Southwest Pacific Tropical Cyclone Outlook

Published: Fri 11 Oct 2019 11:15 AM
NIWA and MetService analyses indicate nine to 12 named tropical cyclones (TC) could occur in the Southwest Pacific basin between November 2019 and April 2020.
This expectation for tropical cyclone activity is close to normal for the region, but with elevated activity east of the International Date Line especially during the late season between February and April.
Tropical cyclones have a significant impact across the Southwest Pacific, with the season officially starting in November and lasting until the end of April.
Vanuatu and New Caledonia typically experience the greatest TC activity, with an average of about two or three named cyclones passing close to those islands each year.
For this season, the expectation for those two islands is normal, while Wallis & Futuna, Tuvalu, Fiji, Samoa, American Samoa, Niue, Tonga and the Southern Cook Islands may experience two or more cyclones. Four severe cyclones reaching Category 3 or higher might occur anywhere across the region.
On average, at least one ex-tropical cyclone passes within 550km of New Zealand each year. For the coming season, the risk for an ex-tropical cyclone affecting New Zealand is considered near normal.
If an ex-tropical cyclone comes close to the country, it is more likely to pass east of the North Island, and landfall of a degrading storm system is possible. Significant rainfall, extreme winds, hazardous marine conditions and coastal damage can occur leading up to and during these events.
At present, sea surface temperature anomalies across the central Pacific are warm (positive), while the eastern equatorial Pacific Ocean is cool (negative). Atmospheric circulation patterns over French Polynesia and northern Australia indicate ENSO (El Niño-Southern Oscillation) neutral conditions, but are leaning toward El Niño. Oceanic and atmospheric forecasts for ENSO indicate neutral conditions for the TC season are most likely, with lower probabilities that some type of El Niño may develop by mid-summer.
It is worth noting that if El Niño develops it may be the Modoki type (central Pacific-based) during summer.
There is no expectation that there will be any reduction from normal tropical cyclone activity for the coming season. Increased TC activity is expected for several islands east of the International Date Line, including Samoa, American Samoa, Tonga, Niue the southern Cook Islands, and especially near the Austral Islands.
Tropical cyclones are categorised in strength from 1 to 5, with 5 being the most intense. For the coming season, about four storms are anticipated to reach at least Category 3 strength, with mean wind speeds of at least 118 km/h.
Past years with conditions similar to present suggest several storms that develop could intensify to at least Category 3 strength. Category 5 strength cyclones, where wind gusts exceed 199 km/h, have occurred in some years with similar conditions leading into the 2019/20 season (known as ‘analogue’ seasons). Therefore, all communities should remain alert and well-prepared for severe events.
New Zealand should also remain vigilant as the season unfolds. During some analogue seasons used in the preparation of this seasonal outlook, multiple ex-tropical cyclones passed within 550 km of the country. Significant wind, waves and rainfall are possible from ex-tropical cyclone interactions. Their effects can be spread over a large area, particularly if the decaying storm system of tropical origin interacts with separate weather systems embedded in the middle latitudes.
All communities, regardless of changes in TC risk, should remain vigilant and be aware if the regional climate situation (including ENSO) changes. As with most years, TC activity is expected to increase during the second half of the season from February-April. Early season TC activity is expected to be near normal.
NIWA, MetService, MeteoFrance, BoM, NOAA and Pacific Island National Meteorological Services will all continue to track the progression of ENSO and TC activity, with an update to this guidance in January 2020 if needed.
It does not take a direct hit or a severe cyclone to cause significant damage or life-threatening weather. When dangerous weather is forecast, please heed the advice of your local civil defence or disaster management offices.
New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and Meteorological Service of New Zealand (MetService) formulated this seasonal tropical cyclone outlook, along with contributions from meteorological forecasting organizations from the Southwest Pacific, the Australian Bureau of Meteorology, MeteoFrance and the Pacific Island National Meteorological Services.

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