An F1 tornado hit the village of Lekárovce in eastern Slovakia on the afternoon of 3 October 2018. The tornado, which occurred outside the main convective season in Slovakia, was not anticipated by the meteorologists of the Slovak Hydrometeorological Institute. The models available to the forecasters simulated an environment of marginal convective available potential energy (CAPE) and weakening vertical wind shear. This paper addresses forecasting challenges associated with events related to a tornado threat. To investigate conditions before tornado formation, observational datasets, including sounding, and vertical-azimuth display (VAD) data from a radar station and surface stations were used. Hodographs based on observational data and a higher-resolution run of the limited-area model showed stronger lower tropospheric shear than was formerly anticipated over the area of interest. The higher-resolution model was able to better represent the modification of the lower tropospheric flow by a mountain chain, which was crucial to maintaining the strong lower tropospheric shear in the early afternoon hours before the tornado’s occurrence. We discuss the importance of using both observational datasets and higher-resolution modeling in the simulation of lower tropospheric wind profiles, which affect the lower tropospheric storm relative helicity as one of the key ingredients in mesocyclonic tornadogenesis.
<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.&#160;</p>
<p>The spatial distribution of supercells is strongly inhomogeneous across Slovakia with a higher frequency of occurrence in several regions in the eastern part of the country. The main aim of the work was to find reasons for this inhomogeneity. This is done by running high-resolution simulations of the ALARO model with a grid spacing of 1 km for all supercell cases in eastern Slovakia between 2017 and 2020. We studied the evolution of pre-convective environments, as well as the lower tropospheric flow patterns to establish the typical scenarios, in which supercells occur in Slovakia. The most important factor was found to be an interaction of large-scale flow with local orography, which affected a whole range of processes in the troposphere. As a result of the various orographic effects, such as blocking, wrapping, gap wind, lee cyclogenesis, and upslope flow, we have detected zones of enhanced convergence and spatial anomalies of ingredients important for the development of deep moist convection. In most of the analyzed cases, we noted a local increase in the vertical wind shear in the lee of the mountains, which increased the probability of supercell formation. In the cases with the prevailing southerly flow and warm-air advection regime, supercells typically formed to the south and east of the Western Carpathians. Convergence lines and upslope flow initiated convection and local enhancement of vertical wind shear was essential for the genesis of supercells. In the cases with the prevailing northerly flow and cold-air advection, blocking allowed for the maintenance of a warm and humid air mass (with non-zero CAPE values) and a generation of convergence zone on the southern flank of Western Carpathians.</p>
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