Storms over Croatia. Credit: Ivan Smiljanic

Destructive convective storms in SE Europe

18-20 July 2023

Photo credit: Ivan Smiljanic

Storms over Croatia. Credit: Ivan Smiljanic
Storms over Croatia. Credit: Ivan Smiljanic

Two days in a row devastating convective storms were bringing severe winds, large hail and heavy showers, resulting in large damage and casualties in SE Europe.

Last Updated

28 July 2023

Published on

21 July 2023

By Natasa Strelec Mahovic, Ivan Smiljanic and Jochen Kerkmann

In the period with extreme heat over southern parts of Europe, strong upper-level westerly winds, with jet-streak located across Central Europe, cooler air aloft, pushing from north of Europe, creating steep lapse rates, and very humid low layers around the Po Valley, conditions ideal for severe storms development were present over southern parts of the continent through several days mid July.

On 18 July a line of convective storms seen in HRV Clouds RGB loop (Figure 1), caused mostly devastating winds, the wind damages being reported from west Austria all the way to south of Bosnia and Herzegovina, as can be seen from reports from European Severe Weather Database.

Figure 1: MSG SEVIRI HRV Clouds RGB, 18 July 04:30-22:30 UTC

Only hours after this line of storms passed, on 19 July, a convective system developed above northern Italy, and proceeded eastwards with a speeds of more than 80km/h, gaining in strength and developing into a Mesoscale Convective System, while passing over Slovenia. The airmass loop from 18 July 07:00 UTC to 20 July 07:00 UTC, depicts very the unstable situation, with many storms developing in the area (Figure 2).

Figure 2: MSG SEVIRI Airmass RGB, 18 July 07:00 UTC-20 July 07:00 UTC

The storm on 19 July initially hit Slovenia, causing large wind-related damage. Then, when passing over Croatia, became more violent, exhibiting supercell characteristics, leaving a swath of destruction.

In Karlovac, in west Croatia, large hail damaged many roofs, cars, fields. The size of hail was up to 13cm, according to witness reports. The video of the supercell approaching (Figure 3) clearly shows rotation in the supercell, and, even, a wall cloud appearing.

Figure 3: Video of the storm approaching Karlovac on 19 July

Fast movement and the linear organisation of the cells in the MCS was seen in the radar composite (Figure 4).

Figure 4: Radar composite precipitation intensity for 19 July, 14:50-16:35 UTC. Source: DHMZ

The largest wind and rain damages were reported in Zagreb, where the wind speed, measured at Zagreb airport, reached 115km/h, and the amount of precipitation was 20 to 35mm in only 10 minutes. As a result of what was, possibly, the strongest storm on the record in Zagreb, the city was left with fallen trees, flooded areas, and widespread damage, and two people died.

In the evening, while moving further east, the storm caused damage and a casualty in the eastern part of Croatia. The last wind speed measured at the automatic weather station Gradiste, in the easternmost part of Croatia was 50m/s (Source DHMZ), after which the station collapsed from the wind.

Figures 5 and 6 show the development and fast movement of the storm, crossing from northern Italy to Serbia in less than 12 hours. Note new storms developing to the west of the domain, which later caused severe hail in northern Italy.

Figure 5: MSG SEVIRI IR10.8 overlaid on HRV, 19 July 05:00-23:45 UTC
Figure 6: MSG SEVIRI HRV clouds RGB, 19 July 08:00 UTC-20 July 08:00 UTC

The features observed on top of the storm indicated the extreme severity of the system. Overshooting tops, above anvil cirrus plume and gravity waves, were clearly seen in SEVIRI HRV images, and were also visible in the MTG FCI Sandwich product (Figure 7).

MTG FCI IR10.5 overlaid on VIS0.6 image,19 July 2023, 1300 UTC
Figure 7: MTG FCI IR10.5 overlaid on VIS0.6,19 July 13:00 UTC

The storm was well forecast and warned about in Meteoalarm. Automated Severe Weather Guidance by ESSL (Figure 8) shows that a very precise forecast of the event was available a day before, with high probability of large hail and lightning.

Automated Severe Weather Guidance by ESSL from 18 July 2023
Figure 8: Automated Severe Weather Guidance by ESSL, 18 July 12:00 UTC, valid for 19 July 06:00 UTC-20 July 06:00 UTC

When it comes to convection monitoring, visible imagery from satellites are most useful to assess the ‘topography’ of the cloud tops. This is important, as the features that can be spotted through that imagery (eg overshooting tops, above-anvil cirus plumes, or gravity waves) are directly related to a storm severity, and the specific processes that are taking place within convective clouds.

SNPP VIIRS True Colour RGB, 19 July, 12:28 UTC
Figure 9: S-NPP VIIRS True Colour RGB, 19 July 12:28 UTC

Most important feature for assessing the topography of the clouds are, indeed, shadows. Hence, during midday, with high Sun angles, shadows are very faint and cloud-top features are hardly seen (Figure 9). This can be mitigated by choosing combination of visible and near-IR channels (eg looking at Cloud Phase RGB), since the clouds have different transparency in different spectral regions (Figure 10, compared to 9).

SNPP VIIRS Cloud Phase RGB, 19 July, 12:28 UTC
Figure 10: S-NPP VIIRS Cloud Phase RGB, 19 July 12:28 UTC

The other way is by utilising a more slant viewing angle, with geostationary satellite v polar-orbiting imagery (Figure 11, compared to 9).

MTG-I1 FCI VIS0.6, 19 July, 12:10 UTC
Figure 11: MTG-I1 FCI VIS0.6, 19 July 12:10 UTC

In the infrared region, most important cloud-top assessment is related to temperature distribution (Figure 12). Temperatures on top of the clouds are also related to internal processes directly connected to maturity, intensity, and further development of the convection.

S-NPP VIIRS IR11.5 channel, 19 July, 12:28 UTC
Figure 12: S-NPP VIIRS IR11.5 channel (colour-coded, with minimal temperature founded around 191.2 K), 19 July 12:28 UTC

Additional content

Europe weather: Six dead after Balkans hit by powerful storm as heatwave bakes continent (Sky News)
Storm like hurricane blows away everything! Major damage in Zagreb, Croatia (YouTube)