The Severe to Extreme storms over Bosnia and Herzegovina in July 2023

Abstract: In July 2023, the western Balkans were struck by a serias of supercells and squall lines, that resulted in several fatalities, numerous injuries, and extensive property damage. In each of these cases, a trough at the 500 mb level facilitated the of a colder air mass from northern Europe, which collided with a warmer air mass from the south. This interaction, combined with surface heating and high Convective Available Potential Energy (CAPE) and a low Lifted Index (LI), led to development of cumulonimbus (Cb) clouds, which caused significant damage. These atmospheric instabilities produced hailstones exceeding 5 cm in diameter, wind gusts between 15-25 m/s (locally even stronger), heavy localized rainfall, and intense lightning activity. For most days with severe to extreme weather phenomena, numerical weather models provided accurate forecasts. However, on July 18, the Weather Research and Forecasting (WRF) with ARW core version 4.3.3 underestimated the event. The model, configured with Thompson microphysics and a horizontal resolution of 8 x 8 km (initialized with GFS data at 12 UTC on July 17, 2023), predicted the 500 mb trough and associated jet stream to be located farther west and north than the actual location. In reality, the suqall line affected most parts of northern Croatia, Bosnia nad Herzegovina, southewestern Serbia and Montenegro moving quickly from the Alps. As a result, real-time radar and satellite monitoring proved to be crucial for tracking the system. RGB composites, along with WV and IR satellite channels, were particulary effective in monitoringthe squall line on that day. Most of the supercells and instabilities observed during July 2023 followed a similar trajectory, originating over the Alps and progressing toward the Balkans and the Pannonian Plain.