EVALUATION OF ETA-RSM ENSEMBLE PROBABILISTIC PRECIPITATION FORECASTS

The accuracy of short-range probabilistic forecasts of quantitative pr ecipitation (PQPF) from the experimental Era-Regional Spectral Model e nsemble is compared with the accuracy of forecasts from the Nested Gri d Model's model output statistics (MOS) over a set of 13 case days fro m September 1995 through January 1996. Ensembles adjusted to compensat e for deficiencies noted in prior forecasts were found to be more skil lful than MOS for all precipitation categories except the basic probab ility of measurable precipitation. Gamma distributions fit to the corr ected ensemble probability distributions provided an additional small improvement. Interestingly, despite the favorable comparison with MOS forecasts, this ensemble configuration showed no ability to ''forecast the forecast skill'' of precipitation-that is, the ensemble was not a ble to forecast the variable specificity of the ensemble probability d istribution from day-to-day and location-to-location. Probability fore casts from gamma distributions developed as a function of the ensemble mean alone were as skillful at PQPF as forecasts from distributions w hose specificity varied with the spread of the ensemble. Since forecas ters desire information on forecast uncertainty from the ensemble, the se results suggest that future ensemble configurations should be check ed carefully for their presumed ability to forecast uncertainty.