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Southwest Pacific Tropical Cyclone Outlook

Forecast – NIWA

Meteorological forecasting centres across the Southwest Pacific are predicting near average numbers of tropical cyclones (TC) for the 201415 season (November 2014 to April 2015). The 30-year average number of all (named) of TCs from 1981-2010 is …15 October 2014

Southwest Pacific Tropical Cyclone Outlook: Near average tropical cyclone numbers for the region is likely, with increased activity from February onward

Meteorological forecasting centres across the Southwest Pacific are predicting near average numbers of tropical cyclones (TC) for the 2014–15 season (November 2014 to April 2015). The 30-year average number of all (named) of TCs from 1981-2010 is 12.4 (10.4) in the Southwest Pacific each season from November to April. The outlook indicates that 8 to 12 named TCs are expected for the coming season. TC activity for Vanuatu and New Caledonia is anticipated to be below normal for this season, while elevated activity is expected for Samoa, Tuvalu, Tokelau, Niue, and the Southern Cook Islands. There is also an outlook of normal or above normal activity for countries situated close to the International Date Line (such as Wallis et Futuna and Tonga).

It should be recognised that the season-long forecast reflects an expectation of overall reduced activity during the early season (November to January) and net increased activity in general during the late season (February to April). Note that the TC activity outlook for islands like New Caledonia, Vanuatu, Fiji, and Tonga indicates two or more cyclones could interact with each of those countries during the season despite small differences from normal. At least one or more severe TCs (Category 3 or higher) could occur anywhere across the Southwest Pacific during the season. All communities should remain vigilant and follow forecast information provided by their national meteorological service.

On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. For the coming TC season, the risk for New Zealand is slightly higher than normal. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it has a higher probability of passing east rather than west of Auckland city.

Outlook analysis
Conditions within the ENSO-neutral range are indicated by sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean, and the atmospheric circulation patterns that exist over French Polynesia and northern Australia. There is an expectation amongst a number of international forecast centres of a weak El Niño developing in the coming months and this is supported by a few of the climate models. Taking this climate scenario into account, near normal TC activity can be expected for many islands in the Southwest Pacific during the 2014–2015 season, with 8 to 12 named TCs forming across the region during the November 2014–April 2015 period.

Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the coming TC season, at least four storms are predicted to reach at least Category 3, with mean wind speeds of at least 64 knots or 118 km/h (so-called ‘hurricane force’ winds). Of those systems, three storms may reach at least Category 4 strength, with mean wind speeds of at least 86 knots or 159 km/h. While Category 5 strength TCs (winds greater than 106 knots or 196 km/h) have not been prominent for ENSO neutral seasons like the current one, this type of event is still possible. Therefore, all communities should remain alert and prepared for severe events.

Tropical cyclones have a significant impact across the Southwest Pacific from year to year. Vanuatu and New Caledonia typically experience the greatest activity, with an average of 2 or 3 TCs passing close to land each year and there are indications that activity may be below average this season for those countries. The outlook for this season indicates near normal TC activity for the 2014–15 season for many islands, with reduced risk for Vanuatu and New Caledonia. Elevated risk is forecast for countries situated close to the International Date Line including Tokelau, Samoa, Niue, Tuvalu and further east in the Southern Cook Islands, although it should be recognized that during ‘normal’ seasons, significant TCs have affected those countries in the past.

On average, New Zealand usually experiences at least one interaction per season with an ex-tropical cyclone during ENSO neutral conditions. Most of the analog seasons identified for this forecast (1978/79; 1979/80; 1980/81; 1986/87; 1990/91; 2001/02; 2009/10 and 2012/13) show an ex-tropical cyclone coming close (within 550 km) to the country. Significant wind, waves and rainfall are possible from these systems. Their effects can be spread over a larger area when the ex-tropical cyclone meets a higher latitude high pressure system.

Even though TC activity is expected to be near normal or below normal for some countries, historical cyclone tracks (see supporting information for this forecast, Figure 2) indicate that TCs can affect parts of French Polynesia (including the Society Islands and the Austral Islands), especially late in the TC season. As with the majority of other years, the late TC season (February–April) is expected to be the most active time in the Southwest Pacific.

All Pacific Islands should remain vigilant in case conditions in the equatorial Pacific change during the TC season. Past ENSO neutral seasons have seen TC tracks with increased sinuosity (irregular or looping motions rather than a curvilinear trajectory), which means they have potential to impact a large area.

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New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and Meteorological Service of New Zealand (MetService) along with meteorological forecasting organizations from the Southwest Pacific, including the Australian Bureau of Meteorology, and the Pacific Island National Meteorological Services have prepared this tropical cyclone outlook.

ENDS

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