Could the 2018 Amsterdam heatwave have been more extreme? A climate risk storyline of plausible extreme heat

The Royal Netherlands Meteorological Institute (KNMI) recently published nine different climate risk storylines to prepare for climate hazards in the current and near future climate. This study specifically zooms in on the climate risk storyline for a heatwave in Amsterdam. The summer of 2018 was exceptional, leading to the first code Orange for extreme heat. In this study we investigate whether, and how, the heatwave of 2018 could plausibly have evolved into a more extreme event. Using ensemble forecasts from ECMWF, we identify an alternative but physically consistent meteorological evolution in which the cooling front of late July 2018 did not reach the Netherlands. This alternative scenario, termed ‘Heatwave XL’, is dynamically downscaled using the regional climate model RACMO, with corrections for model bias. Urban heat island diagnostics are applied to derive spatially explicit heat exposure across Amsterdam. Sectoral impact knowledge from impact partners is then integrated to assess potential societal impacts and cascading effects. The Heatwave XL storyline results in several additional days of extreme daytime temperatures exceeding 35 °C, combined with persistently hot nights, likely exacerbating societal impacts already seen in 2018. This case demonstrates the value of storyline approaches for stress-testing preparedness and supporting anticipatory decision-making under uncertainty in a warming climate.