A study of a damaging hailstorm episode in the Basque Country: The Vitoria-Gasteiz case of July 6, 2023

In this work, we present a study of a severe weather event that occurred on July 6, 2023, in the Basque Country, particularly affecting its capital (Vitoria-Gasteiz), where it caused numerous incidents due to hail, including material damage and even injuries from hailstones impacts. A synoptic, mesoscale, and local analysis of this hailstorm event is conducted, utilizing numerical models at different scales and various observations sources, including data from Automatic Weather Stations, disdrometers, and notably radar data available in the area. The ultimate goal of this study is to contribute to a deeper understanding of deep convection and associated hazards in our region.

During this episode, the overall situation is characterized by relatively low thermal pressures over the Iberian Peninsula, with southwest winds aloft and a slight trough located to the west. Instability indices indicate a significant level of instability, with lifted index (LI) values below -3 ºC, a CAPE exceeding 600 J/kg, and a TTI around 50 ºC. Within this unstable synoptic environment, highly active convective cores or cells develop during the afternoon, moving from southwest to northeast. However, at certain moments and with certain cells, there is a pronounced west-east displacement, as seen in the particular case of the storm affecting Vitoria-Gasteiz. This storm enters Araba territory from the west and moves from west to east until reaching Vitoria-Gasteiz, maintaining its activity with radar reflectivities of 55-60 dBz, occasionally reaching 60-65 dBz. Subsequently, after passing Vitoria-Gasteiz, the convective system loses activity and shifts northeastward.

During the event intense showers accompanied by large hail, up to 4-5 cm in diameter are produced. Precipitation intensities recorded at various network stations range from 20 mm/h to 30 mm/h. This high impact episode causes numerous incidents and extensive damage in Vitoria-Gasteiz, including 11 injuries with head lacerations, damage to thousands of cars, street and road collapses, sewer and factory damage, flooding in commercial premises and garages, etc. Crop damage are also reported in other areas of Araba.