Aucklanders are being warned to prepare for more downpours tonight, amid potential for a second “atmospheric river” sweeping into the upper north later in the week.

Severe thunderstorms are expected to hit the region this afternoon, with MetService issuing a severe thunderstorm watch for Tai Tokerau and Auckland from 7pm today through to 10am on Monday as a slow-moving front drifts west over the regions.

Thunderstorms were possible with this front - some potentially severe – along with downpours reaching intensities of 20 to 40mm per hour.

MetService meteorologist Georgina Griffiths told a media conference today that Auckland faced a “hang-in-there type of situation”.

 “We are very vulnerable in the region at the moment to any rainfall ... rainfall that normally would not cause problems is exacerbating slips and causing localised flooding.

“We’ve already had some minor rainfall heavy showers in the southeast this morning, and we’ve had some problems.”

Because of that, MetService had lowered its normal thresholds for heavy rain for the region.

“Of particular note is that the showers on the south and east have eased somewhat, but do continue on and off for the remainder of today,” Griffiths said.

 “Some of those showers will pop up with modest rainfall amounts in central, and north and east of, Auckland later today.

“Most of these won’t cause issues, but if we see any rainfall rates of 10mm or 15mm an hour, we may see some brief localised, ankle-deep type flooding.”

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Still, this activity would be brief, and unlike what occurred on Friday.

She said the main concern for the remainder of the day was for areas north of Orewa, including Warkworth and Wellsford.

“We are under heavy rain watch till 6am tomorrow morning. We also have risk of downpours and the MetService has issued a severe thunderstorm watch for the north of the region, north of Orewa,” Griffiths said.

“If we see those thunderstorms form tonight and overnight, and if we receive rainfalls in the 20mm to 40mm an hour range - which is getting up to what [some stations saw] on Friday, then we will see impacts in that northern region.”

Griffiths said Monday would bring a reprieve – and she encouraged Aucklanders to use the break to prepare for a “potentially significant” system arriving later Tuesday or during Wednesday.

“Given the region’s vulnerability, we may not need much rain in Auckland to see some impacts from a flooding slip point of view.”

Niwa Weather forecaster Ben Noll said details of that later-week event were still coming through, “but it looks like we could be in for another intense atmospheric river event for the North Island”.

What are atmospheric rivers?

It’s a phenomenon that many Kiwis would already be all too familiar with after three unusually wet and wild years – and they’ll likely be hearing about it again this week.

Put simply, these rain-makers are just what they sound like: rivers in the sky.

The monster storm that put Auckland underwater on Friday night, in what was the city’s wettest day in history, was an atmospheric river in action.

Snaking thousands of kilometres over oceans, and when bridged by visiting low-pressure systems, they form highways of rain between the balmy subtropics and New Zealand.

Each year, about 40 of these long, thin filaments of atmospheric moisture - capable of carrying double the average flow of the Amazon River, or 200 times that of the Clutha – make landfall here, typically around summer.

But these giant rain-makers can strike in winter, too: as dramatically shown by a deluge that put swathes of Canterbury farmland underwater in 2021, and another that forced the evacuation of half of Westport the same year.

Recently, Kiwi scientists found the characteristics of atmospheric rivers could differ, depending on where in the country they struck.

In the south, their main driver appeared to be the strength of westerly winds across the Southern Ocean, while, in the north, the biggest factor was moisture coming from the subtropical Tasman Sea.

That’s precisely what we saw happen on Friday, when Aucklanders were hammered by rain siphoned direct from latitudes more than 2000km above us.

Of course, the formation of an atmospheric river wasn’t the only remarkable aspect of the big deluge, which brought an entire summer’s rain in a single day.

Meteorologists have described a freak combination of elements that aligned to create one of the most extreme weather events ever observed in this country.

Among them, converging winds; a long band of thunderstorms squeezed within an already dense subtropical low; a blocking high to the south-east that slowed it as it moved over Auckland; and an accompanying low-level jet that piled on yet more moisture as it glided just above the surface.

They’re also pointed to some of those bigger-picture drivers: notably a persistent marine heatwave, a La Niña system that’s ripened the ocean-atmosphere state to bring rain-makers to the north-east, and climate change that almost certainly made the rainfall more extreme.

‘Mother Nature’s energy drink’

But the timing of Friday’s event, and what’s coming this week, owed to something else that we hear much less about than lows, atmospheric rivers, La Niña or global warming.

That’s the Madden-Julian Oscillation (MJO) – the largest element of the intra-seasonal variability in the tropical atmosphere.

Like a never-stopping freight train, this eastward-moving pulse of rain and thunderstorms circled the globe near the equator every 30 to 60 days.

As this happened, the MJO effectively dissected the planet into two halves: one in which it was enhancing rainfall through convective activity, and another in which that convection was being suppressed.

Only discovered in the last half-century, some of the largest atmospheric river events New Zealand has seen have coincided with the MJO rolling by high above us and fuelling convection.

“Right now, we’re in the MJO phases that are associated with rainfall extremes in the northern North Island, specifically Auckland,” Noll said.

“So, we’re in the window that we’d look for big events, like we saw on Friday.”

Within this window, Noll said jet stream winds around New Zealand were moving slower than average, providing little to move away large blocking high-pressure systems to the southeast.

“That’s enabling the formation of several subtropical lows off to the northwest of the country, but not allowing them to move along as quickly as they normally would.”

Critically, this also came at a time that a long-lingering La Niña had “piled up” warmer than average water across the West Pacific.

“Right across the South Pacific, whether you look at the Coral Sea – and especially eastward from New Caledonia to Fiji and Tonga – that area is running really warm right now,” Noll said.

“And this is actually the direction from which we’re expecting a plume to be dropping down onto the north and north-east of the country on Tuesday.”

That region also happened to be an important source region for atmospheric rivers, especially in this phase of the MJO.

“We’ve now seen the MJO move over these warm waters a couple of times ... and you could almost liken it to an energy drink for Mother Nature.”

Whether the coming subtropical low brought as much rain depended on how swiftly it moved over Auckland – and the latest modelling indicated this could prove a faster-moving system than Friday’s.

Unfortunately, Noll said this wouldn’t be our last taste of MJO-fuelled rainfall this summer.

“After this one passes through, it looks like it comes through again in late February or early March – so there’ll be another opportunity for big rainfall events then.”