The Use of Probabilities in Subjective Quantitative Precipitation Forecasts: Some Experimental Results

Abstract

This paper summarizes the results of an experiment in which National Weather Service forecasters formulated probabilistic quantitative precipitation forecasts (QPFs) during a 17-month period in 1981?82. These forecasts expressed the likelihood that certain threshold amounts of precipitation would be equaled or exceeded in 12-hour periods at four locations in Texas. The forecasters had no previous experience in quantifying the uncertainty in such forecasts, but they did receive feedback regarding their collective performance at the end of the first year of the experiment. In the evaluation of the experimental results, particular attention is focused on three issues: 1) the reliability and skill of the subjective QPFs; 2) the effects of feedback and experience on the quality of these forecasts; and 3) the relative performance of the subjective probabilistic QPFs and objective probabilistic QPFs produced by the model output statistics system. The subjective probabilistic QPFs possess positive skill, although they exhibit considerable overforecasting for larger precipitation amounts. Moreover, the feedback provided to the forecasters evidently contributed to modest increases in the reliability and skill of their forecasts. In this regard, the quality of the subjective and objective QPFs is generally comparable in the first year of the experiment. However, after the receipt of the feedback, the skill of the subjective forecasts exceeded the skill of the objective forecasts. These results are considered to be encouraging regarding the ability of forecasters to formulate reliable and skillful probabilistic QPFS, but more extensive experiments should be undertaken to investigate this and related issues in greater detail.