An unprecedented Mediterranean tropical cyclogenesis attempt above extreme sea surface temperatures?

On 3-4 October 2026, a strong and organized mesoscale convective system (MCS) developed over the Balearic Sea in the Western Mediterranean, and produced torrential rainfall in the Barcelona area, with up to 248 mm at Granollers (230 mm in 8 hours; 65 mm in an hour). The event occurred without discernible large-scale forcing from an upper-level cut-off low or trough, while western Mediterranean Sea surface temperatures (SSTs) were about 2.5 °C above the 1991–2020 mean. Satellite imagery showed a compact, quasi-circular cloud shield resembling a tropical disturbance, and Doppler radar showed low-level winds compatible with a cyclonic circulation near the coast. We assess how the MCS and its rotation depended on the SST anomaly with five 1-km convection-permitting HARMONIE-AROME simulations with SST taken respectively from the operational IFS analysis, the ERA5 1991–2020 climatology, and subtracting to the IFS analysis 2.4, 2.6 and 2.9 °C. With the observed SST, the model reproduces an MCS similar to the observed one, with a coherent column of cyclonic vorticity extending from 500 to 925 hPa and accumulated precipitation exceeding 250 mm offshore. With any of the cooler SSTs, neither the MCS nor the vortex forms, and the precipitation in the Barcelona surroundings nearly vanishes. These results are consistent with the hypothesis that tropical cyclogenesis occurred, and suggest that Mediterranean warming may be opening a new cyclone formation pathway associated with slight to no upper-level forcing that has not been documented in the region before.