  During the night between 9 and 10 September 2017, multiple flash floods
associated to a heavy-precipitation event affected the town of Livorno, located
in Tuscany, Italy. Accumulated precipitation exceeding 200 mm in two hours,
associated with a return period higher than 200 years, caused all the largest
streams of the Livorno municipality to flood several areas of the town. We used
the limited-area Weather Research and Forecasting (WRF) model, in a
convection-permitting setup, to reconstruct the extreme event leading to the
flash floods. We evaluated possible forecasting improvements emerging from the
assimilation of local ground stations and X- and S-band radar data into the
WRF, using the configuration operational at the meteorological center of
Tuscany region (LaMMA) at the time of the event. Simulations were verified
against weather station observations, through an innovative method aimed at
disentangling the positioning and intensity errors of precipitation forecasts.
By providing more accurate descriptions of the low-level flow and a better
assessment of the atmospheric water vapour, the results demonstrate that
assimilating radar data improved the quantitative precipitation forecasts.
