Urban flash flood in Zagreb

By Natasa Strelec Mahovic (EUMETSAT), Tanja Renko, Izidor Pelajic and Tatjana Vujnovic (DHMZ)

In a short period of just a few hours on the evening of 24 July 2020, 60 to 90mm of rain fell in the city of Zagreb, causing severe urban flooding. At Zagreb-Gric meteorological station, located in the very centre of the city, two-hourly precipitation of 77mm was recorded. The return period for that kind of intensity at the station is more than 400 years.

Flooding caused damages at many facilities, including the children's hospital, already previously badly damaged in the catastrophic earthquake in March, and one firefighter died.

The situation was well forecast by various NWP models and, using satellite, lightning and radar data, forecasters from Croatian Meteorological and Hydrological Service (DHMZ) were able to issue timely warnings for thunderstorms, rain and flash floods for north-western parts of the country (Figure 1). Hydrological warnings were also issued, since hydrological models forecast high probability of flash flooding.

Synoptic situation

The infrared images in Figure 2 follow the severe convective storms that developed on 23 July and overnight into 24 July, first over north Italy where they brought large hail and strong winds in many places.

On 24 July storms started to develop on the northern Adriatic coast, with hail recorded over Istria peninsula . Later very severe storm development, triggered by the Dinaric Alps, caused heavy rainfall, hail and severe lightning over Bosnia and Herzegovia, with flash flooding and severe wind damage in the city of Tuzla. The severity of the storms was indicated by many overshooting tops, as well as cold-ring and cold-U/V features occurring on top of the storms.

Synoptic forcing was provided by an upper-level trough passing over the Alps on the afternoon of 24 July. The trough moved eastwards, as can be seen in the animation in Figure 3.

Looking at the wind at 700hPa one can notice that the wind field suggests a presence of a small-scale cyclone, moving ahead of the trough. Notice also a small short-wave trough forming at 500hPa and passing over north west Croatia, which, after aligned with the 700hPa cyclone, gave an additional synoptic 'push' for the storm development.

The ingredients for deep moist convection, moisture, instability, lifting mechanism and wind shear, were all in place and the setup was supportive for the storm development. Moderate to high values of CAPE were present all over the area, mostly with very little or negligible convective inhibition, as seen in the Sqew-T log-P diagram of Zagreb radiosounding at 12:00 UTC (Figure 4).

Lift was very well supported by the approaching trough, with topography of the Alps and the Dinaric mountains playing an important role. Due to a southwesterly flow of moist and very unstable air towards the Dinaric Alps, divergence in the upper levels and significant cyclonic vorticity advection, vertical motion was quite strong, helping the storms to consume available energy. Deep layer shear (between 10 and 20m/s) helped the storms organise, especially during the afternoon over Bosnia and Herzegovina, and later on over continental Croatia.

Storm over Zagreb

The initiation of the storm over Zagreb happened quite late in the day, when the forecasters thought the warning level they issued might have been too high and the precipitation signal overestimated. This delay in the development may have been caused by the anvils of the storms over the north Adriatic, which cut the insolation and slowed the initiation. However, with the arrival of the trough new cells started to develop behind a more intensive and damaging storm system over northwest Croatia. Comparison of the satellite and radar depiction of the storm can be seen in Figure 5.