A comparative study of long term hydrologic forecasts: Deterministic vs. probabilistic

National Weather Service (NWS) long term hydrologic forecasts traditionally have provided a single crest value. Recent studies (Pielke, 1999) have shown that this format does not provide users adequate information on associated forecast risk levels. The NWS North Central River Forecast Center (NCRFC) is implementing the Ensemble Streamflow Prediction (ESP) System, utilizing climatological temperature and precipitation time series, to generate new long term forecasts. ESP provides hydrologic products extending out weeks, and even months, in a probabilistic format. The purpose is to provide users with more information in planning protective measures and mitigating flood losses. This study defines a correlation between the single crest value, based on snow water content plus normal precipitation, from historical forecast techniques and an associated probability value from the ESP products. Furthermore, a verification procedure using observed crests approximates relative uncertainties inherent in both methodologies.