'L'alluvione di Firenze del 1966': an ensemble-based re-forecasting study
Capecchi, Valerio ; Lang, Simon ; Buizza, Roberto
During the first few days of November 1966, several Italian regions were hit by a storm that produced intense and persistent precipitation over north-east and central Italy. The exceptionally intense rain on 3-4 November followed weeks of wet conditions and led to disastrous flooding. The historical city of Florence (Firenze) was one of the most severely affected. The Arno River flooded the city, causing huge damage to its cultural and artistic heritage and enormous economic losses. At the same time, Venice experienced the maximum ever recorded storm surge levels. The storm is considered to be one of the most severe weather events to have affected Italy in the last century, with more than 110 fatalities caused by weather-induced conditions.
To mark the 50th anniversary of the Arno River flooding, we have revisited this rainfall event by applying cutting-edge global and regional NWP modelling to it using an ensemble approach. The results show that the ensemble approach provides added value compared to single forecasts by providing objective confidence measures and helping to estimate the probability of high-precipitation values, in this case up to three days in advance. The WRF (Weather and Research Forecasting) limited-area model (LAM) predicts higher probabilities of intense precipitation in the area than the global model, but neither ECMWF's current global model nor the LAM correctly predicts the intensity of the rainfall observed in the Arno catchment area. Part of the explanation may be the limited observations available to initialise the forecasts as well as the relatively coarse resolution of the global model.
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