence also attributed to last winter's Canterbury floods.
Parts of Kaeo in Northland were under water late on Tuesday. SH10 north of the town had almost a metre of water across the road in places. Photo / David Fisher
Noll said there'd been unusual warmth with the wet over July – especially in the North Island – but Niwa wouldn't be publishing its official climate summary for the month until next week.
The first half of 2022 was New Zealand's second hottest on the books.
Looking forward, Noll said more stormy weather was expected over the first week of August, with potential for heavy rain in the west of the South Island, particularly.
"But over the upcoming three months, we're likely to see a trend toward less low pressure and more high pressure, which, in a broad sense, should see the return of dry spells for some regions," he said.
"So, we look to be transitioning from this pattern we've seen over July to something a bit less unsettled."
But he added the IOD could still keep influencing our weather – as could the increasingly-likely prospect of a third consecutive La Niña, bringing rainy conditions to north-east parts of the country.
"We've got a couple of recent La Niña summers already under our belt, so things like marine heatwaves and dry spots around the country will probably be in the mix once again."
