Seasonal forecasts of river flow conditions can be of significant value in water resource planning and decision-making from the local to regional level. Composite analysis techniques allow the computation of conditional probabilities for high, normal, or low flow regimes based on the Climate Prediction Center forecasts of El Nino/Southern Oscillation (ENSO) conditions in the eastern, tropical Pacific Ocean. Using 1950-2005 hydroclimatological data from sites on the Pearl River in Mississippi, high and low flow events are first counted, then ranked by month and year to develop terciles, and later sorted by ENSO episode using the Oceanic Nino Index (ONI). The result is a calculation of station-specific composite probabilities. A risk analysis is then performed to indicate the relative probability for a given flow regime based on the tails of the statistical distribution. Tests of statistical significance are used to indicate confidence in the relationship. Probabilistic forecasts are then generated using the CPC forecasts of sea surface temperatures (SSTs) in the Nino 3.4 region for the upcoming three-month season.

As a result, the Lower Mississippi River Forecast Center is evaluating the application of the historical composites as a new long-range forecast tool that may be provided as additional hydrologic guidance to weather forecast offices (WFOs) and regional partners (e.g. USACE, USGS). To comprehensively assess the composite analysis methodology, hindsight forecasts for each three month season during the 24-year period from 1982-2005 is performed. A verification process details the statistical level of confidence in the seasonal forecasts over the 24-year period and is assumed to represent the long-term climatological tendency. The current project highlights the product evaluation, current year forecasts, results of the hindcast verification, and major findings related to ENSO forcing of river flow conditions in the Pearl River Basin. Based on the results, the application may be expanded to include the remaining hydrologic basins in the Lower Mississippi River Forecast Center area of responsibility.