Standardized Precipitation Index Outlook


The SPI Outlook is based on the CariCOF Precipitation Outlook. Here, the gamma distribution is fitted to the historical rainfall according to the four time scales 1-, 3-, 6-, and 12- months and divided into the three tercile ranges. The boundaries (of rainfall) of these three tercile ranges for the three forecasted months are (in this first case July to September) used to project the boundaries of the index. These are displayed in the table below.
As an example of how to read, we notice that from the precipitation forecast for July to September 2009 that Cayman Islands has a high (45 %) probability of below normal rainfall for the three month period. This being predicted, one may suggest that there is a high probability of the index for Roberts Airport for the six month period (April to September) experiencing at least moderate to extreme drought (-2.83 – -1.24).
Based on the reality that the terciles divide the data in the lowest 33.3, the highest 33.3 and the other 33.3 in the middle, the most useful application of the index projections would be for the longer time scales 3-, 6- and 12- months. Future work will also provide index projections on deciles, which will be more applicable to the shorter (3 month) time scale.