Ex-tropical cyclones and New Zealand
How tropical storms lose their warm heart on the way south - and why they can still be so destructive.
An ex-tropical cyclone is a tropical cyclone that has moved out of the tropics, lost its warm core and merged with the mid-latitude westerlies - but it often keeps dangerous rain and wind. New Zealand sits far enough south that tropical cyclones rarely arrive intact; on average around one ex-tropical system tracks close to the country each year, usually between December and April.
What is an ex-tropical cyclone?
Tropical cyclones are warm-core storms that draw their energy from very warm ocean water, typically above 26.5 degrees Celsius. They form in the tropical South-West Pacific near Fiji, Vanuatu and the Coral Sea during the cyclone season. As one of these systems tracks south towards New Zealand, it crosses cooler water and encounters the westerly winds of the mid-latitudes. It loses its tropical characteristics and is reclassified as ex-tropical - but crucially, it can still carry huge amounts of tropical moisture and pack damaging winds.
Why New Zealand rarely sees a "true" tropical cyclone
By the time a South Pacific cyclone reaches New Zealand's latitudes, the sea surface is usually too cool to sustain its tropical structure. The warm ocean fuel that powers the storm simply is not there. That is why New Zealand almost never experiences a landfalling tropical cyclone in the way that Fiji or northern Queensland does. Instead it deals with the transformed, extratropical remnant - which is often larger in area even as its core weakens.
Extratropical transition: what changes
The transformation is called extratropical transition. Several things happen at once: the storm's compact warm core spreads out and cools, it connects to fronts and the jet-level westerlies, its wind field broadens, and its heaviest rain often shifts to one side of the track. The result is a system that may no longer qualify as a tropical cyclone yet can dump prolonged, intense rainfall over a wide area and drive damaging, long-fetch winds and coastal swells. Because the moisture is tropical in origin, rainfall totals can be extreme where the flow is forced up against ranges.
How often does New Zealand get hit?
NIWA's long-term guidance is that, on average, at least one ex-tropical cyclone passes within about 550 kilometres of New Zealand each year, with a higher risk in La Nina years when the South-West Pacific is more active. Not every one causes major damage - many pass to the north or east, or weaken to little more than a rain band. But in a bad year, one that tracks directly over populated regions can become a national disaster.
Notable ex-tropical cyclones
| Storm | When | Main impact |
|---|---|---|
| Cyclone Bola | March 1988 | Extreme rainfall and flooding in Gisborne and the East Cape; one of the country's worst rain events |
| Cyclone Gita | February 2018 | Damaging winds and flooding, worst felt around the top of the South Island |
| Cyclone Gabrielle | February 2023 | Catastrophic flooding across the North Island's east; one of New Zealand's costliest and deadliest weather disasters, prompting a national state of emergency |
Cyclone Gabrielle is the defining recent example: as an ex-tropical system it delivered devastating rain to Hawke's Bay, Tairawhiti and Northland, and MetService issued Red rain warnings during the event. It underlined that "ex-tropical" does not mean harmless.
Why they're so hard to forecast
Extratropical transition is one of the trickiest forecasting problems there is. The exact track, timing and where the heaviest rain lands can hinge on subtle interactions between the decaying tropical core and the incoming westerly trough - and global models can disagree markedly on all three several days out. This uncertainty is precisely what a model-comparison approach is built for. checkweather lines up ECMWF, GFS and ICON and expresses their level of agreement as a confidence score; when a transitioning cyclone is approaching and the models cluster on one track, confidence is high, and when they fan out, that spread is your early warning to plan for a range of outcomes.
Staying safe: watches, warnings and lead time
For an approaching ex-tropical cyclone, watch the MetService escalation ladder: a Severe Weather Outlook flags the threat up to about six days ahead, a Watch follows as confidence grows, and Warnings - Orange or, in the worst cases, Red - are issued when severe weather is expected. Because these systems are large and slow-moving, the rain hazard often outlasts the wind. Heed local Civil Defence instructions, avoid unnecessary travel, and remember that river flooding and slips can peak well after the storm's centre has passed.