Quantitative precipitation forecasts over the United States by the ECMWF ensemble prediction system

The performance of the ECMWF Ensemble Prediction System (EPS) is assessed for probabilistic forecasts of 24-h accumulated precipitation over the eastern United States. Daily forecasts for the priod 1 January 1997 to 31 January 1999 are verified for projections of 1-10 days. Verification is performed separately for the cool and warm seasons, and the impact of changes to the EPS that occurred during the study period is assessed. Analyses of rain gauge data from the River Forecast Centers of NOAA are used for verification. Skill is measured relative to long-term climatic frequencies, and the statistical significance of differences in the accuracy and skill among forecasts is estimated. Overall, EPS forecasts are more skillful during the winter than the summer. The EPS produces significantly skillful forecasts to past 1 week for a threshold of 1 mm in both seasons. Accuracy decreases as the threshold increases, until forecasts of 50 mm are not significantly skillful at 1 day. The implementation of evolved singular vectors in the EPS appears to have minimal impact on skill during the summer. The addition of both evolved singular vectors and stochastic processes in the EPS appears to improve short-range performance for thresholds between 1 and 20 mm during the winter, but results for higher thresholds (50 mm) are equivocal.