Tropical rainfall outlook: May to July 2005
The tropical Pacific is in a neutral but borderline El Niño state. Because the forecast period is for the transitional months, the rainfall forecasts does not show a very strong pattern in the Pacific region.
However, enhanced convection is expected over the Solomon Islands, Vanuatu, and the Southern Cook Islands where rainfall is forecast to be near or above average.
Suppressed convection is likely over the Marquesas Islands, where rainfall is expected to be below average. Another region of suppressed convection is expected over Tuvalu and Tokelau, where rainfall is forecast to be near or below average.
Rainfall is expected to be near average for most of the other countries in the forecast region. Since the forecast period coincides with the transition from the wet to dry seasons, confidence of the global forecast models is low to moderate.
Rainfall outlook map for May to July 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 |
|---|---|---|
| Solomon Islands | 20:40:40 (Near average or above) | Low – moderate |
| Vanuatu | 20:40:40 (Near averae or above) | Low – moderate |
| Southern Cook Islands | 20:40:40 (Near average or above) | Low – moderate |
| Papua New Guinea | 25:40:35 (Near average) | Low – moderate |
| Western Kiribati | 20:45:35 (Near average) | Low – moderate |
| Eastern Kiribati | 25:50:25 (Near average) | Low – moderate |
| Wallis and Futuna | 20:45:35 (Near average) | Low – moderate |
| New Caledonia | 30:40:30 (Near average) | Low – moderate |
| Fiji | 35:40:25 (Near average) | Low – moderate |
| Tonga | 25:45:30 (Near average) | Low – moderate |
| Niue | 30:45:25 (Near average) | Low – moderate |
| Samoa | 20:45:35 (Near average) | Low – moderate |
| Northern Cook Islands | 20:50:30 (Near average) | Low – moderate |
| Society Islands | 20:50:30 (Near average) | Low – moderate |
| Austral Islands | 35:45:20 (Near average) | Low – moderate |
| Tuamotu Islands | 35:40:25 (Near average) | Low – moderate |
| Pitcairn Island | 20:45:35 (Near average) | Low – moderate |
| Tuvalu | 40:45:15 (Near average or below) | Low – moderate |
| Tokelau | 40:40:20 (Near average or below) | Low – moderate |
| Marquesas Islands | 45:30:25 (Below average) | 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.