Universität Wien, Institut für Meteorologie und Geophysik - Austria
2 Austro Control - Aviation Weather Service - Austria
Thunderstorm development in Austria is highly influenced by orographical forcing especially in summer. However, the most severe thunderstorms occur in the lower northeastern and southeastern parts 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 cold front approaches from West (e.g. from Switzerland) and when unstable air is advected from the Mediterranean by southwesterly flow.
During such conditions, a convergence line is frequently formed north of the Alps by a cold low-level airflow. This convergence zone moves faster to the east than the cold front and triggers new cells or intensifies existing cells in the unstable air and produces severe thunderstorms especially in the later afternoon.
Two case studies will be presented based on the analysis of data from the Austro Control Weather Radar System. On May 17, 2001, a supercell developed on the southern flank of a convergence line over the Czech Republic. Near the Austrian town Krems, heavy rain and hail caused damage in a region of 8000 hectares of vineyards and fields. The second case is the evolution of a mesoscale convective system (MCS) on August 03, 2001. Severe Thunderstorms approached Austria from Bavaria which evolved into a MCS with a pronounced bow echo. In the whole northern part of Austria, severe hail and strong winds were reported over several hours.