Tropical rainfall and SST outlook: September to Novermber 2013

The dynamical models continue to indicate drier conditions than normal in the eastern Pacific along the Equator, while the September to November period is forecast to be slightly wetter than normal in the western and central Pacific south of the Equator (consistent with a SPCZ forecast to be positioned south of normal).

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

Near normal rainfall is expected for the Austral Islands, New Caledonia, Pitcairn Island, the Society Islands, Tokelau, the Tuamotu Archipelago, Tuvalu, Vanuatu, Wallis & Futuna and the Federated States of Micronesia.

Normal or below normal rainfall is forecast for Eastern Kiribati and Western Kiribati, the Northern Cook Islands and the Marquesas.

No guidance is provided for the Southern Cook Islands.

As was the case last month, the global model ensemble forecast for SST does not provide much guidance for several island groups. A consistent feature across models however is the persistence of above normal SSTs for parts of the southeast Pacific.

Near or above average SST is forecast for Tonga and Fiji.

No Guidance is provided for Niue, New Caledonia, Wallis and Futuna, the Southern Cook Islands, Western Kiribati, Pitcairn Island, the Society Islands and the Tuamotu Archipelago.

Near normal sea surface temperatures are expected elsewhere.

The confidence for the rainfall outlook is generally moderate to high, except for the Southern Cook Islands where uncertainty is greater.

The average region–wide hit rate for rainfall forecasts issued in September is 66 %, 3 % higher than the long–term average for all months combined. For the Island groups for which SST guidance is provided, the confidence is generally high. 

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 September-November 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 at

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 September to November 2013
SST anomaly outlook map for September to November 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. *If conditions are climatology, we expect an equal chance of the rainfall being in any tercile.
The last six months rainfall anomalies for each Island group alongside the latest ICU rainfall forecast for the September-November 2013 period.