Forecasting of thunderstorms in the pre-monsoon season at Delhi

Accurate prediction of thunderstorms during the pre-monsoon season (April-J une) in India is essential for human activities such as construction, aviat ion and agriculture. Two objective forecasting methods are developed using data from May and June for 1985-89. The developed methods are tested with i ndependent data sets Of the recent years, namely May and June for the years 1994 and 1995. The first method is based on a graphical technique. Fifteen different types of stability index are used in combinations of different p airs. It is found that Showalter index versus Totals total index and Jeffer son's modified index versus George index can cluster cases of occurrence of thunderstorms mixed with a few cases of non-occurrence along a zone. The z ones are demarcated and further sub-zones are created for clarity. The prob ability of occurrence/non-occurrence of thunderstorms in each sub-zone is t hen calculated. The second approach uses a multiple regression method to pr edict the occurrence/non-occurrence of thunderstorms. A total of 274 potent ial predictors are subjected to stepwise screening and nine significant pre dictors are selected to formulate a multiple regression equation that gives the forecast in probabilistic terms. Out of the two methods tested, it is found that the multiple regression method gives consistently better results with developmental as well as independent data sets; it is a potential met hod for operational use.