Variability conceals emerging trend in 100yr projections of UK local hourly rainfall extremes

Extreme precipitation is projected to intensify with warming, but how this will manifest locally through time is uncertain. Here, we exploit an ensemble of convection-permitting transient simulations to examine the emerging signal in local hourly rainfall extremes over 100-years. We show rainfall events in the UK exceeding 20 mm/h that can cause flash floods are 4-times as frequent by 2070s under high emissions; in contrast, a coarser resolution regional model shows only a 2.6x increase. With every degree of regional warming, the intensity of extreme downpours increases by 5-15%. Regional records of local hourly rainfall occur 40% more often than in the absence of warming. However, these changes are not realised as a smooth trend. Instead, as a result of internal variability, extreme years with record-breaking events may be followed by multiple decades with no new local rainfall records. The tendency for extreme years to cluster poses key challenges for communities trying to adapt. Climate projections at km-scale show that local hourly precipitation extremes in the UK become 4-times more frequent by 2070, while they do not intensify gradually with warming, but tend to cluster in time.