A damaging supercell hailstorm on 6 July 2023 in Vitoria-Gasteiz, the Basque Country

Abstract. We analyze a severe supercell that affected Vitoria-Gasteiz (Basque Country, northern Spain) on 6 July 2023, producing 4–5 cm hail and rainfall rates close to 30 mm h−1. The main objective of this work is to improve the forecasting and early-warning capabilities for similar high-impact storms in the Basque Country. This region lies on the northern edge of one of the most active supercell areas of the Iberian Peninsula, but remains relatively understudied. The synoptic and convective environment is examined using GFS analyses and the 1 km operational WRF configuration of Euskalmet, while the storm evolution and surface impacts are characterized through radar, rain gauge and disdrometer data. Environmental parameters were substantially above the median of Spanish very-large-hail supercells, with surface-based CAPE near 2000 J kg−1, 0–6 km bulk shear of 20–25 m s−1 and 0–3 km storm-relative helicity slightly above 200 m2 s−2. The storm originated from a splitting cell and rapidly intensified as a right-moving supercell, reaching cloud-top heights of 14–15 km and reflectivity values of 60–65 dBZ. This case also shows that convective initiation and storm splitting west of Basque Country must be monitored, as right-moving cells from these areas can affect the Basque Country when the vertical wind profile shows cyclonic curvature.