A detailed survey has been made from available records of all past synoptic situations which resulted in heavy rainfall in different subcatchments of Ganga basin in Bihar in different months of the monsoon season. Most of these spells associated with either a low pressure system or break monsoon condition. While the heavy rain associated with break condition is confined to the immediate vicinity of the monsoon trough, effect of a low pressure system extends much beyond its location, It has been possible to delineate an area for each month and each individual subcatchment, location of a low pressure system within which is fraught with risk of heavy rainfall in that particular subcatchment. This area has been termed as the risk zone for the subcatchment.