Flash and urban flooding in Rijeka, Croatia on 28 September 2022

Synoptic situation

This section provides an overview of the synoptic situation on 28 September 2022.

On 28 September 2022, western Croatia and surrounding countries were under the influence of a strong south-westerly flow ahead of a deep trough and an upper-level low over central and northern Europe. At the edge of the trough, whose axis was in the north-south direction, there were several short-wave troughs, at least one of which moved over the western part of Croatia. At the same time a surface low was positioned over northern Italy (Figures 4 and 5).

Figure 4: ECMWF geopotential height analysis charts: - AT500 (lines) and temperature (°C) at 500 hPa isobaric surface (colour shades) for 28 (left) and 29 September (right) 2022, at 00:00 UTC.

Figure 5: Surface pressure charts and frontal analysis, 28 September 00:00 UTC (source: DWD).

A large amount of moist and unstable air arrived in south-westerly flow from the Mediterranean, where the conditions for the formation of deep moist convection (DMC) were met. The necessary conditions for DMC are moisture in the lower layers of the atmosphere, instability of the air (with cooler air advected at higher levels, creating sufficiently pronounced lapse rates), and a mechanism for raising moist air to a height (LFC, level of free convection) from which it is possible to use all the available convective potential energy (CAPE).

Flash and urban flooding was the result of quasi-stationary convective storms that occurred in the evening hours of 28 September in Istria and around the city of Rijeka.

It started raining in the wider Rijeka area late in the afternoon, and shortly before 18:00 local time the convective processes began to intensify. The first lightning was registered around 18:30 local time.

Figure 6: Lightning during the period from 28 September 2022 at 15:00 local time to 29 September 2022 at 04:00 local time. It is possible to follow the path of cells from the southwest to the northeast of Croatia. Source: nowcast GmbH.

Then, around 20:00 local time, more pronounced thunderclouds began to form along the western coast of Istria and move through the interior of Istria in the south-westerly flow.

Figure 7: Meteosat-11 satellite image in the infrared part of the spectrum (IR 10.8 µm). Colours indicate temperature (from 200 K (dark red) to 240 K (blue)), on 28 September 2022, 19:15 UTC (21:15 h local time). Circled in red are convective cells over western Croatia. (Source: EUMETSAT).

In addition to the continuous formation of convection at the same location, the orography also contributed to the strengthening of the processes and the intensity of the precipitation, demonstrated by radar reflections often intensifying with air passing over Učka mountain and rising towards Gorski Kotar. Clouds bringing rain and lightning continued to form until after midnight (Figure 7).

Figure 8: Composite radar image from radars at Goli (Istria, Croatia) and Pasja Raven (Slovenia) showing the radar reflectivity in dBz, on 28 September 2022 at 19:15 UTC.

Figure 9: Hourly precipitation accumulation measured by radar on 28 September at 21:00 and 22:00 UTC.

The analysis of the radiosonde measurement (Figure 10) at the Italian station Rivolto, which was representative for this case (depicting characteristics of the air mass that was advected over the area of the northern Adriatic in the south-westerly flow), showed that the important processes took place in the warm part of the troposphere and that saturation (high humidity) occurred within a thick layer of the atmosphere. These conditions were necessary for the formation of a large amount of precipitable water, with a value of 27 mm observed, along with a relative humidity greater than 80%. There was also a cyclonic curvature of the flow in the lower layers of the troposphere, although a low jet stream, which would also favour abundant precipitation, was not recorded at Rivolto at that moment.

Figure 10: Radiosonde measurement (thermodiagram) from Rivolto (Italy), 28 September 2022 at 18:00 UTC (20:00 local time).