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.
28 July 2023
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.
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).
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.
Fast movement and the linear organisation of the cells in the MCS was seen in the radar composite (Figure 4).
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.
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.
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).
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.
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.
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).
The other way is by utilising a more slant viewing angle, with geostationary satellite v polar-orbiting imagery (Figure 11, compared to 9).
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.