Point and Area Precipitation Probability Forecasts: Some Experimental Results

An experiment was conducted at the National Weather Service Forecast Office in St. Louis, Missouri, to investigate the ability of forecasters to differentiate among different points in a forecast area with regard to the likelihood of the occurrence of measurable precipitation and the relative ability of forecasters to make point and area precipitation probability forecasts. On each forecasting occasion in the experimental period (November 1972 to March 1973), the forecasters made an average point probability forecast for the St. Louis metropolitan area, point probability forecasts for five specific points in the area, an are a probability forecast, and an expected areal coverage forecast. The results indicate that the forecasters did not differentiate among the five points very often, but that this absence of differences among the point probabilities was justified by the lack of variability exhibited by the observations of precipitation occurrence at these points during the experimental period. Evaluations of the average point probability forecasts, individual point probability forecasts, and expected areal coverage forecasts reveal that these forecasts were quite reliable and accurate and that they were also internally consistent. The area probability forecasts, however, tended not to be consistent with the other forecasts, and the average area probability forecast was considerably lower than the relative frequency of occurrence of precipitation "somewhere in the area." The implications of these results for precipitation probability forecasting in meteorology are briefly discussed.