Tropical rainfall outlook: August to October 2005
Enhanced convection is expected from Papua New Guinea extending east to the Marquesas Islands, including the Solomon Islands, Wallis and Futuna and the Northern Cook Islands, where rainfall is expected to be near or above average.
Near or below average rainfall is forecast for Western and Eastern Kiribati, Fiji, the Southern Cook Islands and the Pitcairn Island.
Rainfall is expected to be near average elsewhere in the region.
The skill of the global rainfall forecast models remain in low to moderate range.
Rainfall outlook map for August to October 2005.
Probabilities of rainfall departures from average
Broad-scale rainfall patterns and anomalies in the southern tropical Pacific area are estimated from the state of large-scale regional climate factors, such as La Niña or El Niño, their effect on the South Pacific and Tropical Convergence Zones, surface and sub-surface sea temperatures, and computer models of the global climate.
Rainfall estimates for the next three months for Pacific Islands are given in the adjacent 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.
The probabilities shown express the expected shift in the distribution from the long-term average, based on predictions of oceanic and atmospheric conditions. The amount of inter-model forecast consistency is indicated by the levels of confidence expressed in the table.
| Island group | Rainfall outlook | Outlook confidence |
|---|---|---|
| Papua New Guinea | 20:40:40 (Near average or above) | Moderate |
| Solomon Islands | 20:40:40 (Near average or above) | Moderate |
| Wallis and Futuna | 20:40:40 (Near average or above) | Low - moderate |
| Northern Cook Islands | 20:40:40 (Near average or above) | Low - moderate |
| Marquesas Islands | 20:40:40 (Near average or above) | Low - moderate |
| Tuvalu | 25:50:25 (Near average) | Low - moderate |
| Tokelau | 20:45:35 (Near average) | Low - moderate |
| New Caledonia | 30:40:30 (Near average) | Moderate |
| Vanuatu | 25:45:30 (Near average) | Low - moderate |
| Samoa | 20:45:35 (Near average) | Low - moderate |
| Tonga | 25:50:25 (Near average) | Low - moderate |
| Niue | 30:45:25 (Near average) | Low - moderate |
| Society Islands | 20:50:30 (Near average) | Low - moderate |
| Austral Islands | 30:50:20 (Near average) | Low - moderate |
| Tuamotu Islands | 25:50:25 (Near average) | Moderate |
| Western Kiribati | 45:40:15 (Near average or below) | Low - moderate |
| Eastern Kiribati | 40:45:15 (Near average or below) | Moderate |
| Fiji | 45:40:15 (Near average or Below | Moderate |
| Southern Cook Islands | 40:45:15 (Near average or below) | Low - moderate |
| Pitcairn Island | 40:40:20 (Near average or below) | Low - moderate |
Rainfall outcomes as estimated from models and historical records. The third column indicates the probability of bottom (below), middle (average) or top (above) tercile rainfall, where a percentage is given. The rainfall outlook (second column) is subjectively estimated from the probabilities of bottom, middle and top terciles.