Effects of Variable Stability in a 1000-mb. Graphical Prediction Model

The most striking shortcomings of previous 1000-mb. forecast models have been over-intensification of pressure systems, especially anticyclones, and inferior predictions in and near mountains. In this article a graphical-numerical two-level prediction model that incorporates a variable mean stability is developed and tested. When the mean stability is small, the 1000-mb. prediction is determined primarily by the 500-mb. steering and height changes. As the stability increases the 500-mb. control decreases and the effective mountain wind exerts more control over the 1000-mb. changes. Since the stability is generally larger in anticyclones than cyclones thc anticyclones are steered by a smaller percentage of the 500-mb. wind and are more influenced by the mountain topography. Twenty-four-hour forecasts were handproduced daily for the month of September 1965. Predictions were also prepared using a constant stability model, and the forecast5 of the two models were compared statistically by rigorous verification techniques. It was found that a definite improvement in the overall product can be expected by application of the variable stability model as opposed to the constant stability technique, especially in the mountainous areas and around anticyclones.