Tropical rainfall and SST outlook: March to May 2014
The dynamical models indicate drier conditions than normal for the March to May 2014 period in the eastern equatorial and south Pacific.
Wetter than normal conditions are expected in the equatorial west Pacific.
Near or above normal rainfall is forecast for the Federated States of Micronesia, the Austral Islands, the Southern Cook Islands, Western Kiribati, Niue, Papua New Guinea, Pitcairn Island, Vanuatu, and the Federated States of Micronesia.
Near normal rainfall is expected for Eastern Kiribati, New Caledonia and Wallis & Futuna.
Normal or below normal rainfall is forecast for Samoa, the Society Islands, Tokelau, the Tuamotu Archipelago, Tuvalu and the Northern Cook Islands.
Below normal rainfall is forecast for the Marquesas. No clear guidance is available this month for Fiji, Tonga and the Solomon Islands.
The global model ensemble forecast for SST indicates that the large area of positive SST anomalies in the central and eastern Pacific that has been a persistent feature in the region over more than a year will persist over March – May 2014.
Near normal or above normal SSTs are forecast for Fiji, Western Kiribati and Tonga.
Normal SSTs are forecast for the Austral Islands, New Caledonia, the Northern Cook Islands, Pitcairn, Papua New Guinea, Samoa, the Society Islands, the Solomon Islands, the Southern Cook Islands, Tokelau, the Tuamotu Archipelago, Tuvalu, Vanuatu and Wallis and Futuna.
No clear guidance is provided for Eastern Kiribati, the Marquesas, the Federated States of Micronesia and Niue.
The confidence for the rainfall outlook is generally high, uncertainty is greater for Western Kiribati, Eastern Kiribati and the Solomon Islands. The average region–wide hit rate for rainfall forecasts issued in March is 63 %, similar to the long– term average for all months combined. Confidence for the SST forecasts is high where guidance is available.
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 March-May 2014 period.
The past 6 months rainfall anomalies are based on the near-real-time TRMM (Tropical Rainfall Measurement Mission) merged satellite product.
TRMM →
The data has been downloaded at 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".
