It wouldn't be news to anyone in rain-soaked Auckland that this month has been miserable.
Now meteorologists say the big drivers that gave winter a depressing third act also cooled down seas which have been running warm since 2017.
August might also mean the end to a 30-month streak of above-average temperatures - as well as a record number of rainy days in the City of Sails.
Auckland is on track to see a total 29 rain days this month – breaking a record set only last month – and Saturday may bring only August's second clear day over the city.
Niwa figures showed, up until Sunday, Auckland had received 182mm of rain for the month – that's 165 per cent of normal – while the mean daily temperature had been running at a slightly-above-average 12.1C.
Auckland's rainy month had eclipsed that of other centres, such as Wellington (90mm so far), Christchurch (37mm), Hamilton (124mm) and Tauranga (124mm).
Niwa meteorologist Ben Noll likened New Zealand's weather up to the start of the month as a smooth motorway.
July had clocked in as the second hottest on the books and came after a balmy six months that also brought one of the country's warmest starts to the year.
"We were trucking along easily – but now we're on a different road now, and it's rocky."
So what drove winter's grim third act?
Noll explained that the Southern Ocean, usually hugely influential in flavouring our winter weather, had been in a largely dormant state throughout the first half of the season.
"We weren't seeing those cold air masses coming up and generating that instability which causes low pressure to form in the Tasman Sea, and moisture to then be dragged here to New Zealand."
But that changed when meteorologists began noticing how stronger activity in the jet stream – a band of fast-moving air sitting about 10km above us – was bolstering low pressure systems as they approached the country.
"That partly explained why we saw a lot of severe weather in various parts of the country over the month."
At the same time, stronger southwesterly airflows across August saw sea surface temperatures around the country plunge.
Noll noted that these sea temperatures had been sitting at above average for the past two and a half years – and the cool change had finally allowed colder masses from the south to reach us without being "muted" by the balmier waters before they got here.
"Now that they seem to have been taken out of the equation, these systems that track northward from the south were really able to show off their true colours."
The result: chilly, wintery weather.
Another big indicator of August's bleak turn was that the Southern Annular Mode, or SAM, had largely been stuck in a negative phase throughout most of the month.
The SAM can generally be described as a ring of climate variability that encircles the South Pole, but stretches far out to the latitudes of New Zealand.
First identified in the 1970s, it involves alternating changes in windiness and storm activity between the middle latitudes, where New Zealand lies, and higher latitudes, over the southern oceans and Antarctic sea ice zone.
In its positive phase, the SAM is associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.
It was a predominantly positive SAM that had contributed toward our past two unusually balmy summers, as well as two marine heatwaves that dramatically warmed up the oceans surrounding New Zealand.
But in the negative phase – as we've experienced for weeks now - the westerlies increase over New Zealand, with more unsettled weather, while windiness and storm activity eased over the southern oceans.
It remained to be seen whether August's colder weather would be enough to break a 30-month run of above average temperatures.
All of that has come as meteorologists are closely monitoring a rare polar phenomenon that may bring more freezing weather our way during the first part of spring.
What's called a sudden stratospheric warming event, or SSW, occurred when the temperature of the stratosphere high above the South Pole climbed by more than 25C.
That could weaken the polar vortex – a swirling, freezing air mass that usually kept harsh, wintry conditions locked up tight around Antarctica – and send cold blasts in the form of "streamers" northward toward us.
Today is a busy day at NIWA as we compile our climate outlook for spring 🌱
— NIWA Weather (@NiwaWeather) August 27, 2019
Heads up, September is looking like it could be a particularly bumpy month! pic.twitter.com/vZmVEGxaDz
Noll said if these effects interacted with those from a system expected from the north early next month, temperatures might dive dramatically.
"We would expect it to be felt most notable between September 7 and 14, with some cold snaps possible for most of the country – and potentially some more snow as well."
Between now and October, Niwa has forecast temperatures to be near average or above average for all regions.
A Sudden Stratospheric Warming event is still forecast to peak on Friday 30 August over Antarctica.
— NIWA Weather (@NiwaWeather) August 26, 2019
🗣️ Impacts take time to trickle down through the atmosphere, but particularly unsettled weather conditions may unfold around NZ starting in the 1st or 2nd week of September. pic.twitter.com/WD3wKHPpdT
Rainfall was expected to near normal or above normal in the west of the North Island and north of the South Island, with near normal rainfall forecast for all remaining regions.
In the shorter term, meanwhile, MetService has forecast more rain moving over the country across the next day, as another cold front moved up from the south.
Colder temperatures in the atmosphere were making for some snow - possibly down to 500m in places the Far South.
Road snowfall warnings were in place for Milford Road, Crown Range Road and Lindis Pass.
