Despite that fact that the storm occurred entirely over the sea, various types of in-situ and remote sensing observations are available to document the most interesting part of the evolution from the end of the occlusion until dissipation. A cloud-free central area was observed in satellite imagery and matched a ring-shaped pattern of rain in ground-based radar reflectivity. Reaching an intensity equivalent to a tropical storm, an axially symmetric wind field was observed from a space-borne scatterometer. The time series of pressure and wind values recorded by the buoy “Gascogne”, over which the storm center passed, is reminiscent of a tropical cyclone with a distinct, small-scale inner-core structure. Cyclone phase space diagrams (Hart, 2003), adapted to the specific characteristics of this storm, clearly corroborate the transition from a frontal cold-core to a symmetric warm-core system, often referred to as “tropical transition” (TT). To the best of our knowledge, this is the first-ever documented case of this kind over the Bay of Biscay. The ECMWF deterministic forecast predicted well the track, intensity, and TT of the storm four days ahead.
We speculated that the tropical transition of Stephanie was fostered by anomalously high sea surface temperatures (SSTs) present in September 2016 over the Bay of Biscay. While these high SSTs are likely related to the prevalence of anticyclonic weather in the weeks preceding the storm, average SSTs over the Gulf of Biscay increased by about 1°C in the last 35 years and weather systems of this kind may become more frequent in a future warmer climate.