Trends in the Quality of National Weather Service Forecasts

Abstract

This paper describes the results of a study of trends in the quality of National Weather Service (NWS) forecasts from 1967 to 1985. Primary attention is focused on forecasts of precipitation probabilities, maximum temperatures, and minimum temperatures A skill score based on the Brier score is used to verify the precipitation probability forecasts, whereas the temperature forecasts are evaluated using the mean absolute error and percentage of errors greater than 10°F. For each element, trends are examined for objective forecasts produced by numerical-statistical models and for subjective forecasts formulated by NWS forecasters. In addition to weather element, type of forecast, and verification measure, results are stratified by season (cool and warm), lead time (three or four periods), and NWS region (four regions and all regions combined). At the national level, the forecasts for these three weather elements exhibit positive and highly significant trends in quality for almost all of the various stratifications. Exceptions to this general result are associated solely with the minimum temperature forecasts, primarily for the 60 h lead time. These national trends are generally stronger for the objective forecasts than for the subjective forecasts and for the cool season than for the warm season. Regionally, the trends in quality are almost always positive and are statistically significant in a majority of the cases. However, nonsignificant trends occur more frequently at the regional level than at the national level. As a result of the positive trends in performance, current levels of forecast quality for these weather elements are markedly higher than the levels that existed 15?20 years ago.