1News weather expert Daniel Corbett explains what we can expect to see as a La Nina weather pattern takes over.
It’s that time of year again – tropical cyclone season, which runs from November 1 to April 30.
The recent, catastrophic Hurricane Melissa in the Caribbean Sea has put the devastation major storms can bring back in the forefront of our minds. The continuing warmer than normal seas around the globe are adding fuel to the fire for the development of tropical cyclones.
Elevated risk of ‘tropical troublemakers’ – Watch on TVNZ+
The 2025-2026 southwest Pacific cyclone season could be an interesting one in that there could be a couple of wild cards putting the squeeze on our summer holiday plans. La Nina and the warmer than normal seas around New Zealand will play their part.
This season there could be close to the average of between four and nine tropical cyclones, with the potential for two to four severe ones, over the entire region. Across New Zealand we always seem to have one or two tropical cyclones to contend with over the season. This could be the case again this year.
What is a cyclone?
First a quick recap. Tropical cyclones, or hurricanes as they are called in many parts of the Northern Hemisphere, are not like the typical fronts and rain makers we experience around New Zealand during the winter and spring.
These types of weather systems develop from a contrast with height within the weather system. That is usually colder above and warmer below, which enhances rising warm air and subsequent cloud growth and rain.
In tropical systems, the development is due to very warm seas, upper-level energy and favourable light winds aloft. It is not driven by a temperature contrast in the atmosphere but more the energy release from the intake of water vapour and moisture over the very warm seas.
This is in turn converted to cloud and rain.
The upper-level energy and favourable light winds allow a twist to develop in the mass of cloud and rain. A bit like a tornado but on a much bigger scale.
Think of tropical cyclones as being like big industrial atmospheric hoovers. They suck up water and water vapour and convert it into energy. This grows into a massive rotating storm with the strongest winds along the walls of the storm’s centre, the eye wall.
Tropical systems also have copious amounts of moisture wrapped up in them so they can bring torrential rainfall more easily.
What’s different this year?
This season the factors that could come into play include the very warm seas in the waters of the southwest Pacific, a brief and weak La Nina, upper energy from the Indian Ocean and the fading Polar Vortex.
Tropical cyclones in our part of the world typically develop during active phases in the tropics that tend to occur every 30-40 days. One active phase has just moved through with potential for the next one at the end of November or early December.
After that it could be during the holiday season and into the new year. Sometimes the timing is not exact. As always, that’s something to bear in mind when you head off to pitch your tent at your favourite beach campground.
The fading Polar Vortex that has played a part in the very active spring weather of late might still send surges of cold and stronger winds into the subtropics in the next couple of weeks. This can put a dampener on any early tropical development due to increased wind or shear in the atmosphere.
La Nina weather patterns typically push our summertime highs further south down across the South Island, so the north of the country gets exposed to more of the humid northeasterlies.
Along with that, the warmer air in the atmosphere form La Nina weather patterns tends to sit north and east of the country which can affect the steering of the incoming lows. It can cause them to linger and cut off, bringing rain for several days.
These sorts of weather patterns can be a real pain for a beach holiday!
Weak and short-lived La Nina
Another interesting wild card that could happen this year is, even though the La Nina will be weak and short lived, we could still have some of the La Nina type effects in our weather systems through the summer.
There could also be the risk of a hybrid tropical system that lingers in one location for several days. This is something that happened with Tropical Cyclone Alfred which hit the Queensland coast in March this year.
Another thing to watch out for!
So, in a nutshell – enjoy the summer but keep the wet weather gear and the plan B handy in case the weather turns because of a rain maker from the tropics.
Watch Dan’s cyclone outlook on TVNZ+
