The anticipated sudden stratospheric warming (SSW) above Antarctica NIWA warned about last week is happening, and its scale is reportedly "impressive".
But that doesn't necessarily mean New Zealand is going to experience the kind of polar blast that froze Europe last year, killing dozens of people.
During an SSW, the air above the icy continent warms more than 25C above normal and the usual westerly winds in the stratosphere - 30 to 50km above the ground - reverse direction, disrupting the stable status quo.
The vortex of "stormy and freezing weather" surrounding Antarctica weakens, letting cold air from the south to rush north. The last time this happened - in 2002 - New Zealand suffered temperatures on average between 2C and 3C colder than normal.
"The event is unfolding as forecast last week," NIWA forecaster Ben Noll told Newshub on Thursday. "It does look like things are going to turn quite unsettled as we go into next week... It takes time to drift downward through the atmosphere. It goes beyond a weather timescale - it's a climactic event and takes time."
A low-pressure is also on its way from the Southern Ocean, and Noll fears that will "tangled up" with the SSW.
"If that happens, the weather around New Zealand could get quite volatile as we go into the second week of September... The beginning of the spring season looks like it's going to be quite rocky."
But the dire forecast is being rubbished by independent forecaster Philip Duncan of WeatherWatch, who said all NIWA can predict this far out is that "spring weather is going to arrive in the middle of spring".
"We can't see anything in the long-range maps at the moment that looks anything different to previous springs," he told Newshub.
"We're looking out to the middle of September now which is when NIWA said this would be flaring up - nothing's jumping out yet that looks dramatic."
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The problem he says is it's impossible to predict day-to-day weather this far out, regardless of what the climate is doing.
"I think this Antarctica thing is fascinating to watch, but no one knows what it means weather-wise. So to me, all it does is it makes a lot of farmers really stressed out that we're going to be hit by an Antarctic blast, when that may or may not happen in any spring in any given year."
NIWA said earlier this week computer models might struggle to cope with the sheer amount of warming taking place above Antarctica.
"If I see a major blast, I'll be the first to say 'here we go, it's happening'," said Duncan.
Noll said if the SSW does send a polar blast northwards, "the Southern Hemisphere is a big place" and we might be spared.
"It could easily go into the Indian Ocean, it could go towards South America."
Duncan and WeatherWatch have been in a long-running battle with NIWA over access to weather data. Duncan said it was "really unusual" taxpayer-funded NIWA would be making predictions about the weather, when we already had the MetService for that, also taxpayer-funded.
"It would be like the fire service doing police warnings. They're both emergency services with flashing lights, but they have two different jobs."
He said the SSW was "fascinating" enough for NIWA to report on its own, without having to make the story about its potential effects on New Zealand weather.