Subtropical Storm Stephanie was a unique warm-core depression that formed over the Bay of Biscay, which produced sustained wind-speeds equivalent to those of a Tropical storm, according to the private weather office Keraunos. The storm initially formed as a low connected to a trough, and on September 14 separated from the trough. It developed a warm core and was officially given the name "Stephanie" by the Free University of Berlin on September 15. At 1700Z, September 15, several sources began making mention of it. A few hours later, the storm further intensified, peaking at 50 mph, just as it made landfall on the Basque coast.