Initial Weather Regimes as Predictors of Numerical 30-Day Mean Forecast Accuracy

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Abstract
A set of thirty 30-day mean 500-mb height anomaly forecasts run from National Meteorological Center initial analyses by the NCAR Community Climate Model is examined in order to learn if the forecast accuracy can be estimated with the initial conditions. Defining initial weather regimes by a 500-mb geostrophic zonal index anomaly difference between 50°W and 10°E discriminates between the best and worst 30-day mean forecasts in the sample. Initial regimes characterized by anomalously high zonal index (500-mb geostrophic westerlies) at 50°W and low index at 10°E yield on average lower 30-day mean forecast-observed anomaly correlation than initial regimes with opposite conditions (anomalously low zonal index at 50°N and high index at 10°E). It is suggested that initial regimes with abnormally fast geostrophic 500-mb westerlies at 50°W are followed in time by intense and poorly forecast synoptic-scale cyclones over the Atlantic Ocean. It is shown in a case study that the local synoptic- to planetary-s... Abstract A set of thirty 30-day mean 500-mb height anomaly forecasts run from National Meteorological Center initial analyses by the NCAR Community Climate Model is examined in order to learn if the forecast accuracy can be estimated with the initial conditions. Defining initial weather regimes by a 500-mb geostrophic zonal index anomaly difference between 50°W and 10°E discriminates between the best and worst 30-day mean forecasts in the sample. Initial regimes characterized by anomalously high zonal index (500-mb geostrophic westerlies) at 50°W and low index at 10°E yield on average lower 30-day mean forecast-observed anomaly correlation than initial regimes with opposite conditions (anomalously low zonal index at 50°N and high index at 10°E). It is suggested that initial regimes with abnormally fast geostrophic 500-mb westerlies at 50°W are followed in time by intense and poorly forecast synoptic-scale cyclones over the Atlantic Ocean. It is shown in a case study that the local synoptic- to planetary-s...

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