Tropical cyclone risk altered as La Niña back

MEDIA RELEASE - TROPICAL CYCLONE UPDATE 9 JANUARY 2009

Tropical cyclone risk altered as La Niña back

Conditions in the tropical Pacific moved towards a La Niña state during December, and these conditions are likely to alter the chances of tropical cyclone activity for several tropical South Pacific countries between January and May. However, in every season there is always a risk of tropical cyclones across the South Pacific requiring preparedness for disaster prevention and mitigation.

For the remainder of the tropical cyclone season, from January to May 2009, we are likely to see an increased risk of occurrence for those areas of the South Pacific west of the Date Line. These countries include Vanuatu, Solomon Islands and northern New Zealand. However, a reduced risk of tropical cyclones is likely in parts of the South Pacific east of the Date Line, such as Samoa, Tonga and Niue. The risk of occurrence is slightly increased for New Caledonia.

Climate forecasting organisation in the Pacific are in general agreement that La Niña conditions have redeveloped in the tropical Pacific and are expected to prevail into autumn. This brings about an increased normal risk of tropical cyclones in the Coral Sea. On average six or seven tropical cyclones can be expected over the entire Southwest Pacific region during a weak La Niña season.

For New Zealand, the weak La Niña conditions will not have much effect on the chances of experiencing an ex-tropical cyclone. There is a slightly higher than average of an ex-tropical cyclone passing within 500 km of the country between January and May, with the highest risk districts being Northland and Gisborne. When these reach the country they are no longer classified as tropical cyclones, but can still cause strong winds and heavy rainfall. The most common months for ex-tropical cyclones affecting New Zealand are January to March.

Southwest Pacific tropical cyclones are grouped into classes ranging from 1 to 5, with 5 being the strongest. On average four per season reach at least class 4 with mean wind speeds of at least 64 knots or 118 km/h, while two usually reach class 5 with mean speeds in excess of 90 knots or 167 km/h.

Class Description Mean wind speed (km/h) Central Pressure (hPa) 1 Tropical Depression <62 >995
2 Gale 62-88 995-986
3 Storm 89-117 985-975
4 Hurricane 117-167 <975 5 Major Hurricane >167 <945

Classification of tropical cyclones

ENDS

© Scoop Media