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.

Azores subtropical storm
View of the storm from space on 4 October 2005. Though located over the open Atlantic Ocean and Middle East, the Azores are visible on the northern side of the image.
The storm at peak intensity near the Azores on 4 October
Meteorological history
Formed4 October 2005
Dissipated5 October 2005
Subtropical storm
1-minute sustained (SSHWS/NWS)
Highest winds50 mph (85 km/h)
Lowest pressure997 mbar (hPa); 29.44 inHg
Overall effects
FatalitiesNone reported
Areas affectedAzores
IBTrACS
Part of the 2005 Atlantic hurricane season

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
Map plotting the storm's track and intensity, according to the Saffir–Simpson scale
Map key
Saffir–Simpson scale
  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)
  Unknown
Storm type
triangle Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression

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 nautical miles (740 kilometres; 460 miles) southwest of São Miguel Island in the Azores.[1] Early on 4 October, convection increased as the surface low organized itself, and the system became a subtropical depression.[2]

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 (53 mph) 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.[2] The newly absorbed system would separate from the dissolving frontal system and become Subtropical Storm Vince on 8 October.[3]

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,[4] until an unrelated cold front passed the Azores.[5] 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.[2]

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 (49 mph) with gusts to 94 km/h (58 mph).[6] Ponta Delgada faced 61 km/h (38 mph) winds, with the peak recorded gust being 85 km/h (53 mph).[1] No damage or fatalities were reported.[2]

The 2005 Azores storm was not classified as a subtropical storm until April 2006, after a reassessment by the National Hurricane Center.[7] Had it been operationally classified as such, it would have been named Tammy.[8] 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.[2] 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.[9][10] The previous record holder was an unnamed 1933 tropical storm that developed on 26 October.[9] It held this distinction until 2020, when Hurricane Teddy attained tropical storm strength on 14 September.[11][12]

See also

References
  1. Beven II, John L.; Lixion A. Avila; Eric S. Blake; Daniel P. Brown; James L. Franklin; Richard D. Knabb; Richard J. Pasch; Famie R. Rhome & Stacy R. Stewart (March 2008). "Annual Summary: Atlantic Hurricane Season of 2005" (PDF). Monthly Weather Review. 136 (3): 1131–1141. Bibcode:2008MWRv..136.1109B. doi:10.1175/2007MWR2074.1. Archived from the original (PDF) on 10 September 2008. Retrieved 8 September 2008.
  2. "Tropical Cyclone Report: Unnamed Subtropical Storm" (PDF). National Hurricane Center; National Oceanic and Atmospheric Administration. 10 April 2006. Retrieved 31 October 2006.
  3. "Tropical Cyclone Report: Hurricane Vince" (PDF). National Hurricane Center; National Oceanic and Atmospheric Administration. 22 February 2006. Archived (PDF) from the original on 24 May 2006. Retrieved 4 May 2006.
  4. "History for Santa Maria, Azores: Week of October 2, 2005 through October 8, 2005". Weather Underground. 8 October 2005. Archived from the original on 5 October 2017. Retrieved 8 August 2008.
  5. "History for Santa Maria, Azores: Thursday, October 6, 2005". Weather Underground. 6 October 2005. Archived from the original on 17 August 2016. Retrieved 8 August 2008.
  6. "History for Santa Maria, Azores: Tuesday, October 4, 2005". Weather Underground. 4 October 2005. Archived from the original on 5 October 2017. Retrieved 7 August 2008.
  7. "Tropical Weather Summary - 2005 Web Final". National Hurricane Center; Central Pacific Hurricane Center. Archived from the original on 16 October 2020. Retrieved 23 October 2020.
  8. Masters, Jeff (14 April 2006). "Should-have-been-Tammy | Category 6". Weather Underground. Archived from the original on 26 October 2020. Retrieved 23 October 2020.
  9. "Atlantic hurricane best track ("HURDAT")". National Oceanic and Atmospheric Administration; NHC Hurricane Research Division. 1 January 2008. Archived from the original on 16 September 2008. Retrieved 10 August 2008.
  10. "Atlantic storm could become Epsilon; possible Zeta forming in southwest Caribbean". Avoyellestoday.com. Avoyelles Publishing Company. 19 October 2020. Archived from the original on 22 October 2020. Retrieved 23 October 2020.
  11. Stewart, Stacy (14 September 2020). "Tropical Storm Teddy Discussion Number 7". Miami, Florida: National Hurricane Center. Archived from the original on 16 September 2020. Retrieved 14 September 2020.
  12. Marchante, Michelle; Harris, Alex (14 September 2020). "With newly formed Tropical Storm Teddy, NHC tracking five named systems at once". The Miami Herald. Chatham Asset Management. Archived from the original on 14 September 2020. Retrieved 14 September 2020.
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