The probabilistic forecast is supplied by a Regional Ensemble Prediction System (REPS), based on the WRF-ARW model, implemented and developed by the Epson Meteo Centre (EMC). The REPS used in this project has a grid mesh size of 18 km, 36 vertical levels and twenty members; boundary conditions and base data are provided by a global ensemble prediction system (GEPS) based on a modified version of the WRF-ARW applied to the global scale, which has a grid mesh size of 200 km and the same number of vertical levels as REPS and uses the same initial conditions (before perturbation) provided by the 12UTC, 0.5 degree analysis as the GFS model. The forecast has a lead time of 30 days and the output fields are produced every 12 hours. Each perturbation of the ensemble is produced by an algorithm developed by EMC. The combined system GEPS-REPS is carried out every two days. The REPS data are used starting at 00UTC, the same initial time of the hydrological simulation. For a detailed description of WRF model, the reader is referred to Skamarock and Klemp (2007) [11].