General Aspects of Extended-Range Forecasting

Abstract

Absolute definition of the term "extended-range fore-casting" is not possible and probably will not be for many years. As understood today it vaguely refers to those predictions which go beyond the limit imposed by ordinary standardized forecast techniques. The latter forecasts, disseminated by meteorologica! services the world over, generally cover periods of 24, 36, or at most 48 hr. With this in mind, "extended-range forecasting" presumably embraces any atmospheric prediction in-volving any time period beyond 48 hr and up to the geological epochs including the ice ages. The field of extended-range forecasting thereby becomes most in-viting to people both within and without professional meteorology, particularly in view of the tremendous po-tentialities it offers in the direction of improving the lot of man. Considering the extent and complexities of the prob-lems posed by the vast spectrum of long-range weather forecasts, meteorologists have made small and slow progress. This spectrum may be divided arbitrarily into the broad bands: medium range (covering periods from three or four days to a week), monthly, seasonal, annual, decennial, centurial, and so on to millennial. In spite of the fact that many workers have spent arduous years attempting to find solutions applicable to these periods, no system has evolved which even re-motely approaches reliability. Unfortunately, the same comment can be made in regard to the short-range forecasts covering as little as 24 hr in advance. This sad state of affairs reflects the lack of basic knowledge of the workings of the atmosphere and points up the need for periods of stocktaking as exemplified by this Compendium. But if objectives appear distant, it does not mean that progress has not been made. Indeed, medium-range forecasts covering periods from a few days to a week are already proving. economically valuable in many countries. Forecasts of general weather conditions for periods a month in advance ha ve shown promise. There are even some optimistic meteorologists who believe that seasonal, annual, or even decennial forecasts of some degree of reliability may be possible in the not-too-distant future. For the most part, this optimism cannot be traced to statistically significant results in the prediction of weather phenomena. If we look at the problem through the harshly revealing magnifying glass of statistics, it appears that, of the spectrum of ex-tended-range forecasts cited, only the medium-range and perhaps monthly forecasts have demonstrated some degree of skill. The term skill is used here in its rigid statistica! sense, that is, that the predictions are correct more often than are predictions made from many years of climatic records, or that forecasts must in the long run show a higher proportion of successes than the climatic probability of occurrence of the event being forecast. The use of this yardstick makes it possible to restrict this article to only those two forms of extended-range forecasting, medium range and monthly, which in the knowledge of the author have met this criterion over a period sufficiently long to be considered significant. It must be admitted that this manner of selection may be somewhat unfair to those who claim ability to forecast for decades, centuries, and longer, for adequate me-teorologica! records are as yet unavailable to verify these predictions.