Error charts for the numerical barotropic forecasts prepared at the Joint Numerical Weather Prediction Unit since October 1957 have revealed retrogressive patterns of very long wavelength. These errors are shown to be due to changes in the large-scale components predicted by the numerical model. These components are actually quasistationary in the atmosphere. Forecasts prepared with an approximation to these components held unchanged show significantly increased accuracy. Finally, some of the

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... fficulties in developing a more acceptable physical approach to this problem are outlined. Problems Group, Fleet Weather Central, Suitland, Md.