## 3 Background of the Event
### Meteorological Background  
The synoptic meteorological situation between 10 and 13 August 2019 was characterised by a low surface pressure (995 hPa) moving North from Ireland to Norway related with a high troposphere trough from Northern Atlantic Ocean (Iceland) to France. The axis of the trough was rotating eastward, and a cold front passes the Alps on the 10 August (Figure 3a). This front caused sparsely distributed showers on the Lepontine Alps and Northern Ossola Valley. The mean wind from 700 hPa to 300 hPa was from West-South-West. The satellite images on North Italy showed thin clouds, comparable to  
Figure 2: The investigated slope nearby the San Domenico hamlet and the Alpe Ciamporino ski area. (**a**) Geological map (source ARPA Piemonte layers; geologia e dissesto, 1:250,000; source [[https://webigis.arpa.piemonte.it/Geoviewer2D/index.html?config=other-configs/geologia250k_config.json](https://webigis.arpa.piemonte.it/Geoviewer2D/index.html?config=other-configs/geologia250k_config.json)]([https://webigis.arpa.piemonte.it/Geoviewer2D/index.html?config=other-configs/geologia250k_config.json](https://webigis.arpa.piemonte.it/Geoviewer2D/index.html?config=other-configs/geologia250k_config.json))); (**b**) Hillslope model elaborated from a DTM of 5 m of resolution (source [[http://www.geoportale.piemonte.it/cms/](http://www.geoportale.piemonte.it/cms/)]([http://www.geoportale.piemonte.it/cms/](http://www.geoportale.piemonte.it/cms/))); (**c**) Corine Land Use 2012–Level IV (source Geoportale Nazionale; [[http://www.pcn.minambiente.it/mattm/servizio-wms/](http://www.pcn.minambiente.it/mattm/servizio-wms/)]([http://www.pcn.minambiente.it/mattm/servizio-wms/](http://www.pcn.minambiente.it/mattm/servizio-wms/))); Background courtesy of Environmental Systems Research Institute (ESRI) DigitalGlobe & GeoEye (GIS User commonly; [[http://microsites.digitalglobe.com/arcgis/](http://microsites.digitalglobe.com/arcgis/)]([http://microsites.digitalglobe.com/arcgis/](http://microsites.digitalglobe.com/arcgis/))). small afternoon thunderstorms, in fact, the Figure 3e does not indicate deep convection, and the type of clouds is low and middle (Figure 3g). Figure 3: (**a,b**) Surface pressure chart analysis, with station data, fronts and isobars; (**c,d**) satellite HRV (High-Resolution Visible) image; (**e,f**) satellite RGB convection-enhanced product (red and orange indicate high convection); (**g,h**) satellite RGB Dust product (yellow: low clouds; green: middle clouds; black-dark red: high clouds; red: convective clouds). Blue lines on satellite images represent geopotential heights at 500 hPa in decametres. Left panels (**a,c,e,g**) refer to Saturday 10 th August 2019 at 1200 UTC, while right panels (**b,d,f,h**) refer to Monday 12 th August 2019 at 0600 UTC. Source DWD Archive for panels a and b, Eumetrain for satellite images. After this first passage, only low clouds persisted over the investigated area for the following 36 h, when the progressive expansion of a high-pressure field from the Azores Islands modified the mean flow to South-West and increased the instability over the Po Valley. During the 11 th afternoon, a huge thunderstorm was registered between Novara and Milano, and it vanished before the Ossola Valley. The changed circulation advected warm and moist air from the Mediterranean Sea over the Western Alps and Apennines and generated a stationary front that insisted on the Alps between 0600 UTC and 1200 UTC of the 12 August (Figure 3b). The maximum cloud cover was reached between midday and afternoon when the most intense precipitation was registered on the neighbouring plains (Novara, Vercelli and Milano) and piedmont areas. This front caused the precipitation that between 0600 UTC and 0620 UTC provoked the debris flow along the Croso stream. Between 10 and 13 August the maximum Convective Available Potential Energy (CAPE) measured at Milano Linate sounding occurred on 12 August (1200 UTC, 2282 J/kg), indicating the most unstable conditions among all these days.