2005 Azores Subtropical Storm - Meteorological History

Meteorological History

The system originated out of an upper-level low just west of the Canary Islands on September 28. The low organized itself over the next several days, producing several bursts of convection. While remaining non-tropical with a cold core it moved gradually west to northwest. On October 3, it became a broad surface low about 400 nautical miles (460 mi, 740 km) southwest of São Miguel Island in the Azores. Early on October 4, convection increased as the surface low organized itself, and the system became a subtropical depression. Around the same time, the depression turned northeast into a warm sector ahead of an oncoming cold front and strengthened into a subtropical storm. The system continued to track northeast and strengthened slightly, reaching its peak intensity of 50 mph (85 km/h) as it approached the Azores that evening. After tracking through the Azores, the storm weakened slightly as it moved to the north-northeast. Through an interaction with the cold front early on October 5 the subtropical storm became extratropical. The system was fully absorbed by the front later that day. The newly absorbed system would separate from the dissolving frontal system and become Subtropical Storm Vince on October 8.

At the time, the system was not believed to have been subtropical. However, there were several post-season findings that confirmed that the system was indeed a subtropical storm. The first was the cloud pattern, in which it had deep convection around the center and was better organized with a well-defined center of circulation. In addition, the system had a warm core more typical of tropical cyclones as opposed to the cold core of extratropical cyclones. The warm-core nature also meant that there were no warm or cold fronts attached to the system, as temperatures did not change ahead of and behind the system until the unrelated cold front passed the Azores. Satellite imagery suggested that the system was briefly a tropical storm as the warm core was found; however, the widespread wind field and the presence of an upper-level trough confirmed that it was merely subtropical.