Quality Control for Probability Forecasts

The meaning of probabilistic weather forecasts is discussed from the point of view of a subjectivist concept of probability. The prior degree of belief of probabilities of the weather in question, for a given forecast statement, is expressed analytically as a beta function. Bzyes’ theorem is used to modify this degree of belief in the light of experience, producing a posterior degree of belief which is also in the forin of a beta function. By establishing an arbitary criterion that one should always be able to assign at least as much belief to the probability interval implied by the forecast as to any other equivalent interval, a method of quality control for probability forecasts is developed. Appropriate tables are given to permit application of the method, and the implications of the method, for both forecaster and forecast user, are discussed.