Auckland floods: Despite weather warnings, size of record storm 'couldn't be predicted'
Saturday, 28 January 2023
Although meteorological services were forecasting heavy rain for Auckland days in advance, officials say the size of its record rainfall “couldn’t have been predicted”.
On Friday, Tāmaki Makaurau experienced its “wettest day on record”, with severe flooding which claimed three lives. A fourth person is missing.
Hundreds of people had to be rescued from their homes, workplaces and cars, trapped by floodwater, slips and fallen trees, and water and wastewater operations remain “significantly” impacted.
MetService meteorologist Angus Hines said the forecasted heavy rain featured in its severe weather outlook early last week, and was reflected in multiple severe weather watches and warnings from Thursday morning onwards.
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Rain began before sunrise on Friday, and MetService issued an ‘orange’ heavy rain warning for the entirety of the day – warning of a risk of thunderstorms and isolated downpours.
At 9.10am on Friday, MetService issued a severe thunderstorm watch for Northland and Auckland, valid to 7pm. This warned that “bands of heavy rain and embedded thunderstorms” were expected to move across Northland and Auckland, and that “some of these thunderstorms may be severe, producing localised downpours of 25-40 mm/h”.
MetService advised rainfall of this intensity could cause surface and/or flash flooding, especially in low-lying areas, and slips. It advised of potentially “hazardous” driving conditions.
It continued to reissue severe weather advisories for Tāmaki Makaurau throughout the day, with updates landing roughly every couple of hours.
Things quickly escalated: rainfall across Auckland turned “extreme and unprecedented”, Hines says.
The heavy downfalls and embedded thunderstorms were caused by warm air descending from the Tropics.
Lisa Murray, head of weather communications at MetService, says while people may not have heard any thunder, it was the long band of thunderstorms producing downpours which “exacerbated the severe situation”.
With torrential downpours not abating, MetService – in consultation with Auckland Council – elevated the warning to ‘red’ for the entire region: only issued in the most extreme weather events.
Auckland Emergency Management deputy controller Rachel Kelleher says at this time of year, it’s “not unusual” for Auckland to experience weather systems that “could potentially bring heavy rain”.
Agencies work closely with MetService to understand the likely severity of these events.
However, “the intensity of Friday’s event escalated rapidly, and localised rain rates were unprecedented”.
Kelleher says they took action as soon as they received intelligence that the storm was escalating, setting up its Emergency Coordination Centre by 5pm.
“Our response teams were quickly deployed and worked alongside our emergency service partners to respond to what has been a devastating storm event, the size of which couldn’t have been predicted.”
Kelleher says the activation and response was “well underway” before a local state of emergency was declared by Auckland Mayor Wayne Brown about 9.30pm.
While a declaration affords AEM more powers, such as enforcing evacuations if needed, “it does not signal the start of a response”.
“Our focus was, and continues to be, on supporting Aucklanders who have been impacted.”
On Saturday afternoon, Emergency Management controller Andrew Clark said the event was beyond anything we've ever seen.
The priority was rescuing people and getting shelter for people who were displaced, and 'we had to find emergency centres in the matter of minutes'.
How unusual was Friday’s storm?
Angus Hines, from MetService, says the numbers were “really remarkable”. The “extremely torrential” rainfall amounts recorded were “unprecedented” for Auckland’s weather stations.
The Auckland Airport weather station has had an unbroken record of observation since 1962. Prior to Friday, its previous record for wettest day saw 161.8mm of rain.
Between 9am Friday and 9am Saturday, the same site recorded 245mm – surpassing the previous record by more than 50%.
Other weather stations across Tāmaki Makaurau also tallied likely ‘record’ rainfall, with many spots noting between 250-300mm of rain within the day.
For context, anything over 6mm within an hour is classified as heavy, and 40mm or more per hour is classified as torrential.
Parts of Auckland saw 80mm per hour on Friday.
At the Auckland Council-owned Harbour Road station, 82.5mm of rainfall was measured in one hour, 80mm at Albert Park, and 71mm at Auckland Airport – all in the hour between 7-8pm, Hines said.
Hines said forecasters can never really be totally sure of the extent of a severe weather event until you start to see what is happening at weather stations.
“We knew of the risk that these [thunderstorms and localised downpours] would be coming through, but you can never be sure of the exact extent.”