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Flash floods induced by extreme rainfall events represent one of the most life-threatening phenomena in the Mediterranean. While their catastrophic ground effects are well documented by postevent surveys, the extreme rainfall events that generate them are still difficult to observe properly. Being able to collect observations of such events will help scientists to better understand and model these phenomena. The recent flash floods that hit the Liguria region (Italy) between the end of October and beginning of November 2011 give us the opportunity to use the measurements available from a large number of sensors, both ground based and spaceborne, to characterize these events. In this paper, the authors analyze the role of the key ingredients (e.g., unstable air masses, moist low-level jets, steep orography, and a slow-evolving synoptic pattern) for severe rainfall processes over complex orography. For the two Ligurian events, this role has been analyzed through the available observations (e.g., Meteosat Second Generation, Moderate Resolution Imaging Spectroradiometer, the Italian Radar Network mosaic, and the Italian rain gauge network observations). The authors then address the possible role of sea–atmosphere interactions and propose a characterization of these events in terms of their predictability.

Gaining a deeper physical understanding of the high-impact weather events which repeatedly affected the Western Mediterranean Basin in recent years on the coastal areas of easternSpain, southern France and northern Italy is strongly motivated by the social request to reduce the casualties and the economical impacts due to these highly localized and hardly predictable phenomena.In October 2014, an extreme event hit Genoa city centre, less than 3 years after a very similar event, which occurred in November 2011.Taking advantage of the availability of both observational data and modelling results at the micro-α meteorological scale, this article provides insights about the triggering mechanism and the subsequent spatio-temporal evolution of the Genoa 2014 back-building Mesoscale Convective System. The major finding is the effect of a virtual mountain created over the Ligurian Sea by the convergence of a cold and dry jet outflowing from the Po valley and a warm and moist low-level southeasterly jet within the planetary boundary layer.

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Abstract. Forecast verification is a long-standing issue of the whole meteorologists' community. A common definition of a truly satisfying prediction skill has not been achieved so far. Even the definition of "event", due to its spatio-temporal discontinuity, is highly affected by uncertainty.Moreover, decision-making demands numerical weather prediction modellers to provide information about the "inner" uncertainty, i.e. the degree of uncertainty related to the choice of a specific setting of the model (microphysics, turbulence scheme, convective closure, etc.).Most European Mediterranean countries, due to dense development, steep coastal orography and short hydrological response time of the drainage basins, have to deal very frequently with flash floods and sudden shallow land sliding impacting on urban areas. Civil protection organizations are in place to issue early warnings in order to allow local authorities and population to take precautionary measures. To do so in Mediterranean catchments, hydrologists are required to use numerical rainfall predictions in place of rainfall observations on large European catchments.Estimating the measure of uncertainty is for this reason crucial.The goal of this work is to propose an objective evaluation of the performance of the currently operational weather prediction model COSMO-I7 over quite a long time period and to check forecast verification at different space-time scales by the comparison of predictions with observations.Due to large investments in the last years, in fact, Italy has built up one of the most dense hourly-reporting network of rain gauges. The network has a mean space density of about 1/100 km 2 , very similar to the horizontal resolution of currently operating limited area models.Correspondence to: L. Molini (luca.molini@cimafoundation.org) An objective procedure to identify and compare the extreme events of precipitation has been applied to the full set of rainfall observations and over the severe events forecast by COSMO-I7 and announced in official warnings by Italian Civil Protection Department.The procedure allows to classify rainfall events as longlived and spatially distributed or as having a shorter duration and a minor spatial extent. We show that long-lived events are less affected by overall uncertainty than short-lived ones, yet the inner uncertainty of the event affects both.