In this paper we analyse the probabilistic behaviour of heatwaves (HWs) in the main coastal cities of the Basque Country (Bayonne, Bilbao and Donostia-San Sebastián) in the twentyfirst century. We estimate HW behaviour using data from eight climate circulation models under two representative concentration pathways (RCP 8.5 and RCP 4.5). We model HWs according to three factors: number per annum, duration and intensity, including correlations, and find very different results for each climate model. This highlights the problem of using a single model. Under RCP 8.5, we find an expected mean excess over the 30ºC temperature threshold of 4.19ºC for Bayonne, 4.05ºC for Bilbao and 4.14ºC for Donostia-San Sebastián in 2100. These expected values are based on incomplete information, so we also calculate several risk measures.