An ex-tropical cyclone potentially veering toward New Zealand early next week is the last thing that storm-battered residents of the soaked upper North Island will be wanting to hear. But how certain – or uncertain – is it that we’ll get an unwelcome visit? Jamie Morton explains the tricky science of tracking cyclones.
First off, what do we know about this coming system?
Meteorologists emphasise that we don’t yet have an actual tropical cyclone - let alone one beating a furious path toward the North Island – but rather the strong potential for one to form out of an intensifying, westward-rolling tropical low in the Coral Sea dubbed 14U.
That’s likely to happen around Thursday, before potentially moving down toward the east and south of New Caledonia - and then perhaps on into our neighbourhood.
“Current models do show it close to the North Island on Monday, but there’s still quite a lot of moving parts between now and then – and plenty of uncertainty regarding its track,” MetService meteorologist Andrew James told the Herald.
Eyes on the tropics, potential impacts for Aotearoa late in the weekend 👀
A tropical low in the Coral Sea is likely to develop into a tropical cyclone in the coming days and current information brings it toward our shores later in the weekend. https://t.co/7wKJumOqMX
If it did happen to veer close to New Zealand, we could expect impacts like heavy rainfall, strong winds and big seas.
At this stage, MetService was forecasting the potential for southeast winds to strengthen around Northland on Sunday, with rain spreading down the North Island early next week.
What are tropical cyclones, anyway?
We can think of them as lows that develop extreme characteristics as they amass energy over warmer tropical waters, high above New Zealand.
Gale-force winds - or those higher than 63km/h - are found at low levels near their swirling centres, but can fan out for hundreds of kilometres.
Each year, over the November-to-April season, about 10 tropical cyclones form in the Southwest Pacific basin.
Only a few of those ever reach category 4 strength - where mean wind speeds reach more than 159km/h, or higher – and category 5 events like 2015′s Pam, or 2017′s Donna, were rarer still.
Models suggest this coming system could become a category 3 event.
Vanuatu and New Caledonia typically experience the greatest activity - with an average of two or three named cyclones passing close to land each year - while at least one system typically came within 550km of Aotearoa, usually around February and March.
But this season began with a slightly elevated forecast risk to the country; Hale’s visit last month caused millions of dollars in damage in Coromandel, while leaving beaches and waterways around the Tairawhiti region strewn with slash and debris.
The aftermath of Cyclone Hale near the mouth of the Waiapu River, with the beach buried under layers of slash and the sea a milk chocolate brown full of silt and sediment. Photo / Graeme Atkins
Why do we call local events ex-tropical cyclones?
To get down here, cyclones have to make their way over much colder waters, while hitting strong upper-level winds as they move out of the tropics.
By the time they arrive, they’re almost always re-classified as “ex-tropical” cyclones – as this one will be if it travels here.
That doesn’t mean they’ve weakened or been downgraded, but have morphed into a completely different type of beast.
And ex-tropical cyclones can still pack the potential for severe weather: under the right conditions, they can intensify and even muster lower pressures than they had before being reclassified.
Many of our most severe storms - such as 2017′s Debbie, which brought the deluge that pushed a Rangitaiki River stopbank to breaking point, flooding Edgecumbe - have been ex-tropical cyclones.
Flooding in Edgecumbe in April 2017. Photo / NZME
In the tropics, the strongest winds and most intense rain associated with a tropical cyclone usually occur just outside the “eye”, or cyclone centre.
But after it has been transformed in what’s call an “extra-tropical transition”, the systems lose their symmetrical cloud patterns.
The strongest winds and heaviest rain can then be found hundreds of kilometres from the cyclone’s centre - usually in a large area south of the centre.
For meteorologists, that means the position of the cyclone centre is no longer a good indicator of where the most severe weather will hit.
In 1988′s catastrophic Cyclone Bola, for example, the heaviest rain and strongest winds over New Zealand occurred well away from the centre.
Across the globe, any cyclone activity is closely tracked by six “regional specialised meteorological centres” (RSMCs), and another six “tropical cyclone warning centres” (TCWCs).
Tropical cyclone specialists track systems using several “ensemble” models that combine global and high-resolution regional models.
One pulls together more than 50, while another combines more than 100.
Model “runs” are made about twice a day, which forecasters use to produce new bulletins, and over busy periods, forecasters monitor the situation day and night.
The stronger co-relating patterns in the models become, the more confident forecasters are in predicting where tropical cyclones will move around the region.
Like many agencies around the world, MetService draws on the “big three” of the world’s numerical weather prediction, or NWP, models.
Those are the Global Forecast System, from the National Centres for Environmental Prediction in the United States; the British Met Office’s Unified model; and the renowned European Centre for Medium Range Weather Forecasting (ECMWF).
The latter two models run every 12 hours, while the US one runs every six. The models typically take between six and eight hours to run, and cover about a week into the future.
While these three combined systems could capture global weather in reasonably high resolution, conditions could always change.
And tropical cyclones happened to be notoriously variable systems embedded within the already complex system that was our weather.
What happens once they’re formed?
Assuming this system reached the criteria for a tropical cyclone, Australia’s Bureau of Meteorology (BOM) would give it a name - drawn from a kept list spanning from Anika to Zelia – and create a forecast track.
Tracks were typically produced every six hours out to 72 hours, or three days – yet the period for which MetService’s meteorologists produced forecasts for New Zealand stretched to double that time.
🌀The next tropical cyclone poses a threat to New Zealand - but it's still too far away to lock in.
Most reliable modelling puts the storm near NZ this Sunday. It's a wait-and-see situation for the next few more days as tracking for our part of the world becomes more reliable. pic.twitter.com/94KMM7BOTX
That was why ensemble forecasting was crucial, as it acted to minimise changes between model runs.
While details and timing at that point often changed slightly - especially with longer lead times - the overall picture tended not to.
James said that, while there were early indications the system could be travelling near New Zealand at the end of the weekend or early next week, it could be in our local TCWC’s region of responsibility by Friday.
Should any severe weather be expected to impact New Zealand, MetService will issue detailed outlooks, watches and warnings for affected areas.
