Three-month outlook

Tropical rainfall outlook: September to November 2008

Rainfall outlook map for September to November 2008. (Click for enlargement)

Sea surface temperature outlook map for September to November 2008. (Click for enlargement)

La Niña conditions have dissipated in the equatorial and subtropical southwest Pacific. During the September–November 2008 forecast period, a region of suppressed convection is likely to encompass Western Kiribati and Tuvalu, and also Tokelau, the Northern Cook Islands, Pitcairn Island, and the northeastern half of French Polynesia, with average–to–below or below average rainfall expected for those countries.

Enhanced convection is expected to be centralised near Vanuatu and north of New Caledonia and east of the Solomon Islands with above normal rainfall. Near–to–above average rainfall is forecast for Fiji, Niue, Tonga, Papua New Guinea, the Solomon Islands, and New Caledonia for the coming three-month period. No clear precipitation guidance is offered for Eastern Kiribati, Tokelau, Samoa, the Southern Cook Islands, or the Austral Islands.

SSTs are expected to be normal to above normal in a band extending from near Papua New Guinea, southeast to the Austral Islands and Pitcairn Island, including Fiji, Vanuatu, New Caledonia, Tonga, and Niue. Normal to below normal SSTs are forecast for the northeastern sector of French Polynesia, including the northern Tuamotu archipelago and the Marquesas Islands.

The confidence in the forecast model skill for this seasonal rainfall outlook is moderately high for most Pacific Island countries. In the past, the average region-wide hit rate for forecasts issued in September is 63%, 2% higher than the long-term average for all months combined. The SST forecast confidence is moderately high to high.

NOTE: Rainfall estimates for Pacific Islands for the next three months are given in the table. The tercile probabilities (e.g., 20:30:50) are derived from the interpretation of several global climate models. They correspond to the odds of the observed rainfall being in the lowest (driest) one third of the rainfall distribution, the middle one third, or the highest (wettest) one third of the distribution. On the long-term average, rainfall is equally likely (33% chance) in any tercile.