Tropical rainfall and SST outlook: August to October 2014
The dynamical forecast models continue to predict drier than normal conditions in large regions of the Pacific south of the Equator for August – October 2014.
Normal or below normal rainfall is forecast for Samoa, the Society Islands, Tonga, Wallis & Futuna, the northern Cook Islands, Fiji, the Marquesas, New Caledonia, Tokelau, Tuvalu and Vanuatu.
Normal or above normal rainfall is forecast for Niue, the southern Cook Islands, eastern Kiribati, western Kiribati and the Federated States of Micronesia.
Near normal rainfall is expected for the Austral Islands and Pitcairn Island. No clear guidance is available this month for Papua New Guinea and the Tuamotu archipelago.
The global model ensemble forecast for SSTs still indicate higher than normal SSTs in the central and eastern Equatorial Pacific.
Above normal SSTs are also forecast to persist from previous months to the east of New Zealand.
Normal or above normal SSTs are forecast for Western Kiribati, the Federated States of Micronesia and the Solomon Islands.
Near normal SSTs are expected for the Austral island, Fiji, the Marquesas, New Caledonia, the Northern and Southern Cook Islands, the Society Islands, the Tuamotu archipelago and Vanuatu. No guidance was available this month elsewhere.
Climatological probabilities for Papua New Guinea and the Tuamotu archipelago are typically associated with moderate confidence.
The average region–wide hit rate for rainfall forecasts issued in August is 60 %, three points lower than the average for all months combined.
Confidence for the SST forecasts is generally high, but several Island groups are lacking strong guidance from the ensemble of models forecasts.
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 August-October 2014 period.
The past 6 months rainfall anomalies are based on the near-real-time TRMM (Tropical Rainfall Measurement Mission) merged satellite product available from http://trmm.gsfc.nasa.gov. The data has been downloaded and available on the link below:
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 2001 – 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.
IMPORTANT NOTE:
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".
