New Zealand could be in for "an early visit from the polar jet stream", NIWA warns in new projections.
The weather will be varied in the coming week, but the forecaster predicts a polar jet may emerge from the Southern Ocean come the weekend.
This could bring "unsettled and cooler weather, maybe even some South Island snow," NIWA said on social media on Sunday.
NIWA said the potential polar jet is due to "an active pulse of the Madden-Julian Oscillation over Africa and the western Indian Ocean".
The Madden-Julian Oscillation is characterised by "an eastward spread of large regions of enhanced and suppressed tropical rainfall, mainly observed over the Indian and Pacific Ocean".
The pulse is injecting momentum into the jet stream which could see it "extend from the Indian Ocean, across the south of Australia, and eventually into New Zealand," said NIWA.
Philip Duncan from WeatherWatch said on Monday that next week "looks very autumn-like for a time, with a bit of a storm passing across the South Island".
Meteorological autumn begins on Friday in New Zealand. As New Zealand leaves summer, there is much more distance between the areas of high pressure and the country is entering into one.
"Look at one week from now, bit of a storm on the way," Duncan said. "Welcome to autumn."
Duncan made clear that the prediction was not locked in this far out but said that if the predictions ring true there could be "severe gale force winds across the country".
The wet and windy weather is not expected to last long though.
Duncan predicts that, by Wednesday next week, another high will roll through, warming the country back up.
"This doesn't mean the summer weather is over," Duncan said.
"But it does mean there will be more unsettled days in the mix - which is good news if you need rain."
