A Case Study of Perfect Prog vs MOS POP Forecasts

Since 1987 the Techniques Development Lab (TDL) has been providing probability of precipitation (PoP) guidance based on the Perfect Prog (PP) statistical technique. At the same time TDL has continued to provide PoP forecasts based on the Model Output Statistics (MOS) statistical technique. The PP guidance uses Nested-Grid Model (NGM) output to produce the PoP forecasts, while the MOS guidance uses the Limited-Area Fine-Mesh Model (LFM) output. In the case study that follows, the NGM and LFM MEAN RH and P AMT forecasts were accurate and also very similar. Yet, the PP guidance produced a PoP of only 30% at Albany, while MOS had a 70% PoP. The reason for this can be found in the predictors in the equations used to produce the PoP forecasts. This paper will show that there is an important developmental characteristic of the PP equations which can produce a large underforecasting bias in the FP PoP forecasts, under certain conditions.