Severe thunderstorm in the central part of Croatia on 15th September 2022

Severe thunderstorm with gale-force and most likely hurricane-like wind gusts, heavy rain showers and locally large hail affected area of Bjelovarsko–bilogorska county in the central part of Croatia in the afternoon hours on 15^th September 2022. Some locals also mentioned the possibility of a tornado. The thunderstorm caused significant damage to property, crops and forests estimated at approximately 8 million EUR. A natural disaster was declared in two cities and five county municipalities.

Due to very high impact and even the possible occurrence of a tornado, detail synoptic and mesoscale analysis of severe thunderstorm was performed. Moreover, for the first time in recent history, an official on-site meteorological inspection of affected areas was done. That resulted in abundant photo and video materials which were used for the analysis of damage patterns and determination of the phenomena that caused the damage.

On 15th September 2022, the ridge over Croatia weakened and the southwesterly flow strengthened bringing a significant amount of moisture. The dew point temperature was between 16 and 18 °C. A cold front was approaching from the northwest, and in the upper levels a colder air was advected above the very heated surface, due to which the instability of the atmosphere increased. According to radio sounding measurement in Zagreb–Maksimir at 12 UTC, the most unstable CAPE (MU CAPE) was 888 J/kg, and 0-6 km bulk shear was 24.5 m/s which are characteristic values of the environment where supercells can form. The wind shear was mainly concentrated in the lower layers of the troposphere, from 0 to 3 km (23.6 m/s), and it was also significant in the layer from 0 to 1 km (12.1 m/s), which favors the formation of tornadoes.

Radar reflectivity analysis confirmed the presence of a supercell, and a bow-shape was detected what is characteristic for storms with damaging winds. In the satellite imagery during the mature phase of the thunderstorm, the overshooting tops were observed indicating very strong convective updraft. By analyzing the forest damage areas from the ground and using the aerial drone footage, no evidence of wind spinning was found. All trees were knocked down in the approximately same straight-line direction, only slightly divergent damage pattern was observed. Such a damage pattern is usually associated with downbursts.