How the maps of precipitation persistence are generated

These maps are generated each month based on past few months' observed precipitation and the IRI forecast for the next season. Shaded areas are regions where rainfall observations fall into below- or above-normal terciles and also have an enhanced porbability for below- or above-normal precipitation forecast.

NCEP rainfall data for 30 years (1969 to 1998) are used as the observational data reference. CAMS-OPI monthly gridded precipitation is used for the observations for the past 3 and 12 months. First, the rainfall observation for a chosen time period (most recent 3 months and 12 months in our case) at any gridpoint is ranked into a 0 - 1 scale, with the 0 as the driest event in 30 years for the same 3- or 12-month period, and 1.0 the wettest event. A value of 0.333 and less is considered dry (below-normal), and a value of 0.667 and greater is considered wet (above-normal). Secondly, the forecast for the next season for the same location is examined. A forecast with a 40% or greater probability for the below-normal category is considered an enhanced likelihood of dry forecast, and a forecast of 40% or greater for the above-normal category is considered an enhanced likelihood of wet forecast.

Based on the observation and forecast, two sets of maps are plotted. One represents areas with enhanced likelihood of continued wetter or drier than average. The maps' relative likelihoods of persistence are indicated for either wet or dry, depending on whether the forecast probaility is 40-50% (lower intensity; labled "Enhanced"), or 55% and higher (high intensity; labled "Greatly enhanced").

The second set of maps represents areas with enhanced likelihood of reversed tendency regarding the most recent precipitation observation. Such regions have either a below-normal observed rainfall and an above-normal forecast for next season, or vice versa.

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