 The Global Flood Awareness System, GLO FAS, was designed to provide an overview of upcoming floods in large world river basins. The system uses distributed hydrological simulation of numerical ensemble weather predictions with global coverage and compares these forecasts to climatological simulations to detect probabilistic exceedance of warning thresholds. The system was evaluated over a two-year test period and a case study of the Pakistan flood in summer 2010 was conducted. Results showed that hazardous events in large river basins could be skillfully detected with a forecast horizon of up to one month. Additionally, it was found that an accurate simulation of initial model conditions and an improved parameterization of the hydrological model were key components to reproducing accurately the streamflow variability in the many different runoff regimes of the earth. This article was authored by L. Al Fieri, P. Burak, E. Dutra, and others.