Only few countries operate systematically national and regional forecasting services for rainfall-induced landslides (i.e., debris flows, debris avalanches and shallow slides), among them Norway and Italy. In Norway, the Norwegian Water Resources and Energy Directorate (NVE) operates a landslide forecasting service at national level. In Italy, the Regional Agency for Environmental Protection, ARPA Piemonte, is responsible for issuing landslide warnings for the Piedmont region, located in northwestern Italy. A daily hazard assessment is performed, describing both expected awareness level and type of landslide hazard for a selected warning region. Both services provide regular landslide hazard assessments based on a combination of quantitative thresholds and daily rainfall forecasts together with qualitative expert analysis. Daily warning reports are published at http://www.arpa.piemonte.gov.it/rischinaturali and http://www.varsom.no, last access: 7 May 2018.

In spring 2013, ARPA Piemonte and the NVE issued warnings for hydro-meteorological hazards due to the arrival of a deep and large low-pressure system, called herein Vb cyclone. This kind of weather system is known to produce the largest floods in Europe. Less known is that this weather pattern can trigger landslides as well.

In this study, we present the experiences of NVE and ARPA Piemonte in the late spring of 2013. The Vb cyclone influenced weather throughout Europe over a long period, from the end of April until the beginning of June 2013. However, major affects were observed in the first half part of this period in Piedmont, while in Norway, major damage was reported from 15 May to 2 June 2013. Floods and landslides significantly damaged roads, railways, buildings and other infrastructure in both countries.

This case study shows that large synoptic pattern can produce different natural hazards in different parts of Europe, from sandstorms at low latitudes, to flood and landslides when the system moves across the mountain regions. These secondary effects were effectively forecasted by the two landslide warning services, operating in different parts of Europe. The landslide risks were also properly communicated to the public some days in advance. This analysis has allowed the establishment of fruitful international collaboration between ARPA Piemonte and NVE and the future exchange of experiences, procedures and methods relating to similar events.