A forecaster has likened this weekend's wild weather across Canterbury to a classic West Coast deluge, but in reverse - and stretched out over days because of an "atmospheric traffic jam".
The event has so far brought widespread flooding across the South Island's east coast, closed highways, and forced a local state of emergency to be declared in the Ashburton District for seven days.
Niwa forecaster Seth Carrier said the atmospheric set-up behind the storm came with several notable and unusual features.
The main reason the rainfall was so intense was that the low-pressure system driving it, and currently centred off the coast of the North Island, was tapping into moisture in the subtropics, and even the tropics, via a 4000km-long "atmospheric river".
"Basically, the position of the low pressure is kind of ideal to wrap this moisture into the east side of the South Island."
That moisture was swirling in a clockwise flow around the low- pressure system, and being swung straight on to the east coast of South Island.
The Canterbury coast north of Ashburton is currently under a Wind Warning for gale southeasterly winds until 11am Monday morning.
— MetService (@MetService) May 30, 2021
The rest of the the Canterbury region is under a Wind Watch for the same period.
Further details available here: https://t.co/qHyE5zhh6X ^Ash pic.twitter.com/wysIi0rbxe
"This is sort of like a West Coast rain event, but in reverse," Carrier said.
"Normally, we have a westerly wind flow much of the time and that brings that heavy rain into the West Coast.
Update on predicted Canterbury river flows from the @niwa_nz Water Model 🌊
— NIWA Weather (@NiwaWeather) May 30, 2021
📈 Already running high & will continue to rise
⛔ Don't attempt to cross floodwaters!
🚨 Follow advice of emergency officials
🟪 = extremely high river flows pic.twitter.com/QkwYprCDOu
"When the moisture hits the west side of the Southern Alps, the moisture rises, condenses and then we get really heavy rain on the West Coast.
"Then, as these air masses get over the top of the alps and comes back down the east side, they've usually dried out and Canterbury gets very little of the rain."
The fact the opposite had occurred in this case was unusual.
"Right now we have easterly winds instead of westerly winds, and we have a really moisture-rich air mass, bringing that heavy rain into the Canterbury instead of the West Coast.
"The air mass is moving up the east side of the alps, causing it to condense, and it's squeezing all that heavy moisture on to the eastern foothills."
This morning, Canterbury Snow and Weather Watch storm chaser Josh Oliver said it had been pouring in the region for the past 24 hours, and there was still about 30 hours of the weather warning period to go.
👀 A glance at some of the top rainfall totals over the last 24hrs
— NIWA Weather (@NiwaWeather) May 29, 2021
🌧️ More than a month’s worth of rain has already fallen with more to come
❗Rivers have begun to respond to the rain & rise rapidly - keep up to date with latest info from emergency services pic.twitter.com/H7Si0RBce5
"We've just heard that Mt Somers has already hit 300mm, which was the upper level of what the warning was for ... we're well on task to hit 600mm, which in New Zealand is almost unheard of, let alone in Canterbury."
Another notable feature of this event was how slowly the low-pressure system - currently centred off the coast of Waikato - was moving.
"On Monday, the centre of the low is likely going to cross over Auckland as it starts moving to the east," Carrier said.
"But effectively, we are talking about 48 hours of the low not moving very much. And that is allowing that moisture-rich fetch to keep coming around the east side of the North Island, and then slam right into the eastern South Island."
Carrier said the sluggish speed of this low likely made it what meteorologists call a "cut-off low".
These were deep low-pressure systems that tended to drift slowly, and sometimes even stall, because they were too far from a jet stream current that would otherwise zoom them along faster.
"It basically happens where the low pressure at the surface is actually driven by what's going on up higher in the atmosphere," he said.
This video is of the south branch of the Ashburton river up at Inverary Station, sent in by Liv Thomson.
— MetService (@MetService) May 29, 2021
Thank you for sharing this footage with us, Liv, and we hope you're staying safe!
^Al pic.twitter.com/6JRTeOkGOC
"It's a level that we call 500 millibars, which is lower than the height that jetliners fly, but it's way up there - probably 6000 or 7000 metres."
Because the low pressure at the surface behaved in the same way as what was happening high above - and moved slowly - the wild weather that came with hung around longer.
"You can almost think of it as like an atmospheric traffic jam, and we can expect that's going to take longer than normal to move."
Even further in the background was the influence of a climate driver called the Madden-Julian oscillation (MJO).
The largest element of the intra-seasonal variability in the tropical atmosphere, the MJO was a pulse of rain and thunderstorms that circled the globe every 30 to 40 days.
In certain phases, it could power weather patterns and drive big downpours over New Zealand.
Without the more predictable flavours of climate systems El Nino or La Nina, the Pacific lingered in an in-between state called El Nino/La Nina Southern Oscillation (ENSO) neutral.
"When you're dealing with ENSO Neutral, which we are right now, and probably will be for the foreseeable future, it's the smaller-scale features like the MJO that can drive the weather," Carrier said.
"When it comes through, it can kick off these areas of low pressure, which is the situation we're in right now."
MetService meteorologist Ashelee Parkes said the wild weather wasn't expected to stop soon.
New Zealand is set to endure a mixed bag of weather next week as more severe rain, warm temperatures and heavy snow are set to batter the country.
The east coast of the South Island should expect a "good southerly" on Monday, but the lower half of the North Island, including Wellington and the Wairarapa, along with the Gisborne region, is set to be doused with more rain.
As the top half of the country is bracing for more heavy rain, the South Island may get some heavy snow.
Parkes said heavy snowfall was expected above 1000m, meaning those areas would be hitting cooler temperatures.
"Going into the rest of the week, eastern areas should be mainly fine. There may be scattered high cloud and certain areas will see some showers."
Intense rain events like this weekend's are also expected to become more frequent under climate change.
Police closed Lewis St at 10.45am for investigation.