Is it spring? Is it winter? Call it "sprinter".
After a burst of thunderstorms tracked across Northland and Auckland early this morning, packing more than 1300 lightning flashes, rain and cold is predicted for much of the North Island today.
Cool conditions may also bring snow to low levels over the South Island, and could affect the highest North Island roads.
Monday and Tuesday aren't looking much better, as a slow-moving front expected to affect the central North Island brings rain to many areas.
On top of that, MetService said the weather system sweeping in from the Tasman this weekend had also collected smoke and dust from bushfires in northern New South Wales.
While it wouldn't pose any health threat as it arrived on our shores today, MetService reported it might help produce some colourful sunrises and sunsets over the next few days.
The troll from the pole
If it feels like there's an extra chill in the air for this time of year, forecasters have pointed to the background influence of a strange event that's opened Antarctica's freezer door upon New Zealand – and it may be here until the second half of spring.
The last time the country was affected by a major rare stratospheric warming event, or SSW, was 2002, when it delivered the coldest October in 20 years.
History might be about to repeat. The latest guidance points to a sluggish, wintry start to spring and a potentially volatile October.
SSW events are remarkably rare in the Southern Hemisphere – just two have been recorded since the 1950s.
They happen when the temperature of the stratosphere 30km to 50km above the South Pole climbs by more than 25C, reversing winds from westerly to easterly.
Usually, the swirling, freezing air mass that is the polar vortex is effective at keeping harsh, wintry conditions locked up around Antarctica.
But an SSW could weaken it or displace it into the stratosphere, sending cold masses filtering down on to the tropospheric polar vortex, where it could then influence our own weather patterns.
The natural phenomenon is more common in the Northern Hemisphere, where one recent SSW unleashed a series of cold blasts from the North Pole to Western Europe and the UK, along with the east coast of the United States.
The media dubbed it "the beast from the east".
Niwa meteorologist Ben Noll said the fingerprints of this latest SSW could already be seen in the chilly conditions over New Zealand since the start of September.
"The cold weather that we are going to be seeing over the next few days is happening in conjunction with this event, which is well and truly under way and has been since late last week," he said.
Noll expected the SSW to be a protracted event, with several major peaks, that would endure through this month at least.
"You can think of this polar set-up as a gate that has basically been swung open, allowing some of those colder air masses to leak northward into the mid-latitudes," he said.
"And New Zealand is certainly one of the recipients of those cold air masses as we go through the weekend. If you are a farmer in the South Island, and you are right in the middle of lambing or calving, it's going to be a really cold time of year.
"If you're a skier or a snowboarder, it's not all bad news, and it's probably good for the tourism industry. But people wanting to get out for tramps at higher-elevation areas are going to want to keep an eye on those daily forecasts."
While spring was notorious for its variable weather, the current anomaly was running at as much as -10C, he said.
"If we look to the longer term, the impact might last from weeks to a month or two, as we saw in the 2002 event."
In that event, although the SSW unfolded in September, it took until October for the stratosphere to "couple" – or begin communicating – with the troposphere below it.
"The guidance we have fresh off the press is showing a volatile October in the Southern Hemisphere," he said.
"One of the things that we look for, in terms of impact, is higher pressure building over the polar region. So, when we have all of this higher pressure forecast near the pole, that means in the mid-latitudes of the hemisphere, we have a tendency to have lower pressure."
Noll expected to see the Southern Annular Mode – a ring of climate variability encircling the South Pole, and a key indicator of storminess in the Southern Ocean – strongly pulse negative, bringing more westerlies and unsettled weather over New Zealand.
"So it looks like the weather will be behaving, in broad strokes, more like winter than spring over the next few weeks for New Zealand."
The bigger picture
It wouldn't just be the SSW event icing the road ahead.
Noll pointed to coastal sea surface temperatures that, after long running hot, had dropped down to near-average levels.
"That means warmer seas won't be there to mute the impacts of those cold air masses that come up from the Southern Ocean."
Further out in the central equatorial Pacific, cooling sea surface temperatures had brought the end to a weak but long-lingering El Nino climate system, and ushered ocean conditions into the neutral range.
But warmer-than-average seas in the west-central tropical Pacific were still expected to occasionally influence New Zealand's weather patterns over coming months, contributing to sub-tropical low pressure systems that could bring heavy rainfall.
Noll also singled out something called the Indian Ocean Dipole.
"That is basically cooler than average sea temperatures north of Australia, and has a role in influencing our climate here in the September to October period."
It wasn't yet clear how this driver would interact with the SSW – but it spelt the potential for cold, dry spells, and also happened to have played a hand in 2002's unusually frosty October.
Plenty of thunder might also be on the cards.
"When air masses emerge from the Southern Ocean and meet that warm air being pulled off Australia at this time of year, it's basically atmospheric fireworks," he said.
"Where this cold and hot air meet is often right in the middle of the Tasman Sea – and New Zealand, sitting direct to the east, could be the recipient of what we'll get from these constant clashes."
Over the wider period, between now and November, Niwa has forecast lower-than-normal air pressure – especially to the south of the country – and more southwesterly winds than usual.
Temperatures are forecast to be near average for most of the country except the north and west of the South Island, where near-average or below-average temperatures are about equally likely.
Rainfall is forecast to be near or above normal for most regions, except for the west of the South Island, where near-normal rainfall is most likely.
Noll expected the cold turn could change the track of what had been an unusually warm year.
"Our year to date has been pretty high up there in terms of the temperature records, mostly thanks to that very, very warm start to the year."
The January to August period had come in at 0.96C above average – second only to the corresponding stretch in 2016, which was 1.10C above average.
"Given the outlook we have for the next couple of months, we may see that average tail off a little bit."