Sudden and savage: Auckland's rainfall sets a 'new benchmark'

Thursday, 2 February 2023

On an average January day, 2.3mm of rain normally falls in Auckland.

In just two minutes, between 7:10pm and 7:12pm last Friday, Māngere in south Auckland recorded nearly twice that.

The total amount of rain that hit Auckland on 27 January, 2023 was unprecedented since records began.

But what caught most people out was the ferocity with which it fell.

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In fact, there were dozens of times during that afternoon and early evening when two-minute rainfall exceeded the 30-year daily average for January.

NIWA meteorologist Tristan Meyers says the rain on Friday was “a new high-water mark” for the country’s biggest city.

The agency’s analysis shows it was the wettest month in Auckland since records began in 1853, smashing the previous 1869 record that had stood for 150 years.

“It wasn’t just for one station,” Meyers said. “It was everywhere.”

The weather peaked at slightly different times around the region, but the pattern was the same: steady rainfall most of the day, followed by a sharp, savage burst that lasted only a few hours but left devastation in its wake.

Below is a chart showing the total rainfall accumulated over the 24-hour period. Even before the worst of the weather, daily totals had already exceeded the January monthly averages for Auckland.

NIWA is now describing the rainfall as a one-in-200-year event.

“It was really, really exceptional,” Meyers said.

“When you do those return levels – the rain that fell in 24 hours, the rain that fell in 12 hours – it was all the same. It was saying at least a one-in-150, one-in-200-year event.”

As well as two-minute intervals, the 10-minute rain record also tumbled at various locations.

Experts say the downpour has set a new benchmark for rain in New Zealand.

Previously, the most rain to ever fall in a 10-minute period at Auckland Airport was 14mm. On Friday, nearby Māngere station broke that record not once but three times.

Of course, Auckland was not the only part of the country to experience extreme rainfall this month.

The most recent rain, together with the rain that fell at the beginning of January in the wake of ex-Cyclone Hale, contributed to the wettest summer period in Tauranga, Whāngarei and Whitianga since the turn of the century.

Meyers says the extremes have not been confined to rain. Invercargill is forecast to reach 32 degrees Celsius on Friday. “It could be the hottest day in 75 years there.”

The extremes of weather being observed would likely prompt “a tough conversation” about the future of some of the country’s infrastructure, he said.

“Stuff like this is concerning, and the preliminary estimates are that we expect climate change to contribute 10 to 20% more rain in events like these [in future].”