Tropical rainfall and SST outlook: July to September 2013

The dynamical models indicate slightly drier conditions than normal in the eastern Pacific along the Equator, while a slightly wetter than normal season is forecast for some Islands groups in the western and central Pacific.

Near or above normal rainfall is forecast for Papua New Guinea, Niue, Samoa and the Solomon Islands.

Near normal rainfall is expected for the Austral Islands, the Northern Cook Islands, the Southern Cook Islands, Fiji, the Marquesas, New Caledonia, Pitcairn Island, the Society Islands, Tonga, the Tuamotu Archipelago, Vanuatu and Wallis & Futuna.

Normal or below normal rainfall is forecast for Eastern Kiribati and Western Kiribati, Tokelau and Tuvalu.

The global model ensemble shows close to normal SST anomalies forecast along the equatorial Pacific.

A region of slightly above normal SSTs are shown in most models for the region extending between Papua New Guinea and the Austral Islands. Near or above average SST is forecast for the Austral Islands, Niue, the Northern Cook Islands, the Southern Cook Islands, Fiji, Western Kiribati, Papua New Guinea, Samoa, the Solomon Islands, Tonga and Wallis & Futuna.

Near or below normal SST is forecast for New Caledonia. Normal sea surface temperatures are expected elsewhere.

The confidence for the rainfall outlook is generally high, except for Eastern Kiribati, where uncertainty is greater. The average region–wide hit rate for rainfall forecasts issued in June is 67 %, 4 % higher than the long–term average for all months combined.

The SST forecast confidence is high across the region except for Eastern Kiribati and the Marquesas, where uncertainty is greater. 

The figure on the bottom right presents the last six months rainfall anomalies for each Island group alongside the latest ICU rainfall forecast for the May-July 2013 period.

The past 6 months rainfall anomalies are based on the near-real-time TRMM (Tropical Rainfall Measurement Mission) merged satellite product available from The data has been downloaded from the ftp server at URL For each Island group, the monthly value is derived from the average of all grid-points (or "pixels") in the TRMM Dataset that intersect a coastline, to ensure that the values correspond as closely as possible to rainfall on land, and excluding rainfall falling on ocean surfaces.

The climatology used has been established over the 1998 – 2012 period. The categories ("Well-below", "Below", etc) are determined according to the percentage of the normal rainfall for that month. The thresholds are indicated in the colorbar at the bottom: to give an example, "Well-below" normal rainfall means the rainfall for that month was under 40 % of the normal rainfall, "Below" normal rainfall means that between 40 and 80 % of normal rainfall was received, etc.


Please note that, while we use the same color-scheme for the past rainfall anomalies and the ICU forecast, the type of information presented is different. In the case of the past 6 months, actual rainfall has been estimated by satellite, and the categories are well-defined by monthly estimated rainfall compared to the long-term mean. The ICU forecast, on the other hand, is probabilistic: it indicates the likelihood (percentage chance) of rainfall being at, above, or below normal for the season as a whole. When the percentage chances in two categories are close to each other, we indicate both categories: for example if the forecast is for 35 % chance of receiving below rainfall, and 40 % chance of normal rainfall, the outcome is "Normal or below". 

Rainfall anomaly outlook map for July to September 2013
SST anomaly outlook map for July to September 2013
NOTE: Rainfall and sea surface termperature estimates for Pacific Islands for the next three months are given in the tables below. The tercile probabilities (e.g., 20:30:50) are derived from the averages of several global climate models. They correspond to the odds of the observed rainfall or sea surface temperatures being in the lowest one third of the distribution, the middle one third, or the highest one third of the distribution. For the long term average, it is equally likely (33% chance) that conditions in any of the three terciles will occur.
The last six months rainfall anomalies for each Island group alongside the latest ICU rainfall forecast for the July-September 2013 period.