Tropical Cyclone guidance – forecast update

Tropical Cyclone guidance – forecast update

Ne ar normal tropical cyclone (TC) activity is expected for most countries in the southwest Pacific during the remainder of the season (February – April 2010). Communities should remain alert and prepared.

As previously forecast, overall activity is expected to be near normal, with 8–11 TCs expected for the total number of storms in the November 2009 – April 2010 period. On average, nine tropical cyclones occur each year for the southwest Pacific region. Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the present season, two or three storms were forecast to reach at least Category 3, and one storm was expected to reach at least Category 4, with mean wind speeds of at least 64 knots or 118 km/h.

Updated projections show an increased risk of tropical cyclones to the east of the Date Line, particularly for the Southern Cook Islands. Increased risk also exists for Niue and Tonga. There is also increased risk for the Solomon Islands during the end of this season. It should be noted that TCs can affect parts of southwest French Polynesia (Society and Austral Islands) during El Niño. These islands should remain vigilant as the event continues to evolve with progression into austral autumn. Though a moderate El Niño exists, the number for TCs entering a 550km radius of the New Zealand coast is expected to remain about normal.

In the Southwest Pacific, tropical cyclones (TCs) usually develop in the wet season, from November through to April, but occasionally occur in October and May, and have even occurred in June. Peak cyclone occurrence is usually from January to March. In seasons with similar background climate conditions to present, several tropical cyclones have occurred in the Coral Sea region between the Solomon Islands and New Caledonia, and near Vanuatu, Fiji and Tonga, while a few affect other areas.

On average, half of the tropical cyclones that developed since the 1969–70 season reached hurricane force with mean wind speeds of at least 64 knots (118 km/h). Since 1969, an average of two TCs have typically occurred in March while one has usually developed in April.

Expected departure from normal of the number of Tropical Cyclones occurring later in the season (February-June) in the southwest Pacific for 2009-10. This guidance is based on analogue years with similar SST anomalies and SOI to the present situation, with a weakly coupled El Niño in the prior Austral autumn/spring. The analogue years selected from the 1957-58 to 2008-09 period. Only three analogue seasons were found within the 1969-70 to 2008-09.