A New Operational System for Forecasting Precipitation Type

Abstract A new system is developed which gives conditional probability forecasts for three precipitation type categories: snow or sleet, freezing rain and rain. Also, the probability forecasts are transformed into categorical forecasts so that a “best category”is provided. The Model Output Statistics (MOS) technique is used with output from the Limited-area Fine Mesh (LFM) model to develop statistical forecast equations for each of several regions in the conterminous United States. To help account for the evaporational cooling effect, predictors such as LFM forecasts of boundary layer and 850 mb wet-bulb temperatures and observed surface and dew-point temperatures are included. Also, joint predictors are designed to help account for predictor interactions. The values of the joint predictors are relative frequencies of the freezing rain or snow categories taken from graphs that show these frequencies as joint functions of various pairs of LFM predictors. Results from a statistical screening procedure vary ...