Subjective probability forecasts of wind speed, visibility and precipitation events for six-hour periods have been prepared on an experimental basis by forecasters at Zierikzec in The Netherlands since October 1980. Results from the first year of the experiment were encouraging, but they revealed a substantial amount of overforecasting (i.e., a strong tendency for forecast probabilities to exceed observed relative frequencies) for all events, periods and forecasters. Moreover, this overforecasting was reflected in a rapid deterioration in the skill of the forecast as a function of lead time. In October 1981 the forecasters were given extensive feedback concerning their individual and collective performance during the first year of the experimental program. The purpose of this paper is to compare the results of the first and second years of the experiment.
Evaluation of the forecasts formulated in the fist and second years of the Zierikzee experiment reveals marked improvements in reliability (i.e., reductions in overforecasting) from year 1 to year 2, both overall and for most stratifications of the results by event, period or forecaster. For example, the reliability of the forecasts increased for all events and periods and for three of the four forecasters. The improvements in reliability are reflected in substantial increases in the skill of the forecasts from year 1 to year 2, with overall skill scores for the second (first) year for the wind speed, visibility and precipitation forecasts of 25.4% (13.9%), 22.4% (12.4%) and 0.5% (−24.7%), respectively. These improvements in performance are attributed to the feedback provided to the forecasters at the beginning of the second year of the experiment and to the experience in probability forecasting gained by the forecasters during the first year of the program.
The paper concludes with a brief discussion of the results and their implications for probability forecasting in meteorology.