Need for improved (spatially and temporally) weather forecasts and more accurate tracking of flooded areas
One of the main challenges for flash flood events during the prevention phase of the disaster management cycle is to have an as accurate as possible estimation of the area where the flash flood event is expected to occur, as well as the time when it is expected. Accurate weather forecasting can support efficient flood early warning systems and a more accurate tracking of the flooded areas, which will foster effective preparedness for a flash flood event and in turn robust event crisis management and resilience.
In areas with intense relief, as are the typical Mediterranean areas, the accuracy of large scale (i.e., regional) weather forecasting may be affected, and local meteorological conditions prevail and may trigger flash floods. Hence, local weather systems strong enough to trigger floods may be underestimated. In addition, thunderstorms that typically trigger floods cannot be assigned with high accuracy to a particular area and this can result either in false alarms in local EWSs (in the area falsely identified to be affected) or in an unexpected flash flood (in the actual affected area). To this end, it is important to ensure increased accuracy in weather forecasting in terms of spatial resolution in order to track more efficiently the location of a resulting flood.
At the same time, for efficient flash flood preparedness, it is also critical to ensure increased accuracy in weather forecasting in terms of temporal resolution. More specifically, increased time lag between rainfall forecast and flood occurrence needs to be provided. This is particularly important for small basins in typical Mediterranean areas, which, due to intense relief and often dense hydrographic networks, have reduced concentration time and therefore frequent, though intense flash floods.
Overall, technological tools that may support the accuracy of weather forecasting need to be developed, while the existing ones need to be improved. During the TCP4 Flash Flood workshop, discussion focused on how to best exploit EFAS services for flash flood forecasting, especially for Mediterranean areas that have the characteristics mentioned above. It was eventually agreed between the invited experts that it would be very helpful to retrieve input from the EFAS system if it is improved in such a way so as to account for more extreme floods (e.g., considering greater return periods).
Apart from the local meteorological conditions, the accuracy in flash flood forecasting depends strongly on other local conditions, such as the geomorphological features of a particular area, local land use and land cover characteristics, as well as other local particularities.