Tropical rainfall and SST outlook: August to October 2013
The dynamical models continue to indicate drier conditions than normal in the eastern Pacific along the Equator, while a slightly wetter than normal season is forecast for some island groups in the western and central Pacific, consistent with a SPCZ positioned south of normal for this time of the year.
Near or above normal rainfall is forecast for Fiji, Niue, Papua New Guinea, Tonga and Samoa.
Near normal rainfall is expected for the Austral Islands, the Southern Cook Islands, New Caledonia, Pitcairn Island, the Society Islands, the Solomon Islands, Tokelau, the Tuamotu Archipelago, Vanuatu and Wallis & Futuna.
Normal or below normal rainfall is forecast for the Northern Cook Islands, Tuvalu, the Federated States of Micronesia, Western Kiribati and the Marquesas.
Below normal rainfall is forecast for Eastern Kiribati.
The global model ensemble forecast for SST does not provide much guidance this month for several island groups. A region of slightly above normal SSTs is nevertheless shown in most models for parts of the southeast Pacific.
Near or above average SST is forecast for Niue and Tonga.
No guidance is provided for Fiji, the Southern Cook Islands, New Caledonia, Samoa, the Federated States of Micronesia, Western Kiribati and Eastern Kiribati.
Near normal sea surface temperatures are expected elsewhere.
The confidence for the rainfall outlook is generally high, except for Eastern Kiribati, Western Kiribati Wallis & Futuna and the Solomon Islands, where uncertainty is greater. The average region–wide hit rate for rainfall forecasts issued in August is 55 %, 8 % lower than the long–term average for all months combined. For the Island groups for which 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 August-October 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 http://trmm.gsfc.nasa.gov. The data has been downloaded from the ftp server at URL ftp://disc2.nascom.nasa.gov/data/TRMM/Gridded/Derived_Products/3B42RT/Daily 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.
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".
