Tropical rainfall and SST outlook: October to December 2013

The dynamical models indicate slightly drier conditions than normal in the far eastern Pacific south of the Equator, while the October to December period is forecast to be slightly wetter than normal in the western and central Pacific south of the Equator.

Near or above normal rainfall is forecast for Fiji, Tonga, Niue, the southern Cook Islands and the northern Cook Islands, Tonga, the federated States of Micronesia.

Near normal rainfall is expected for the Austral Islands, Eastern and Western Kiribati, New Caledonia, Papua New Guinea, Pitcairn Island, Samoa, the Society Islands, the Solomon Islands, Tokelau, the Tuamotu Archipelago, Vanuatu and Wallis & Futuna.

Normal or below normal rainfall is forecast for Tuvalu and the Marquesas.

The global model ensemble forecast for SST indicates that the region of warmer than normal temperatures in the central and eastern Pacific that is currently present (see figure on page 2) is likely to persist over the coming three months. Several models also forecast slightly warmer than normal SSTs in the equatorial eastern Pacific.

Near or above average SST is forecast for Fiji, Niue, the Southern Cook Islands, Eastern and Western Kiribati, Tonga and Wallis and Futuna.

Near or below average SST is forecast for 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 high, except for the Northern Cook Islands, Eastern Kiribati and Western Kiribati and Papua New Guinea, where uncertainty is greater.

The average region–wide hit rate for rainfall forecasts issued in October is 66 %, 3 % higher than the long–term average for all months combined.

The confidence is generally high for the SST forecasts, expect for both the far western (Micronesia) and far eastern (French Polynesia) Pacific. 

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 October to December 2013
SST anomaly outlook map for October to December 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 October - December 2013 period.