Error Growth in Poor ECMWF Forecasts over the Contiguous United States
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Successive improvements to the European Center for Medium-range Weather Forecasting model have resulted in improved forecast performance over the Contiguous United States CONUS. While the overall performance of the model in this region was found to have improved during the period of the 1981-1990 winter seasons, the number of poor forecasts increased over this time. This study uses the Root Mean Square RMS error to measure the performance of 5-day 500 mb winter forecasts over the CONUS. Poor and good forecasts were defined in terms of the 10-year distribution of the RMS values between the 1981 and 1990 winter seasons. Subjective analysis of a subset of poor forecasts yielded no obvious patterns of error growth, location or propagation in the evolution of poor forecasts. A tendency was noted for in situ amplification of forecast errors. Additionally, successive forecasts verifying on the same day were found to have similar error patterns, with increased amplitudes at longer forecast lengths. This implies that the initial conditions are not a significant source of the error in poor forecasts.