The dynamical model forecasts indicate that the central equatorial Pacific is likely to experience above normal rainfall in May – July 2015. In contrast, large areas in the western Pacific south of the Equator, as well as parts of the south eastern Pacific are expected to experience reduced rainfall. Below normal rainfall is forecast for Fiji, Papua New Guinea and Vanuatu.
Normal or below normal rainfall is forecast for the southern Cook Islands, New Caledonia, Samoa, the Society Islands, Tokelau, the Tuamotu Archipelago, Wallis & Futuna, the northern Cook Islands, the Marquesas, Niue and Tonga. Near normal rainfall is expected for the Austral Islands and Pitcairn Island. Normal or above normal rainfall is forecast for Tuvalu. Above normal rainfall is forecast for eastern Kiribati and western Kiribati. No clear guidance is available this month for the Federated States of Micronesia and the Solomon Islands.
The global model ensemble forecast for SSTs indicates higher than normal SSTs over the equatorial Pacific, with maximum anomalies now positioned east of the International Dateline. Above normal SSTs are forecast for Western Kiribati and Eastern Kiribati. Normal or above normal SSTs are forecast for New Caledonia, the northern Cook Islands, Samoa, Tokelau and Tuvalu. Near normal SSTs are forecast for the Marquesas, the Federated States of Micronesia, Pitcairn Island, the Solomon Islands, the southern Cook Islands and Vanuatu. No guidance is available this month for the Austral Islands, Fiji, Niue, Papua New Guinea, the Society Islands, Tonga, the Tuamotu archipelago and Wallis and Futuna.
The confidence for the rainfall outlooks is generally high. The average region–wide hit rate for rainfall forecasts issued for the May – July season is about 58 %, five points less than the average for all months combined. The confidence for the SSTs forecasts 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 May-July 2015 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".
