On Hailstorm Variability in Switzerland: Key Drivers for Seasonal Predictability

The new long-term (1959-2022) reconstruction of Swiss hail days by Wilhelm et al. (2024) reveals substantial natural year-to-year variability in hail activity and a pronounced seasonal cycle. Building on these findings, this study aims to identify the drivers behind this variability, in particular, local and remote atmospheric, ocean and surface conditions that distinguish hail-active from hail-sparse summers. To investigate these mechanisms, we analyze composites of potential drivers, comparing seasons and months with high versus low hail frequency.Our results show that strong hail seasons in northern Switzerland are associated with anomalously warm sea surface temperatures (SSTs) in the Mediterranean, cool SSTs in the northern and eastern Atlantic, elevated skin temperatures and dry soils in Central Europe, and a distinct wave train in the 500 hPa geopotential height field, with a trough upstream over the Atlantic and a ridge over the study region. We find a strong coupling between the large-scale atmospheric circulation, Central European land-surface conditions, and surrounding SSTs. These patterns contrast clearly with those during weak hail seasons and differ between northern and southern Switzerland, indicating regional signals and potential for predictability. Furthermore, we identify precursors of strong hail seasons by examining SSTs, snow cover, and sea ice during the preceding winter and spring. These relationships are then leveraged to build a first seasonal prediction model aimed at classifying hail seasons as strong, average, or weak, providing a valuable step toward anticipating hailstorm risk several months in advance.