Damage survey, environment and storm-scale evolution of the giant hail and F4 tornado producing supercell on 24 June 2021

In the afternoon of 24 June 2021, severe hailstorms affected Austria, Czechia, and Poland and an F4 tornado occurred in southeastern Czechia. Along the 27.1 km long path, it damaged 1200 buildings and caused 6 fatalities and more than 280 injuries. The width of the damage path was extreme for European standards, up to 2500 m across. The zone with significant damage of F2 or stronger was up to 520 m wide. Isolated instances of F4 damage were noted in 3 villages with the destruction of well-constructed brick walls and significant debarking of trees. We discuss the challenges associated with surveying the tornado from an organizational point of view to the strategy onsite. Improvements are proposed to make surveys of such large-scale events more effective.

The event was not well forecast even by expert forecasters present at the ESSL Testbed 2021. Although the environment was clearly conducive for supercells capable of very large hail with high values of CAPE (> 3000 J/kg) and strong vertical wind shear (0-6 km bulk shear > 20 m/s), lower tropospheric shear was forecast to remain fairly weak by most of the NWP models with 0-1 km bulk shear < 10 m/s and 0-1 km SRH < 100 m²/s². The fact that only one tornado (and of such a high intensity) occurred in the area despite numerous supercells present points to the importance of mesoscale modifications to the environment. We address the storm-scale evolution starting from the merger of two storms through updraft intensification with giant hail production, and subsequently, low-level mesocyclone strengthening and tornado production. We also discuss the importance of local mesoscale boundaries and modification to the environment shortly before the tornado.

The event illustrates a number of difficulties with tornado forecasting in Europe. The first is the lack of sufficient data exchange among countries. The tornado passed within 10 km of the borders of Austria and Slovakia and the tornado-producing supercell formed over Austria. No exchange of automatic station surface observations and volumetric radar data between those countries takes place and this likely limited the situational awareness of forecasters. While the tornado occurred over Czechia, the storm was best detected from a Slovakian radar. Another difficulty was the aggressive filtering of doppler velocity data that masked the core of the low-level mesocyclone preventing forecasters to appreciate the intensity of the event as it unfolded.