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West Auckland hit by “incredible” rainfall – Expert Reaction

Parts of Auckland – especially in the west – have had an “incredible” amount of rain in overnight thunderstorms, according to NIWA.

Fifty homes have been evacuated, people trapped, and cars swept away due to flooding. The massive storm and floodwaters come as the city enters at least another two weeks at the highest level of Covid-19 lockdown. A heavy rain warning for Auckland, the Coromandel Peninsula, and Great Barrier Island is in place until 4pm today.

The SMC asked experts to comment.

Dr Emily Lane, Hydrodynamics Scientist, and leader of Mā te haumaru ō te wai, flood resilience Endeavour project, NIWA, comments:

“Kumeu just experienced its second-wettest day since records began – with over 200 mm falling overnight. Unfortunately, intense rainstorms like this are going to become more frequent and more intense as climate change progresses. While this most recent event fell in Auckland, these heavy rainstorms could happen anywhere around Aotearoa. It is the same story that we saw in Canterbury and Westport earlier this year, or in Napier and Taranaki at the end of last year.

“The event last night happened as a double whammy because of the current CoViD-19 Level four lockdown in Auckland, which makes it especially hard. Imminent risk to life from flooding does over-ride the level four restrictions but it is sensible to do it in ways that maintain physically distancing as much as possible so that it doesn’t exacerbate the pandemic further down the track.

“These sorts of cascading hazards – where one hazard happens on top of another, potentially compounding the effects of the two – are also likely to become more common with climate change and often it is those that are already most vulnerable that are hardest hit.

“We are already locked in to some amount of warming but the future could be much worse if we don’t act now. We need to both mitigate the future effects of climate change by reducing our carbon footprint and also work with at-risk communities to help them adapt and cope with the effects of flooding and other climate change impacts in ways that are fair and equitable.”

No conflict of interest declared.

Professor James Renwick, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, comments:

“The heavy rainfall experienced in West Auckland was unusually severe, with up to 200mm or so of rain in 24 hours to 9am Tuesday morning, and considerably more outside that period. That made it the second-wettest day recorded in Kumeu, in the nearly 80 years of records there.

“Extreme events like this are becoming more common around the world as the climate warms, because warmer air holds more moisture (about 7% per degree of warming). So when there’s a storm, there’s more water available to fall out of the sky, so the chances of heavy rainfall go up. That doesn’t mean every storm delivers a record rainfall, but it becomes easier and easier to break rainfall records, as the climate continues to warm.”

Conflict of interest statement: “I am a New Zealand Climate Change Commissioner, I receive funding from MBIE for climate research.”

Lisa Murray, meteorologist and Head of Weather Communications, MetService, comments:

"This has been a devastating flooding event for many in West Auckland who may not have experienced such heavy, persistent downpours in their lifetime. The heavy rain was due to a band of rain with embedded thunderstorms which stalled over the Auckland region, and in 24 hours 150 to 260 millimetres of rain was recorded in areas around West Auckland. Some hourly rainfall totals reached 40mm - and even an hour of rain this heavy will have a significant impact, as seen by the flooding experienced.

"The large band of thunderstorms left a trail of damage through West Auckland while other areas of Auckland saw a lot less rain, for example Auckland Airport recorded just 20 millimetres in the same 24 hours, showing the difficulty of pinpointing the area of impact of a thunderstorm.

"Although climate change has been linked to increases in severe weather events, there is no clear connection with respect to increasing thunderstorms yet. However, climate change and a warming planet mean flooding from rain events is expected to become more intense, but not necessarily more frequent. In general, more heavy rainfall will increase the likelihood of rivers flooding and flash-flooding, especially when drainage systems become overwhelmed.

"Impacts from severe weather events such as flooding and the ensuing clean-up are hard to deal with at the best of times let alone during a Covid lockdown. MetService is deemed an essential service with our meteorologists briefing Auckland Council and NEMA on Monday morning ahead of the event about the uncertainty of the forecast. They continue to monitor this event 24/7 as the rain band moves over Northland."

No conflict of interest declared.

Ben Noll, Meteorologist, NIWA, comments:

“Kumeū in West Auckland has now experienced its 2nd-wettest day on record:

– 208.2 mm of rain fell from 9am Monday to 9am Tuesday

– 201 mm of that fell in 14-hours from Monday night-Tuesday morning

– 149% of the August monthly normal rainfall fell in a single day

– Records in the region date back to 1943

“As air warms, its capacity to hold water increases at the Clausius-Clapeyron rate. That’s approximately 7% per 1C.

“Because warmer air can hold more moisture, this allows more evaporation from the oceans.

“And because of this, with climate change, we expect downpours will become more intense. More intense downpours make flash floods more likely.”

No conflict of interest declared.

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