Final Phase: 15 - 18 July 2021
On 15 and 16 July 2021, the ULL Bernd moves slowly south and crosses the Alps (see Figure 9). During this period the precipitation that took place in the Alpine region was mainly caused by the barrage effect: peaks in precipitation were caused by convection.
Figure 9: Airmass RGB loop from 15 July at 12:00 UTC to 16 July 2021 at 18:00 UTC. Radar reflectivity and geopotential height (cyan) at 500 hPa.
On the morning of 17 July 2021, the center of the ULL was located over the Adriatic Sea. Warm and moist air is conducted around the eastern Alps and then lifted again at the alpine main ridge when arriving from the northeast. Embedded thunderstorms north of the alpine ridge increased precipitation locally, while the barrage effect causes continuous rainfall.
The Bernd system finally moves further south on 18 July so that Central Europe is not under its influence anymore. A weak cold front arriving from the northwest (see Figure 10) activates convection over the alpine region once again.
Figure 10: Airmass RGB from 18 July 2021 at 06:00 UTC. Radar reflectivity and geopotential height (cyan) at 500 hPa.