Semi-implicit higher order version of the Shuman-Hovermale model

The atmospheric numerical model developed by Shuman and Hovermale at the National Meteorological Center has been reformulated for semiimplicit integration. Orography, moisture, friction, and diabatic effects have been incorporated into this new model. Higher order horizontal finite differencing is used in the advective terms of the model equations to reduce truncation error. Forecasts have been made for a number of real data cases using a time sten up to 45 minutes, and the semi-implicit technique has proven stable to at least 7 days. The model is competitive with the operational model at the National Meteorological Center in terms of both forecast accuracv and computation time efficiency.