It’s too early to say for sure, but if an expected El Nino develops by June, more cold southwest airstreams could blow across the country than in recent winters.

Could a proper cold winter be on the way for a change? There are subtle hints that if an expected El Nino develops by June, more cold southwest airstreams will blow across the country than in recent winters.

Kiwis recognise that southerlies or southwesterlies dredging air up from the subpolar regions can bring snow to low levels followed by severe frost.

Meteorological suckers for punishment know putting even the vaguest of such thoughts into the public arena this far out is asking for trouble. The “commentator’s curse” is as alive and well on the weather front as in the world of sport.

But it’s worth a go, given any colder-than-average winter will well and truly buck the trend of the past couple of decades. With our warming climate, it is difficult to get a colder month than average let alone a whole season. 

At the end of February, NIWA’s successor, Earth Sciences New Zealand (ESNZ), alluded obliquely in its autumn seasonal outlook to such a possibility. Having teased, it said it was a subject for another day and another seasonal outlook in a few months’ time.

But The Spinoff’s interest was piqued and we requested more detail. Meteorologist Jon Tunster came back saying winter could see the start of a “formidable” El Nino, with models predicting its “dramatic and rapid turn” during the season.

Tunster cautiously says at this stage a wholesale cold winter looks unlikely, but there might be a notably cold month or two. 

Autumn would continue to feature slow-moving anticyclones across the South Island and occasional rain-bearing tropical or subtropical low-pressure systems moving down from the north. Then winter might flick the switch to more highs sitting further away over Australia and an increase in cold, sometimes stormy, southwesterlies across Aotearoa.

He says that would mean more rain in the south and west of the South Island, and possibly the west of the North Island, but a risk of drier conditions elsewhere. However, if the southwesterlies turn more southerly, that would spread cold showery weather up the east coasts of both islands and into Wellington.

“A southwesterly surplus naturally raises the cold winter question,” Tunster says. “If this pattern did occur, we would see occasional cold snaps, but generally lasting a few days at a time, except in inland parts of the South Island where the cold air can easily linger for much longer periods.

“It has been proving increasingly difficult to produce entirely cold winters nationwide in New Zealand over recent decades. The last such case was in 1992 and required the Mt Pinatubo volcanic eruption to lower global temperatures. More plausible outcomes would be, for example, a single cold month embedded in an otherwise near average season, or a cold season restricted to localised areas.”

Seasonal predictions – those looking months ahead rather than the usual week or two – are fraught with difficulty. A tiny change in today’s weather models compared with yesterday’s will be magnified beyond recognition the further into the future you go.

Most winters in recent decades have been warmer than their long-term average, based on ESNZ’s seven-station national temperature series which started in 1909.

We have to go back to 1997 for our last nationwide colder-than-average winter. Every winter since has had either near, above or well above average temperatures.

Our warmest winter on record was in 2022, with national temperatures 1.4°C above average. The last three winters have all been warmer than average, with 2024 the third warmest yet (1.0°C above average).

We can’t take it for granted that an El Nino will mean a cold season, but we can’t rule it out either. The chance of it happening appears higher this year than in recent years. It mostly hinges on when El Nino develops.

The next major batch of seasonal modelling is expected to come out next week, which could utterly change what forecasters have seen to date.

If a cold winter fails to come to pass, which is entirely plausible, feel free to forget you ever saw this. But if it does. Well, you read it here first.

What is El Nino?

A warming of the central and eastern tropical Pacific Ocean which disrupts normal global weather patterns.

New Zealand experiences more winds between northwest and southwest, with cold, stormy weather in the west and south, and dry or even drought conditions east of the ranges.