18The environments of severe and non-severe thunderstorms were analysed using 16 421 19 proximity soundings from December 2007 to December 2013 taken at 32 Central European 20 stations. The soundings were assigned severity categories for the hazards hail, wind, tornado 21 and rain. For each of the soundings, parameters were calculated representing the instability, 22 vertical wind profile and moisture of the environment. The probability of the various hazards 23 as a function of CAPE and 0-6 km bulk shear (DLS) is quite different for each of the hazards.24Large hail is most likely for high CAPE and high DLS, a regime that also supports severe 25 wind events. A second severe wind regime exists for low CAPE and very high DLS. These 26 events are mostly cold season events. Storms with significant tornadoes occur with much 27 higher DLS than storms with weak or no tornadoes, but with similar CAPE. 0-1 km bulk 28 shear (LLS) does not discriminate better than DLS between weak and significant tornadoes. 29Heavy rain events occur across a wide range of DLS, but with CAPE above the median for 30 non-severe thunderstorms and are most likely when both absolute humidity in the boundary 31 layer and relative humidity in the low-to mid-troposphere are high. LCL height does not 32 discriminate well between the intensity categories of tornadoes, but higher LCL heights were 33 associated with higher probability of severe hail. Storm relative helicity shows similar results 34 to DLS, but with more overlap among intensity categories. 35 36 37

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<p>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.</p><p>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 m<sup>2</sup>/s<sup>2</sup>. 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.</p><p>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. </p>