This research is an effort to understand the heavy rainfall phenomenon which gripped the upper area of Pakistan from 4–9 July 2008. This week lasting event created close to flood situation in Lai Nullah basin of Islamabad. The focus is mainly kept at flooding occurred in the Nullah on 5th July. This study was also an effort to effectively forecast the amount of precipitation expected as a result of such event so that flood like situation can be timely forecasted. The NCEP reanalysis (2.5° × 2.5°) data sets were utilized for this purpose. Different meteorological fields were used to get a picture of atmosphere. It also helped in comparison of both the observed and reanalysis data sets for one particular event. NCEP reanalysis data set though of coarse resolution presented good picture of event in terms of interaction between two main weather systems. The analysis revealed that the south-easterly incursion from the Arabian Sea was activated due to the westerly trough approaching the HKH mountain ranges. The results showed that Vertical wind Velocity (omega) and constant pressure surfaces are good predictors for this particular study.