2005 Azores subtropical storm

The 2005 Azores subtropical storm was the 19th nameable storm and only subtropical storm of the extremely active 2005 Atlantic hurricane season. It was not named by the National Hurricane Center as it was operationally classified as an extratropical low. It developed in the eastern Atlantic Ocean, an unusual region for late-season tropical cyclogenesis. Nonetheless, the system was able to generate a well-defined centre convecting around a warm core on 4 October. The system was short-lived, crossing over the Azores later on 4 October before becoming extratropical again on 5 October. No damages or fatalities were reported during that time. Its remnants were soon absorbed into a cold front. That system went on to become Hurricane Vince, which affected the Iberian Peninsula.

The subtropical nature of this unnamed system was determined several months after the fact, while the National Hurricane Center was performing its annual review of the season. Upon reclassification, the storm was entered into HURDAT, the official hurricane database.

Meteorological history
The system originated out of an upper-level low just west of the Canary Islands on 28 September. The low organized itself over the next days, producing several bursts of convection. While remaining non-tropical with a cold core it moved gradually west to northwest. On 3 October, it became a broad surface low about 400 nmi southwest of São Miguel Island in the Azores. Early on 4 October, 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 85 km/h as it approached the Azores that evening. After tracking through the area, the storm weakened slightly as it moved to the north-northeast. Through an interaction with the cold front early on 5 October, 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 8 October.

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 one. The first finding was the cloud pattern, which had deep convection around the centre and was better organized with a well-defined centre 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 an 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 only subtropical.

Impact, classification and records
Tropical storm-force winds were reported across parts of the Azores, primarily on the eastern islands. The strongest winds were reported on Santa Maria Island, where 10-minute sustained winds reached 79 km/h with gusts to 94 km/h. Ponta Delgada faced 61 km/h winds, with the peak recorded gust being 85 km/h. No damage or fatalities were reported.

The 2005 Azores storm was not classified as a subtropical storm until April 2006, after a reassessment by the National Hurricane Center. Had it been operationally classified as such, it would have been named Tammy. Every year, the National Hurricane Center (NHC) re-analyzes the systems of the past hurricane season and revises the storm history if there is new data that was operationally unavailable. This reanalysis revealed that the storm became a subtropical storm on 4 October, making it the earliest forming 19th Atlantic tropical or subtropical storm on record. The previous record holder was an unnamed 1933 tropical storm that developed on 26 October. It held this distinction until 2020, when Hurricane Teddy attained tropical storm strength on 14 September.