Abstract
<jats:p>Abstract. This study investigates how increasing greenhouse gas concentrations may affect environmental conditions favourable for thunderstorms over Fennoscandia. The future scenarios are based on data produced with the HARMONIE-Climate convection-permitting regional climate model forced by two global climate models at the lateral boundaries. The distribution of thunderstorm-favourable days, defined as days with co-occurring conditional instability and precipitation, in model data compares well with observed thunder days in May–September 2002–2018 across the study domain. By 2081–2100, the area-average frequency of favourable days is projected to increase by approximately 40–185 % in Finland, 30–185 % in Sweden, and 15–210 % in Norway relative to 1986–2005 under the RCP4.5 and RCP8.5 emission scenarios. Projected increases are largest over the northern Fennoscandia and in simulations exhibiting stronger warming. Thunderstorm-favourable conditions occurring simultaneously with strong vertical wind shear are also simulated to become more common, suggesting more frequent potential for severe organized convection in a future climate. More research is needed to understand how the changes in general storm-favourable environmental conditions may affect actual hazard occurrence. The results support the use of the HARMONIE-Climate model for convective storm scenario studies and offer valuable context for climate change adaptation in Fennoscandia.</jats:p>