Removal of distortion error from an ensemble forecast

Large errors developed by 24 h during a 25-member ensemble forecast of quantity of precipitation. The errors could be attributed to an insufficient northeastward motion of the area of precipitation and excessive amounts. This was determined by partitioning of the root-mean-square error into a distortion error, the sum of contributions from incorrect position and magnitude, and a residual error. The distortion error accounted for more than half of the total error. The distortion error occurs on the synoptic scale and can likely be somewhat ameliorated by future improvements in analysis-forecast systems. The residual error occurs at smaller, less predictable scales, and prospects for its deterministic improvement are not so sanguine.