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 Status: Ongoing analysis Material from: Linus, Fatima, Tim H. , Esti, Andrea 

1. Impact

Some extraordinary flash floods hit several parts of Sardinia on 28 November, which for a widespread event was probably only eclipsed in the last decade, on that island, by the events of 18 November 2013. In the 2013 event the peak event rainfall was about 450mm, in the recent event it was closer to 350mm. They were in fact remarkably similar, in synoptic terms, with an 'upside down' cold front moving south to north across the island, over about a day, on the eastern flank of a deep Mediterranean cyclone, and with warm moist air on the NE flank of the cold front, feeding up from a marine source that starts near the N coast of Libya. This airflow delivered periods of heavy rain to the eastern side of the island in particular, where sharply rising mountains can also lead to significant orographic enhancement. Widespread lightning strikes over land (and sea), together with and IFS model output (profiles), together attest to the fact that the 2020 event was highly convective nature of the 2020 eventover Sardinia, notably in the first half of 28th.

2. Description of the event

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This case was also relevant for the MISTRAL initiative (an EU project in which Italian partners and ECMWF have collaborated). In that project we have the "Italy flash flood use case" (see here and here), spearheaded by ECMWF, in which post-processing, of different types, is applied to the ECMWF ENSemble, and also a 2.2km Italy-centred limited area COSMO ensemble. The post-processed outputs are blended together, with lead-time-dependant weighting, to make the final product, which aims to give better probabilistic rainfall forecasts, in particular with a view to providing improved early warnings of flash flood risk (via the association with extreme short period rainfall). Users are encouraged to focus on the higher percentiles (or probabilities of exceeding high thresholds) in the MISTRAL products, to gauge the potential for localised extremes. However some of the plots below illustrate also how the ensemble mean is handled in the raw model and post-processed output, with differences between the two (for a given system) indicating the nature of any bias-correction being applied on the model grid-scale. This plot focusses on a 6h accumulation period, 06-12UTC 28th, when most of the rainfall was diagnosed in the ECMWF model runs to be convective. In contrast Sections 3.2 and 3.3 above deal with a 24h accumulation period.

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