NWS verification of precipitation type and snow amount forecasts during the AFOS era
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NWS verification of precipitation type and snow amount forecasts during the AFOS era

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    "A study was conducted to analyze verification techniques and results of National Weather Service (NWS) subjective and objective forecasts of precipitation type and snow amount. The objective forecast system is based on the model output statistics approach developed by the Techniques Development Laboratory of the NWS. Results and conclusions are based on the data collected by the Automation of Field Operations and Services era verification (AEV) system, which was introduced in October, 1983. Six cool seasons (1983/84 - 1988/89) were included in the period of study. Precipitation type forecasts are stratified into three categories: freezing, frozen, and liquid precipitation. Two sets of observations are used for verification purposes: a verifying time set. which is valid for a specific projection; and a window set, which is chosen from a compilation of precipitation type observations for one hour on either side of the verifying time. The number of cases in the window set exceed the number of cases in the verifying-time set. A difference in the definition of liquid precipitation between subjective and objective forecasts is discussed. Verification results of all 18-, 30-. and 42-hour projections for national and regional precipitation type forecasts are presented. Comparisons and contrasts of subjective and objective forecasts are based on various statistical scores such as the critical success index (CSI) and the Heidke skill score (skill). For national and regional data. CSI scores were much higher for frozen precipitation than for freezing precipitation. Similar results were noted for subjective and objective forecasts. With the exception of the Southern Region, there were few differences in scores among regions, whose results reflected those of the nation. Snow amount forecasts are broken into four categories: 0 (0-1 inch). 2 (2-3 inches). 4 (4-5 inches), and 6 (greater than or equal to 6 inches). Verification results are provided for the complete set of national and regional 24 hour forecasts. In addition to comparisons of subjective and objective forecasts, fluctuations in the scores resulting from both variation in snowstorm frequency and model reliability between seasons are discussed, as is subjective forecast improvement over objective forecasts for category 6. Snow amount forecast results for category 0 were omitted from final analyses. In general, CSI scores were highest for category 6 and lowest for category 4. Subjective forecasts were better than objective forecasts for category 6, but both were similar for categories 2 and 4. For the most part, regional scores reflected national scores. Appendices are provided which describe the AEV system, the MOS precipitation type and snow amount forecast systems, and the definitions of the appropriate verification scores used in the study."
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