An Examination of Nested Grid Model Precipitation Forecasts in the Presence of Moderate-To-Strong Low-Level Southerly Inflow

The performance of the nested grid model (NGM) in predicting heavy rain is assessed for those cases in the cool season when moderate-to-strong low-level southerly inflow from the Gulf of Mexico is present. This study indicates that the NGM underpredicts precipitation maximum for heavier rainfall events, with the underprediction more common at 32 degrees N than at 40 degrees N. The NGM is also shown to have a slight slow bias in moving heavy precipitation bands to the east. Two case studies illustrate the model's difficulties in predicting heavy precipitation but also show that the NGM offers useful information in predicting major rainfall events. Several possible reasons for the NGM underprediction of heavy rainfall over the southern United States are presented.