An Analysis of the Accuracy of 120-h Predictions by the National Meteorological Center's Medium-Range Forecast Model

Abstract

An assessment was made of the 120-h predictions by the medium range forecast (MRF) run of the National Meteorological Center's (NMC's) global spectral model. The ability of the model to forecast surface cyclones and anticyclones was evaluated and compared to shorter range (48 h) forecasts by the NMC Nested Grid Model (NGM) verifying at the same time. The study covers a period from 15 April 1991 through 20 December 1991 over a domain extending from 30°N to 50°N and from 60°W to 130°W. Results showed that the MRF at the 120-h range tended to underforecast the frequency of surface cyclones and anticyclones in the Rocky Mountain region and, in general, to forecast anticyclones and cyclones farther away from the observed systems than did the 48-h NGM verifying at the same time. The forecasts for central sea level pressures averaged within 1.5 hPa of the forecasts of pressure by the NGM. Averaged over regions and seasons, the 120-h MRF errors in many cases were statistically indistinguishable from the 48-h forecast errors. To illustrate model performance, an example of an accurate 120-h forecast is contrasted with a less accurate 120-h forecast following similar initial conditions. In addition, the accuracy of each 120-h MRF prediction for sea level pressure over a domain between 30°N and 60°N from 60°W to 150°W was subjectively determined for the period 14 March 1991 through 20 March 1992. Initial conditions of these forecasts were inspected for common features that could potentially be used to anticipate the accuracy of the 120-h MRF predictions. Typically, the best forecasts were characterized by below-normal 500-hPa heights at 40°N, 50°W in the initial conditions, corresponding to relatively weak 500-hPa geostrophic midlatitude (40°N?60°N) westerlies at 50°W. By comparison, the initial conditions of the worst forecasts featured, in most cases, above-normal 500-hPa heights at 40°N, 50°W corresponding to relatively stronger 500-hPa geostrophic midlatitude westerlies at 50°W.