The likelihood of a tropical cyclone hitting New Zealand's North Island during the oncoming season is "normal-elevated" according to NIWA and MetService.
In its tropical cyclone outlook for the southwest Pacific released today, NIWA said six to 10 named tropical cyclones could hit the Southwest Pacific region between November 2024 and April 2025.
Northern New Zealand was given a normal or elevated activity risk. NIWA said at least one ex-tropical cyclone was expected to reach within 550km of the country every season.
“Two out of five analogue years considered in this outlook had at least one ex-tropical cyclone passing within 550 km of the country,” the report said.
It said that if a cyclone tracked close to New Zealand, there was a “near equal” probability of it tracking to the east or west of the North Island, based on historic climate data.
“Analogue years suggest that a decaying ex-tropical cyclone entering the New Zealand region could affect maritime and coastal areas around the North Island, but an interaction with the South Island cannot be ruled out.”
It said one of these weather events could bring significant rainfall, extreme winds, hazardous marine conditions, and coastal damage.
“The effects of ex-tropical cyclones can also be spread over a large area, particularly if the decaying ex-tropical cyclone interacts with mid-to-high latitude weather systems.”
For the rest of the Pacific, the impact from a tropical cyclone is likely to be higher near the Coral Sea, Solomon Islands, and Papua New Guinea.
Countries likely to see normal or elevated activity are Papua New Guinea, Solomon Islands, and northern New Zealand.
Near normal activity is expected for Vanuatu and New Caledonia.
Tuvalu, Tokelau, Niue, Wallis and Futuna, Samoa, American Samoa, Society Islands, Fiji, and Tonga will likely see normal or reduced activity.
The southern Cook Islands and Austral Islands will see reduced activity.
And Marquesas, Kiribati, northern Cook Islands, Tuamotu Archipelago, and Pitcairn Islands are unlikely to see any activity.
NIWA said three to four tropical cyclones reaching category three or higher may occur anywhere across the Pacific.
“Past seasons with similar conditions to the present, called ‘analogue years’, suggest multiple tropical cyclones could intensify to at least category three strength.
“Category five strength TCs2 , in which sustained winds are 200 km/h or greater, are associated with a majority of the analogue years.
“All communities should remain prepared,” the report said. “It does not take a direct hit or severe TC to cause considerable damage or life-threatening weather.
“When dangerous weather is forecast, please heed the advice of your local meteorological service, civil defence, or disaster management office.”
NIWA also said the El Niño Southern Oscillation — comprised of La Niña, neutral, and El Niño phases — plays an “important role” in the year-to-year development and spatial coverage of regional tropical cyclones and will be a “key factor” in the outlook.
Sea surface temperatures across the eastern and central equatorial Pacific Ocean during early October were “below average and close to La Niña thresholds”.
NIWA said atmospheric circulation patterns related to the El Niño Southern Oscillation over French Polynesia and northern Australia indicated neutral conditions during the start of this month.
“Oceanic and atmospheric forecasts for ENSO suggest La Niña, of weak-to-moderate intensity, has a 60-70% chance of emerging by December 2024.
“La Niña conditions, should they develop, are likely to persist through the back half of the tropical cyclone season in February-April.”
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