Pakistan experienced back-to-back heavy flooding during the summer monsoon season in 2010 and 2011.Both the cases were unique in their nature of weather pattern,intensity,persistence over time and geographical extent.Diagnosis has been carried out in order to find out the causes of heavy rainfall over Himalayan sub-mountainous plains originating cataclysmic flood in Indus River on 28th and 29th July,2010.It was revealed that heavy rain occurred due to the development of meso-scale convective system (MCS) and its evolution was the direct result of interaction between westerly and easterly flows.Tilt and Intensity of Sub-Tropical High (STH) played a crucial role in steering the monsoon low to merge it with the seasonal low.Seasonal low became accentuated due to heavy moisture incursion,both from Arabian Sea and Bay of Bengal,to the northern parts of the country.The subsequent rapid development was the combined result of the presence of the mid latitude westerly’s trough aloft in the north and low level moisture feeding through monsoon flow along the Himalayas and also the direct south-westerly current from the Arabian Sea.The cold air advection at the top of the MCS resulted into subsidence whereas warm and moist air tended to rise.This rise and fall generated strong vertical motion in the presence of favorable orographic features of the terrain.Torential rain occurred over upper KP and combined with enhanced snow/ice melt water developed historic flows in the Indus downstream sparing none of the four provinces from inundation.