The outbreak of severe storms along convergence lines northeast of the Alps. Case study of the 3 August 2001 mesoscale convective system with a pronounced bow echo

Thunderstorm development in Austria is highly influenced by orographical forcing especially in summer. However, the most severe thunderstorms occur in the lower areas north and southeast of the Alps where mountains are not high. The reason is the undisturbed initiation and intensification of thunderstorms due to the low-level wind field which is modified near the edge of the Alps. Typically, severe convective weather events in Austria develop when a slow-moving cold front approaches from west (e.g., Switzerland) and when unstable air is advected from the Mediterranean by southwesterly flow. During such conditions, a convergence zone is frequently formed north of the Alps by a low-level airflow as a result of diabatic heating and evaporative cooling along the cloud edge of the cold front. This convergence line moves faster to the east than the cold front itself and triggers new cells or intensifies existing cells. se study presented in this work is based on analyses of data from the Austrocontrol Weather Radar Network and objective surface pressure analyses. Two supercells initiated along a convergence line due to enhanced low level wind shear on the lee side of the Alps. Both severe storms, linked to the northern slopes of the Alps, approached Austria from Bavaria which evolved into a Mesoscale Convective System with a pronounced bow echo within the orographic gap between the mountain range of Bohemian Forest and the Alps. The case study shows a typical storm configuration and how to improve nowcasting of mesoscale modified thunderstorms north of the Alps.