Beyond blocking anticyclones: Iberian cut-off lows amplified early season heat in UK and France 2026

In May 2026, France and the United Kingdom experienced a record-breaking hot event with daily maximum temperature anomalies exceeding 15 °C. Here, we show that the unprecedented high-pressure system over the region was insufficient to explain the full magnitude of near-surface temperatures in Western France and Southern UK. Using perturbed large ensemble medium-range forecasts from the IFS and AIFS models, we estimate that Iberian cut-off low activity explains 30 % of the additional temperature variance during the heatwave peak, conditional on the presence of the anticyclone. This contributed a local surplus of up to 4.5 °C to daily mean surface temperatures compared to ensemble members that achieved similar 500 hPa geopotential height records. Across similar past synoptic events, we find a comparable impact of cut-off low activity, localized over the French Atlantic coast and southern UK. At the mid-tropospheric level, cut-off low activity drives advective warming from a Saharan branch. In a second stage, the arriving end of this branch is coupled to the boundary layer by increasing subsidence near 850 hPa. While this large-scale circulation pattern is highly unusual for May — being typically associated with late-summer warm spells — the Saharan branch reached an unprecedented southward extent during this event. Furthermore, while no dynamical trend was found for the month of May, June has shown a positive trend in Iberian cut-off low activity since 1985, characterized by a doubling in the occurrence of exceptionally intense cut-off lows. Our findings underscore the importance of cut-off low dynamics in driving extreme early-season heatwaves, a factor that is rarely quantified in previous analyses.