## 3 Results and Discussion
### Hindcast Experiments to Improve the Forecasting Chain  
In an hindcast experiment, the meteorological forecast was run again using the WRF-DA setup, allowing the assimilation of reflectivity radar and temperature data (Figure 14). The WRF-OL configuration already provided a forecast close to the observed event, as demonstrated by the high Total Interest value obtained during the validation process (Table 2). Moreover, in this case, the MODE validation was applied to the WRF-DA forecast to assess the assimilation value added (Table 3). Despite the starting point having already high performing scores, the Total Interest grows for the 150 mm threshold from 0.95 to 0.97 and from 0.91 to 0.95 for the 200 mm  
Comparing Figures 4 and 14, it is possible to highlight that the WRF-DA setup (Figure 14, panel B) allows to reduce the rainfall overestimation provided by the WRF-OL configuration (Figure 4, panel B) in the areas that were not hit by the most intense part of the event, as highlighted by the observed rainfall map (Figures 4 and 14, panels A). Moreover, the WRF-DA was used as input of the Flood-PROOFS hydrological chain to evaluate the benefit and the predictive skills of the meteorological assimilation. Figure 15 shows the stream flow forecast of this configuration, even with the assimilation, and red dots highlight that the peak discharge forecasts were always exceeding the red threshold in all river sections of eastern Sicily in the night between 24 and 25 October. By using the forecast hydrographs of Flood-PROOFS with WRF-DA, a run of the TELEMAC-2D hydraulic model led to forecast floodable areas; Figure 16 is a comparison between the forecasting of the floodable area (on the left) and the flooded area using Flood-PROOFS with ground data. The forecast floodable areas overestimates the ground data flooded areas but, nevertheless, having the information of potentially floodable areas in advance would allow better assessments and better civil protection planning for the post-event phase. Since no streamflow observations are available for the regions, we use the run of the hydrological model fed by observations as benchmark (reference hydrograph), as commonly used in the literature (e.g., [30; 41]). In particular, the peak flows from the benchmark run, which occurred in the morning of 25 th of October, were compared with the peak flows forecast of the 24 th of October. The comparison is performed on a set of basins in Calabria and Sicily, where peak flows forecast or peak flows from the benchmark exceeded the warning thresholds. Since the WRF-OL forecast is performed using a probabilistic approach as in [42], we reported minimum and maximum peak flows of the ensemble. Table 4 reports the results of the comparison; it shows the basins where a streamflow threshold [25] is exceeded, and therefore the section is marked with a red color on the my-Dewetra platform (as explained in Figure 5 caption). In these cases, both the configurations (WRF-OL and WRF-DA) give good results, even if the peaks are slightly underestimated in WRF-DA and overestimated in WRF-OL. Figure 15: OCTOBER 24 09 UTC: Flood-PROOFS stream flow deterministic forecast using WRF-DA of the 24 October at 00 UTC as input. In the cases of the Simeto, Anapo, and Bonamico basins, the overestimation of the forecast is quite high for both configurations, and similar conditions occur in the Amendola basins, even if the WRF-OL configuration has a lower overestimation. The Petrace basin evidences a small underestimation of the Open-Loop configuration, while DA weakly overestimates the peak flow. The selected basins have a drainage area ranging from 50 to 4000 km\({}^{2}\); especially for smaller basins, the intrinsic uncertainties of meteorological forecasts contrast with the spatial scales of the catchments [43; 44].