Temporal clustering of extreme precipitation events on subseasonal time scales is of crucial importance for the formation of large-scale flood events. Here, the temporal clustering of regional-scale extreme precipitation events in southern Switzerland is studied. These precipitation events are relevant for the flooding of lakes in southern Switzerland and northern Italy. This research determines whether temporal clustering is present and then identifies the dynamics that are responsible for the clustering.An observation-based gridded precipitation dataset of Swiss daily rainfall sums and ECMWF reanalysis datasets are used. Also used is a modified version of Ripley's K function, which determines the average number of extreme events in a time period, to characterize temporal clustering on subseasonal time scales and to determine the statistical significance of the clustering. Significant clustering of regional-scale precipitation extremes is found on subseasonal time scales during the fall season.Four high-impact clustering episodes are then selected and the dynamics responsible for the clustering are examined. During the four clustering episodes, all heavy precipitation events were associated with an upperlevel breaking Rossby wave over western Europe and in most cases strong diabatic processes upstream over the Atlantic played a role in the amplification of these breaking waves. Atmospheric blocking downstream over eastern Europe supported this wave breaking during two of the clustering episodes. During one of the clustering periods, several extratropical transitions of tropical cyclones in the Atlantic contributed to the formation of high-amplitude ridges over the Atlantic basin and downstream wave breaking. During another event, blocking over Alaska assisted the phase locking of the Rossby waves downstream over the Atlantic.

Alpine heavy precipitation events often affect small catchments, although the circulation pattern leading to the event extends over the entire North Atlantic. The various For the first time, this study documents an important pathway for Northern Alpine flooding, in which the interaction of synoptic-scale to large-scale weather systems and of long-range moisture transport from the Tropics are dominant. Moreover, the trapping of moisture in a subtropical cut-off near the West African coast is found to be a crucial precursor to the observed European high impact weather.

This article presents a climatological analysis of upper-level synoptic-scale flow structures and low-level moisture signatures associated with extreme precipitation events in north-eastern (NE) and north-western (NW) Switzerland. Extreme daily precipitation events between 1961 and 2010 are identified using a gridded observation-based precipitation data set. ERA-Interim and ERA-40 fields are used to characterize the flow. Large-scale flow configurations over Europe that are common to extreme precipitation events are identified by a K-means clustering approach. For each area, five large-scale flow classes are defined. These flow classes include zonally oriented flows over western Europe accompanied by a ridge over the east Atlantic, amplified upper-level troughs over western Europe, strong westerly or south-westerly flows, wave-breaking events and a broad trough over the Mediterranean. For each class, the main forcing mechanism behind the ascent of moist air, the magnitude of the total precipitable water and the seasonal distribution are presented. The way in which the location and structure of the defined upper-level classes affect the local distribution of precipitation is analysed. Moreover, we analyse the directions of the moisture flux that interacts with the topography and results in extreme precipitation events. The flow conditions range from advective situations in which large amounts of moisture are transported towards the orographic obstacle to flow situations in which heavy precipitation occurs on the lee side of the Alps and ascent is mostly driven by large-scale forcing. The co-occurrence of events between NE and NW Switzerland is presented. Approximately 30% of the events occurred in both areas on the same day or differed in timing by 1 day.