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. This paper presents an extended reanalysis of the rainfall-induced geo-hydrological events that have occurred in the last 70 years in the alpine area of the Lombardy region, Italy. The work is focused on the description of the major meteorological triggering factors that have caused diffuse episodes of shallow landslides and debris flow. The aim of this reanalysis was to try to evaluate their magnitude quantitatively. The triggering factors were studied following two approaches. The first onestarted from the conventional analysis of the rainfall intensity (I) andduration (D) considering local rain gauge data and applying the I–Dthreshold methodology integrated with an estimation of the events' returnperiod. We then extended this analysis and proposed a new index for themagnitude assessment (magnitude index, MI) based on frequency–magnitude theory. The MI was defined considering both the return period and the spatial extent of each rainfall episode. The second approach is based on a regional-scale analysis of meteorologicaltriggers. In particular, the strength of the extratropical cyclone (EC) structure associated with the precipitation events was assessed through the sea level pressure tendency (SLPT) meteorological index. The latter has been estimated from the Norwegian cyclone model (NCM) theory. Both indexes have shown an agreement in ranking the event's magnitude(R2=0.88), giving a similar interpretation of the severity thatwas also found to be in accordance with the information reported in historicaldatabases. This back analysis of 70 years in Valtellina identifies the MI and the SLPTas good magnitude indicators of the event, confirming that a strong cause–effect relationship exists among the EC intensity and the localrainfall recorded on the ground. In respect of the conventional I–Dthreshold methodology, which is limited to a binary estimate of the likelihood of landslide occurrence, the evaluation of the MI and the SLPTindexes allows quantifying the magnitude of a rainfall episode capable ofgenerating severe geo-hydrological hazards.

Analysis of meteorological parameters triggering rainfall-induced landslide: a review of 70 years in Valtellina