An objective method has been developed for forecasting the occurrence of thunderstorms. Four parameters been selected one of which has been derived from the 500-mb contour pattern. The other three parameters (atmospheric pressure, stability and humidity) are related to the center of the forecast area. The method which is based relatively many observations (∼1000) can readily be applied for the day following the 0000 GMT aerological sounding if the 500-mb flow pattern is sufficiently known. From this 500-mb flow a parameter has been derived that is combined with the atmospheric pressure at De Bilt defining a parameter X. By combination of a stability parameter and a moisture variable—both of which have been derived from the aerological sounding at De Bilt— a second parameter. Y, has been defined. Finally, the probability of the occurrence of thunderstorms can be obtained as a function of Xand Y. The system has been designed for the summer season and is based on eleven years (1949 to 1959 inclusive) of dependent data. It was tested for three years (1960 to 1962 inclusive) of independence data yielding a skill factor I=0.48. The method shows more than 2.5 times the skill factor of the persistence forecast.

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