The dependence of short-range 500-mb height forecasts on the initial flow regime

Forecast errors in the 500-mb geopotential height field over North America and adjacent ocean environs are calculated for the National Meteorological Center's Nested Grid Model (NGM). The eight winters 1985/96-1992/93 are examined. Errors are compared for the time-mean flow and for four recurring planetary-scale flow regimes, and the statistical significance of the differences is estimated. Overall, the NGM produces very accurate 500-mb height forecasts out to 48 h, with every forecast cycle of the study period exhibiting useful deterministic skill at 48 h when averaged over the study domain. During the first two winters of operational NGM implementation, the spatially averaged errors were noticeably greater than in subsequent winters. NGM error was essentially constant during the 1987/88-1992/93 winters. -from Authors