## 2 Materials and Methods
### The Meteorological Model: WRF  
The WRF model [9] is used in this work both in operational application without data assimilation (WRF-OL) and in hindcast mode assimilating radar data and in situ weather stations (2 m temperature data, WRF-DA). WRF is a compressible nonhydrostatic model with mass-based terrain-following coordinates that was developed at the National Center for Atmospheric Research (NCAR) in collaboration with several institutes and universities for operational weather forecasting and atmospheric science research. For this study, three two-way nested domains were used in both configurations. The WRF-OL setup adopts a horizontal grid spacing of 13.5, 4.5, and 1.5 km with 50 vertical levels (Figure 3 panel A). The WRF-DA setup has instead a horizontal grid spacing of 22.5, 7.5, and 2.5 km with 50 vertical levels (Figure 3 panel B); all domains' tops reach 50 hPa. The WRF-OL and WRF-DA setup  
Figure 2: 25 October 2021 00 UTC: SLP (hPa) and 10 m wind (knots) with the area of interest highlighted by a red rectangle and the Catania floodplain location highlighted with a dark red dot (**top**); Temperature (°C), Geopotential (dam) and wind (knots) at 500 hPa (**bottom**). Data from ECMWF analysis. have different grid spacing because the configurations used are both already operationally run in CIMA Research Foundation. Thus, for the experiment with data assimilation, the decision was to maintain the setup already running with cappi reflectivity assimilation, with consolidated background error covariance matrix, adding only the 2 m temperature assimilation for the hindcast experiment. All the simulations are performed with the same set of physical parameterizations (Table 1), described hereafter, that have already been successfully tested in other extreme events studies over Italy [10; 11; 12]. The MM5 scheme is adopted as surface layer [13; 14; 15]. A [16] scheme is used to enhance surface fluxes of heat and moisture. The Rapid Update Cycle (RUC) scheme is used as a multi-level soil model (6 levels), with higher resolution in the upper soil layers (0, 5, 20, 40, 160, and 300 cm). This soil model solves the heat diffusion and Richards moisture transfer equations (with a layer approach) and, in the cold season, considers phase changes in soil water [17; 18]. The planetary boundary layer (PBL) dynamics are parameterized with the diagnostic nonlocal Yonsei University (YSU) PBL scheme [19]. The WSM6 microphysics six-class scheme is adopted [20]. Lastly, the radiative processes are parameterized by means of the longwave and shortwave RRTMG schemes [21]. Both model setups in their innermost domains are at the km scale, therefore adopting cloud-resolving grid spacing; thus, they both have an explicit treatment of convection. The initial and lateral boundary conditions are derived from NCEP-GFS (National Centers for Environmental Prediction Global Forecast System) analysis and forecast data available at a horizontal grid spacing of 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) and a 3 h time resolution. The forecasts in this work are initialized at 00 UTC on 24 October 2021. The research forecasts with data assimilation (WRF-DA setup) were implemented with a 3-h cycle 3 DVAR technique of radar reflectivity and personal weather station (Wunderground) data at 18-21-00 UTC before the 48 h free forecast. The WRF data assimilation package WRFDA v3.9.1 is used. For the applied 3 DVAR [12], the reflectivity operator used \begin{table}
\begin{tabular}{c c c} \hline \hline
**Physics Options** & **WRF-OL** & **WRF-DA** \\ \hline Microphysics & WSM6 & WSM6 \\ PBL & YSU & YSU \\ Longwave radiation & RRTMG & RRTMG \\ Shortwave radiation & RRTMG & RRTMG \\ Land surface physics & RUC & RUC \\ Cumulus parameterization & Explicit on inner domain & Explicit on inner domain \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parametrization schemes adopted for subgrid-scale processes. Figure 3: Panel (**A**), the WRF-OL setup: three two-way nested domains with 13.5, 4.5, and 1.5 km grid spacing.