The moisture-packed system soaking the North Island bears some interesting similarities to the freak storm that delivered Auckland’s wettest day on January 27 – along with some other striking weather features.
As with the Anniversary Weekend floods, this system was bringing heavy amounts of rainfall - more than 30mm drenched Auckland in the space of an hour at midday, with lower intensities forecast this evening.
Yet the totals Auckland saw in the January 27 deluge - which dumped 71mm in the space of an hour, and around 250mm in parts of the city over 24 hours, causing widespread flooding and landslides - were far greater.
Weather data from Auckland Airport showed that nearly 150mm had already fallen there this month - adding to a whopping 886mm recorded at the site since the start of the year.
So what did the two rain-makers have in common?
Niwa forecaster Chris Brandolino said an obvious link was that both happened to be fed by moisture stemming from the subtropics above New Zealand.
Heavy rain & thunderstorms will end this evening for Auckland & Northland.
— NIWA Weather (@NiwaWeather) May 9, 2023
However, from BoP to HB and Gisborne heavy rain & thunderstorm threat until later tonight.
A winter-like pattern tomorrow with thundery showers - some heavy - along with South Island elevation snow. pic.twitter.com/trUB1fM98a
Just as with our coastal seas, waters across the West Pacific have been running abnormally warm over three consecutive years of La Niña - building up a potent source of subtropical moisture that’s constantly reached New Zealand’s north in the form of atmospheric rivers.
While La Niña itself had faded, Brandolino said a lag effect in the ocean-atmosphere state – or which he likened to a lingering “La Niña-like cough” - meant we were still seeing its wet and warm characteristics in some visiting systems.
“The vast majority of the biggest heavy rainfall and flooding events in New Zealand occur when there’s a tropical connection – whether that’s an atmospheric river, or an ex-tropical cyclone ... all of those things are coming from the north.”
Again, that moisture was being assisted here with help from a low-pressure system in the Tasman Sea.
Other similar features in the mix included a large convergence zone where different air masses were colliding.
“Basically, we have a low to our west and a blocking high to the east – and the two are acting like cogs, with their air-flows going around each of them,” he said.
“That air happens to be meeting right in our neck of the woods.”
⚠️☔️The heaviest areas of rain are now pushing into the #CoromandelPeninsula, #Hunua/#Miranda area, #Waikato, #Waitomo (white boxes).
— WeatherWatch.co.nz (@WeatherWatchNZ) May 9, 2023
⚠️More general rain for #Auckland & #Northland but can't rule out isolated heavier falls.
📡Track on @MetService Radar: https://t.co/sZ6VGec556 pic.twitter.com/XjJxchTbdv
Convergence zones were also well known for setting up intense rainbands, by enabling convection – the transport of heat and moisture – to quickly develop.
As with January 27, meteorologists were also observing high moisture content in the atmosphere – and low-lying jets of strong wind siphoning in more warmth and rain.
This time, though, the system and its rain was more widespread across New Zealand.
Brandolino said the Anniversary Weekend downpours happened to have more “training” - or more rainfall concentrated on an area over a shorter period of time.
Forecasters have also observed what’s called a “bow echo” - an effect describing how bands of rain showers or thunderstorms “bow out” when strong winds, associated with the storms, reach the surface and spread horizontally.
Tuesday will be an active day for a good part of the country, especially the upper North Island.
— NIWA Weather (@NiwaWeather) May 8, 2023
Here's a look at our high-res forecast - note the bow shaped 🏹 appearance. This indicates...
🌬️ Risk of damaging thunderstorm wind gusts
💧 Heavy rain/flooding are also a concern pic.twitter.com/onKGqhIKsF
The effect was typically associated with intense convection, damaging straight-line winds, thunderstorms and extremely heavy rain.
“What can happen is down-drafts and thunderstorms can tap into higher wind speeds, perhaps 2000 to 3000 metres above the ground – and these wind speeds can then be brought down to the ground, which is what we call momentum transfer,” Brandolino explained.
On rain radars, these forces could be seen pushing out wind and rain laterally ahead of the main thunderstorm - and typically in the semi-circular shape of a bow.
“It’s been only in recent times, with modelling becoming so high-resolution, that we’ve been able to predict [bow echoes] before we see them actually play out.”
Deeper in the background, of course, was the influence of climate change.
In particular, warming sea surface temperatures - as have been observed across the Southwest Pacific and Tasman Sea over the past seven decades – translated to more water vapour.
As air warmed, its capacity to hold water increased at an average 7 per cent per 1C of temperature rise, and because warmer air could carry more moisture, this in turn allowed more evaporation from the oceans – and more fuel for systems like this.
While the tropical air was expected to leave us around the middle of the week, MetService was forecasting the Tasman low and a cold front to our southwest to both reach the country tomorrow.
Rain continues to move through with heavier bursts at times
— MetService (@MetService) May 9, 2023
Northland continues for the afternoon and eases this evening, while Auckland goes until evening and eases tonighthttps://t.co/qHyE5zzql5 pic.twitter.com/ljwmlUwOwC
That was expected to bring much cooler temperatures as far north as Kaitāia, along with further rain and strong winds.
Snow was also expected to lower to about 400 metres over the South Island on Wednesday - and to about 800m over the central North Island on Thursday.
Unfortunately, Brandolino couldn’t rule out another subtropical deluge over coming weeks.
“We probably will get another one – I’m not sure if it’ll be like this – but certainly, the odds for heavy rain look to be higher around late May and early June,” he said.
“After that, we could see a more decided transition away from what we’ve had for the past several months and years.”
That’s down to a pair of forming climate drivers - El Niño and a positive phase of a phenomenon called the Indian Ocean Dipole - working in tandem to bring cooler, drier days over winter, in what should be a sharp shift from the unprecedented warmth and rain the country experienced last year.
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