River flow forecasting is an essential tool to manage the floods in current era especially for flash flooding scenarios in urban areas. This study focuses the flash flooding scenario on basin of Nullah Lai basin which comprises of twin cities of Islamabad & Rawalpindi. Steep slopes in Margalla hills & Islamabad creates high flash floods in lower reaches of Rawalpindi which are densely populated. High intensity rainfall when occurs in steep slopes of Margalla and Islamabad pour down high floods with high velocity which instantaneously reaches the Rawalpindi less sloppy regions in which causes raising of water level in the stream and flooding occurs. Nullah Lai Rawalpindi reach starting from Qatarian bridge to Gawalmandi bridge has always faced the flash flooding over the time. In the period of few hours the water level reaches the several fts level in the nullah that is why it is not possible to timely alert the people living on the banks that problem always ask for the need of forecasting system at Nullah Lai. In current research china metrological agency forecast center (CMA) ensemble forecast data has been utilized to get the forecasted stage in the Nullah Lai. For this purpose, two initial objectives were set to achieve which basic needs to process the data available in grib format at data centers. The digital model of Nullah Lai was made using hydrology tools available in ArcGIS 10.3. The digital equation was obtained from gene expression modeling (GEP) which was later used to generate the ensemble stage forecast against the ensemble rainfall forecast. The results obtained shows that the flash flooding phenomenon in Nullah Lai can be with some uncertainty be predicted well before time. Using 3 days ahead forecast data from CMA same floods were predicted 3-days before the happening of event. This research also provides the procedure to use the ensemble forecast data in developing the automated model to generate the ensemble stage forecast against coming events. This study will help the administrative authorities to better manage the upcoming floods and save live and capital cost lost in flash flooding phenomenon which continuously happen in the basin of Nullah Lai.