NIWA is warning Aucklanders to be wary of trees weakened by Tuesday night’s storm that could be bowled over and result in further outages tomorrow.
The death throes of a massive marine heatwave, a nasty low pressure system and a weird wildcard called a "sting jet": the beast that struck New Zealand yesterday was a stew of dramatic weather and climate factors.
That one of its biggest punches landed straight on the face of ourbiggest city was also a rare event.
Meteorologists have broken the storm, which also handed Auckland, Wellington and Dunedin their coldest night of the year - into three parts.
The 'marine heatwave'
Fascinatingly, the intensity of the storm could partly be attributed to the same reason our surf was so pleasant to swim in over summer.
Since November, sea surfaces around New Zealand, particularly the Tasman Sea, have been warmed by the biggest marine heatwave seen here in 150 years of records.
The marine heatwave was caused by a rare and powerful combination of a La Nina climate system, a Southern Annular Mode locked in a positive phase, a series of persistent highs and the background influence of climate change.
As a cold mass of air moved up across the ocean towards New Zealand from Antarctica this week, it met these warmer waters in the Tasman Sea.
Riding over those unusually warmer ocean waters created a bigger difference in the system's temperature gradient, Niwa meteorologist Ben Noll said.
"The greater the difference in temperature gradient, the stronger the weather system is going to be," he explained.
"If you ran the same weather system over top of the Tasman Sea when it had normal or even colder than normal sea surface temperatures, you would have had a lower impact.
"In this case, if the seas are warmer, there's more water vapour in the air - and that meant more precipitation, and also more strength as well, because of how it had connected with that very chilly cold pocket of air in the upper atmosphere."
It wasn't the first time the big marine heatwave meddled with big weather events that have hit New Zealand.
Meteorologists also blamed it for charging up ex-tropical cyclones that slammed into the country over past months.
Noll expected what was left of the now-fading heatwave had probably been spent on energising this week's storm.
"If you think of it like a bank account, when storms run over those warmer seas, they take out chunks of money and your account goes down.
"This one probably just drained the account and used whatever energy there was left in it."
Co-incidentally, a major study published today found such events had grown longer, stronger and more frequent over the past century - and the trend would continue as the planet warmed.
The low from the west
As the southerly front rolled up the country from south to north, it also unleashed thunderstorms and high wind gusts.
Storm damage at Sandy Bay Rd, Bayswater, Auckland. Photo / Doug Sheering
These hit Taranaki and the central North Island especially hard yesterday morning, downing power lines and tearing roofs from homes.
Meteorologists still weren't sure if tornadoes were actually involved, or just thunderstorm down-bursts or "straight-line winds" that might have appeared similar to people on the ground.
After the front had passed over the middle of the country, there was a brief afternoon lull when the weather improved.
But a deepening low was waiting in the wings west of New Zealand, and made itself known to people in the evening.
When it finally stepped on to the stage, it combined with the southerly front already in play - and most of the drama happened directly over Auckland.
"We had a deepening low moving in from the Tasman Sea; we then had that low making landfall in Waikato and brushing South Auckland which then placed the strongest winds directly over Auckland City," WeatherWatch head analyst Philip Duncan said.
"To help keep the energy, the powerful nationwide Antarctic southerly was roaring up behind it fuelling more winds.
"Finally, on top of all that, these worst winds then tracked across Auckland via a natural wind tunnel."
A lightning strike seen from near Inglewood, Taranaki, yesterday morning. Photo / Steffen Sahin
All of that combined created damaging gales that were a notch above what had been forecast, Duncan said.
At some points, winds over Auckland reached up to hurricane-strength, at more than 200km/h.
MetService meteorologist Tom Adams shed some more light on the one-two punch that hit the city.
"Low pressure systems in the Southern Hemisphere spin clockwise, so if you're slightly to the north of that low, you've not only got western winds, but the low is also moving very quickly," Adams said.
"It's kind of like throwing a tennis ball from a train - the speed of the tennis ball is what you've thrown it at, but also at what speed the train is travelling at.
"So the winds in Auckland were blowing at the speed at which they were going around inside the low, in addition to the speed at which the low was travelling."
Noll said Auckland fell straight in the northwestern portion of the low, and the areas to the south fell in its calmer centre.
"The winds were always going to be maximised on the north-western quadrant - that which passed through Auckland - but underneath the low pressure centre, in Waikato, you had relatively tranquil or lower wind speeds than you did to the northwest."
The polar southerly also doused colder areas in snow, and brought the coldest night of the year so far to Auckland, Wellington and Dunedin.
Temperatures at the centres plummeted to 9C, 7C and 4C respectively.
Adams said a sting jet occurred at the "mesoscale", or the smaller scale of weather systems.
It happened when a zone of strong winds, originating from within the mid-tropospheric cloud head of an explosively deepening depression, were powered up as the jet descended, drying out and evaporating a clear path as it dropped.
Richard Draper had his roof blown off his home on Monte Cassino Place, Birkdale. Photo / Brett Phibbs
This evaporative cooling led to the air within the jet becoming denser, which in turn led to an acceleration of the downward flow.
The jet then began to hook around the system's centre - much like the sting from a scorpion's tail, hence the name - bringing more damaging winds.
"There have been people suggesting that this happened," Adams said.
"I can see on the satellite imagery some things that certainly suggest it happened, but it requires a lot more investigation to know for sure."
Adams said the big picture of the storm was ultimately one of three parts - the climatological picture enhanced by the marine heatwave, the "synoptic" picture that was the storm itself, and the "mesoscale" picture that included the windfunnel and possibly a sting jet.
"So all of those things have been added together and each has made their own contribution," Adams said.
Although that combination wasn't necessarily a freak occurrence, Adams said it was rare to have all of those factors converging at once on a major population centre like Auckland.
"And let's not forget there was significant damage in Taranaki as well - but there's been less in the media, probably just due to the population density."
