Unnamed Subtropical Storm (2013)

Storm Active: December 4-7

*This subtropical storm received no name despite being counted among the total of storms in the 2013 season. This is because though the low pressure system was monitored for development during its period of activity, analysis at the time did not indicate that it had become a subtropical cyclone. In February 2014, this storm was added to the 2013 season during the usual postseason analysis.

This system originated from an extratropical low pressure system formed which formed over the eastern Atlantic on December 3. Though the disturbance was already producing strong winds, there was not significant convection associated with it at the time. After completing a small counterclockwise loop well south of the Azores Islands, the low began to move northward and thunderstorm activity became more concentrated about the center of circulation. It was late in the evening on December 4 when the low was estimated to have developed into a subtropical storm. The cyclone had its peak intensity as a subtropical storm immediately after formation, with wind speeds of 50 mph and a central pressure of 997 mb.

Initially, the cyclone had a nested convective structure in that it had an area of central convection surrounded by a void of dry air around which there were a few outer bands. After persisting for about a day, this structure resolved into more traditional banding on December 6. By this time, the storm had begun to accelerate north-northwestward, approaching the Azores from the south. Wind shear also increased out of the west, and the subtropical storm began to weaken as it moved into cooler water. The unnamed storm lost subtropical characteristics very early on December 7, and dissipated shortly thereafter, bringing only showers and gusty winds to the Azores.

The above image illustrates the nested convective structure of the unnamed storm shortly after its formation.

The unnamed subtropical storm existed only in the far eastern Atlantic. It was one of only 15 December tropical cyclones in the Atlantic on record through 2013 and the first since 2007.