On the evening of Saturday, June 29, 2024, severe thunderstorms impacted Luxembourg, bringing torrential rainfall, very strong wind gusts, and high lightning activity. ACA (The voice of the Insurance and Reinsurance industry) stated that Luxembourg insurers counted €24.5 million in damage claims.
MeteoLux issued a yellow warning for thunderstorms on Friday for Saturday evening, which was upgraded to orange at around 17:25 CEST on Saturday, for the period from 20:00 to 24:00 CEST. This article provides a brief analysis of the weather conditions and the damaging thunderstorm system that affected the Grand Duchy.
Synoptic weather situation
The overall weather situation was defined by an upper-level trough over France moving northward, generating strong large-scale forcing for ascent on its downstream side. This interacted with a volatile air mass over northeastern France and large parts of Germany (Fig. 1, left). At the surface level, a pressure minimum developed over northeastern France (Fig. 1, right), becoming the main area for convective initiation due to increasing low-level convergence.
Fig. 1 : (left) 500 hPa geopotential (m2/s2, black lines) and most unstable CAPE (J/kg) on 29 June 2024 at 18:00 UTC (20:00 CEST) simulated by the ECMWF model initialised on 29 June 2024 at 12:00 UTC. (right) Analysis of the mean sea level pressure (hPa, black lines) and surface fronts for 29 June 2024 at 18:00 UTC (20:00 CEST). Source : Deutscher Wetterdienst.
Regarding the vertical profile of the atmosphere (Fig. 2), sounding measurements taken at Idar-Oberstein around 20:00 CEST indicated strong, deep latent instability (CAPE > 2000 J/kg with slight inhibition) and a significant increase in wind speed (speed shear) between near-surface levels and the low to mid troposphere (around 15 m/s between the ground and 3 km height). These factors suggest the potential for linearly organized or bowing thunderstorm systems. The relatively dry air below 3 km (humidity values between 40% and 60%), combined with high CAPE values within the hail-growth layer (HGL) and a high freezing level (around 4 km altitude), increased the threat of severe wind gusts associated with organized thunderstorms. Additionally, excessive rainfall was a possible threat due to the abundant moisture, with precipitable water around 40 mm.
Fig. 2 : Skew T-log p diagram of a sounding from Idar-Oberstein (WMO 10618). The red (green) curve represents the temperature (dew point), and the yellow curve represents the ascent trajectory of the most unstable air parcel. The most-unstable convective available potential energy (MUCAPE) and downdraft CAPE (DCAPE) are indicated by the area shaded in transparent yellow and transparent blue, respectively. The lifted condensation level (LCL) marks the expected cloud base of a convective storm. The wind barbs are displayed in knots. Source : http://rawinsonde.com/thunder_app/
Evolution of the thunderstorm system
Between 17:00 and 18:00 CEST, widespread thunderstorms began to develop north of Dijon. Satellite imagery revealed numerous overshooting tops surrounded by gravity waves (Fig. 3, left), indicating vigorous updrafts within the storm cells. The rapid development of these cells led to the formation of an extensive cloud shield over the following hours. Overshooting tops remained visible in the imagery, underscoring the severity and persistence of the ongoing deep moist convection (Fig. 3, right).
Fig. 3 : High-resolution satellite image taken by Meteosat Second Generation on 29 June 2024 at 18:30 CEST (left) and at 21:00 CEST (right). Source : EUMETSAT.
Due to the widespread initiation of cells, storm clustering occurred quickly as they moved northward toward Luxembourg. Between 21:00 and 21:45 CEST, a more defined linear storm structure began to form between Verdun and Metz (Fig. 4, left). A few isolated cells developed downstream of this structure, some merging with the approaching bowing convective system near the French-Luxembourg border. By 22:00 CEST, the thunderstorm system reached southern Luxembourg, exhibiting a bow echo structure on radar imagery with maximum reflectivity values around 55 dBZ, indicating heavy rainfall (Fig. 4, right). The storm moved northeast towards Germany, impacting much of the country between 22:00 and 23:00 CEST.
Fig. 4 : Reflectivity (dBZ) measured on 29 June 2024 at 19:30 UTC (21:30 CEST) (left) and at 20:00 UTC (22:00 CEST) (right) by a weather radar located in Neuheilenbach (Germany) with an elevation angle of 0.8°. Data source : Deutscher Wetterdienst.
Excessive rainfall
The highest precipitation quantities were recorded in the southwestern and western part of the country. Some weather stations located in the southwestern and western part the country measured a precipitation amount of more than 30 l/m2 between 22:00 CEST and 23:00 CEST :
- 54.6 l/m2 Koerich (AGE)
- 44.3 l/m2 Reichlange (AGE)
- 43.3 l/m2 Differdange (ASTA)
- 42.9 l/m2 Clemency (ASTA)
- 33.5 l/m2 Schouweiler (ASTA)
At the Airport Luxembourg-Findel, 13.5 l/m2 were measured within 10 minutes during the passage of the thunderstorm.
Damaging wind gusts
The bowing thunderstorm system produced widespread strong wind gusts, driven by a rear inflow jet that descended at the leading edge of the bow echo. Some areas, including Biwer/Wecker, Mersch, Bissen, and Feulen, experienced more severe damage coinciding with the formation of mesovortices. These vortices formed due to rear inflow surges, which on the eastern flank of a vortex (Fig. 5, left), accelerated the rear inflow jet to the surface due to pressure perturbations. The damage patterns in these areas exhibited no clear evidence of tornadic activity since the available images of uprooted or broken trees show a divergent signature. Moreover, the overlap of damage areas with the eastern flanks of the vortices (Fig. 5, right) and the absence of narrow and long-lived velocity couplets in radar imagery suggest that mesovortex-induced straight-line wind events, likely exceeding 100 km/h, were the most plausible cause. For now, all the damages that occurred have been classified as non-tornadic severe wind events by the European Severe Storms Laboratory (ESSL) in the official European Severe Weather Database (ESWD). Some stations across the country recorded wind gusts above 85 km/h between 22:00 and 23:00 CEST :
- 99 km/h Echternach (ASTA)
- 94 km/h Luxembourg-Pescatore (Kachelmannwetter)
- 93 km/h Bettembourg (Kachelmannwetter)
- 86 km/h Useldange (ASTA)
Fig. 5 : Radial velocity (m/s) measured on 29 June 2024 at 20:18 UTC (22:18 CEST) and at 20:33 UTC (22:33 CEST) by a weather radar located in Neuheilenbach (Germany) with an elevation angle of 0.5°. The mesovortices are denoted by white circles. Data source : Deutscher Wetterdienst.
