Seasonal and Geographic Variations in Quantitative Precipitation Prediction by NMC's Nested-Grid Model and Medium-Range Forecast Model

Abstract

This paper assesses the performance of the National Meteorological Center (NMC) Nested-Grid Model (NGM) during a period from March 1988 through March 1990, and the NMC medium-range forecast model (MRF) in two 136-day tests, one during summer made up of two 68-day periods (19 July–25 September 1989 and 20 June–28 August 1990) and one during winter and early spring (12 December 1989–26 April 1990). Seasonal and geographical variations of precipitation bias and threat score are discussed for each model. Differences in model performance in predicting various amounts of precipitation are described. The performance of the NGM and MRF varied by season, geographic area, and precipitation amount. The bias of the models varied significantly during the year. The NGM and MRF overpredicted the frequency of measurable precipitation (≥0.01 in.) across much of the eastern half of the United States during the warm season. Both models, however, underpredicted the frequency of ≥0.50-in. amounts across the South during the cool season. The smooth orography in both models has a strong impact on the models’ precipitation forecasts. Each model overpredicted the frequency of heavier precipitation over the southern Appalachians, over portions of the Gulf-facing upslope areas east of the Rocky Mountains, and to the lee of the Cascade and Sierra ranges of the West. The NGM underpredicted the frequency of heavier amounts on the Pacific-facing windward side of the Cascade Range of Oregon and Washington. Model performance also seems to be related to the synoptic situation. Threat scores were higher when the midlevel westerlies were more active, with the highest threat scores found north of the most frequent track of cyclones during the cool season.