Abstract
The operational model used to generate medium-range forecasts at the National Meteorological Center (NMC) has undergone significant changes in the last few years, resulting in considerable improvement in the skill of its forecasts. The introduction of interactive clouds in late 1988 significantly reduced a cold bias present in model forecasts since April 1985. Model errors during recent Northern Hemisphere summers appear linked to thermal forcing, causing temperatures and upper tropospheric heights over cooler ocean areas to be too low, and heights over the western United States to be too high.
