Significant hail producing storms in Finland

Hail can be produced by various convective storm types. Significant-hail (≥5 cm diameter) events are generally assumed to be produced by supercell thunderstorms. For a forecaster to anticipate a given hail size, forecasting the convective mode is one of the main methods. Therefore, understanding both radar characteristics and environmental conditions during these extreme events, contribute to their better forecasting.

Significant hail has been documented in Finland during the 14-yr period of 1998-2011 during 16 significant-hail days. The significant-hail days are typically characterized by several storms causing severe hail (≥2 cm diameter). In this study, we relate the ground reports of severe hail to the storm producing them. The radar-based convective modes and storm characteristics are determined for all documented storms leading to significant hail in Finland. Additionally, for three supercell cases, the large number of ground reports allowed analyzing the individual hail swaths. The storm environments are studied by using observed soundings.

The results show that several of the significant-hail events in Finland are produced by multicell storms and that most of the events occurred in rather weak buoyancy environment.