Tropical Storm Alpha

[Adriana Gowen]

SubtropicalStorm Alpha was the first subtropical or tropical cyclone ever observed to make landfall in mainland Portugal. The twenty-second tropical or subtropical cyclone and twenty-first named storm of the extremely active and record-breaking 2020 Atlantic hurricane season, Alpha originated from a large non-tropical low that was first monitored by the National Hurricane Center on 15 September. Initially not anticipated to transition into a tropical cyclone, the low gradually tracked south-southeastward for several days with little development. By early on 17 September, the low had separated from its frontal features and exhibited sufficient organization to be classified as a subtropical cyclone, as it approached the Iberian Peninsula, becoming a subtropical storm around that time. Alpha then made landfall just south of Figueira da Foz, Portugal during the evening of 18 September, then rapidly weakened as it moved over the mountainous terrain of Northeastern Portugal. The system degenerated into a remnant low on 19 September, when it was last noted.

Alpha originated from a large, extratropical low-pressure area, which developed over the Northeastern Atlantic Ocean on 14 September.[1] As a strong upper-level trough dug southeastward and became a cut-off low about 500 nmi (580 mi; 930 km) north of the Azores, the interaction between the low and a surface front promoted the formation of a strong frontal low, which rapidly deepened and reached its extratropical peak that day, with maximum 1-minute sustained winds as high as 112 km/h (70 mph) and a minimum central atmospheric pressure of 992 mbar (29.3 inHg).[1] By this time, the extratropical cyclone had a very large radius of gale-force winds expanding over 500 km (270 nmi; 310 mi) from its center of circulation.[1] The low was initially very slow-moving, but began to dip southeastward and weaken by 15 September when the National Hurricane Center (NHC) first began to monitor the system for possible development into a tropical or subtropical cyclone.[1][5] This was because the system was expected to track close to a region of unusually-warm sea surface temperatures to the west of Portugal of around 22 C (72 F), though these temperatures would still typically be too cold to support tropical cyclogenesis.[1]

The size of the low's wind field continued gradually decreasing on 16 September, as some of its frontal features gradually became less defined, although the NHC only highlighted a low (20%) chance of development at this time, operationally.[1][6] Nonetheless, convection, or thunderstorm activity, became more concentrated and organized near the center of the low, and a newly formed central low soon became the dominant feature within the larger extratropical system.[7] In post-season analysis, the NHC estimated that Alpha had developed as a subtropical storm at 06:00 UTC on 17 September, as the thunderstorm activity associated with the smaller low feature became well-organized.[1] Alpha accelerated to the northeast, and a combination of radar imagery from Portugal, scatterometer passes, and satellite-derived wind data revealed Subtropical Storm Alpha had peaked around 00:00 UTC on 18 September, just about 417 km (225 nmi; 259 mi) off the coast of Portugal, while the storm was producing 1-minute sustained winds up to 80 km/h (50 mph).[1][8]

Alpha maintained its intensity up to its landfall about 17 km (11 mi) south of Figueira da Foz, Portugal, around 18:40 UTC that day.[1] The storm's final minimum central pressure estimate of 996 mbar (29.4 inHg) was based on a surface pressure of 999 mbar (29.5 inHg) being recorded in Monte Real, Portugal, well north of the cyclone's landfall point.[9] After landfall, the small low-level circulation associated with Alpha began to quickly decay, as the storm moved inland, and the cyclone weakened to a subtropical depression at 0:00 UTC on 19 September.[1] Alpha degenerated into a remnant low later that day, as it moved over the mountainous terrain of Northeastern Portugal.[1]

The 2020 season was the second (along with 2005) in which an alphabetic list of 21 storm names had been exhausted, necessitating use of the Greek alphabet auxiliary list.[24] In March 2021, the World Meteorological Organization replaced that auxiliary list with a new 21-name supplemental list. As a result, the name Alpha will not be used to name another Atlantic hurricane.[25]

Having reached the end of the alphabetical list of 21 Atlantic tropical storm names for 2020 with Tropical Storm #Wilfred today, the naming protocol for all subsequent storms will now consist of names pulled from the Greek alphabet as decided by the World Meteorological Organization.

Tropical Storm Alpha was the 23rd tropical or subtropical storm of the extremely active 2005 Atlantic hurricane season. It developed from Tropical Depression Twenty-Five in the eastern Caribbean Sea on October 22, 2005. As the 21 pre-designated storm names had been exhausted, it was given the first name on the auxiliary list, which utilized the letters of the Greek alphabet. This was the first hurricane season ever to trigger this naming protocol, and the only one until the 2020 season.

During its brief life, Alpha moved west-northwestward and reached its peak intensity on October 23, but weakened as it neared landfall in the Dominican Republic that same day. Crossing the island of Hispaniola it weakened to a tropical depression, and persisted until October 24, when it dissipated. Its remnant low was absorbed by Hurricane Wilma's large circulation.

Alpha dumped torrential rain on the island of Hispaniola, making it the eighth wettest storm to impact poverty-stricken Haiti. It caused 26 deaths, 17 of them in Haiti, and all of them caused by floods and rain-related landslides. Roads were blocked for weeks and hundreds of houses were destroyed.

Alpha's origins were from a tropical wave that developed near the Windward Islands on October 20.[1] Satellite images indicated that a low pressure center associated with the tropical wave formed near Barbados and moved west-northwest with increasing convective activity. In an area of light wind shear, the convection increased and Doppler weather radar data from Puerto Rico detected a well-defined cyclonic circulation.[1] On October 22, the area of low pressure organized into Tropical Depression Twenty-five, southeast of Hispaniola.[2] Shortly thereafter, satellite imagery indicated that a closed circulation had developed, and the associated convection had started banding.[3] Later that same day, the depression had organized enough to be upgraded to Tropical Storm Alpha; this was the first time the National Hurricane Center had to use a Greek name for an Atlantic hurricane.[4]

When Alpha came within the Doppler weather radar range of Puerto Rico, the radar suggested that an eyewall-like feature had developed.[5] Alpha was tracking along the southwest edge of a subtropical ridge.[5] With nearby Hurricane Wilma's large circulation, Alpha was experiencing strong southerly flow from Wilma.[5] Alpha, with its small circulation, was then faced with the mountainous terrain of Hispaniola, which led forecasters to believe Alpha was soon to dissipate.[6] Shortly after Alpha reached its peak intensity on October 23 its wind decreased. While convection remained in bands, the low level circulation was disrupted by land. This left the center ill-defined and difficult to locate.[7]

Late on October 23, Alpha made landfall in Hispaniola, quickly decreasing in intensity as it did so.[8] After making landfall, the storm weakened, leaving it just between tropical storm and tropical depression status.[8] Because of the weakening, and the strong southerly flow from nearby Hurricane Wilma, it was hard to determine whether a low-level circulation was present.[8] However, satellite imagery suggested that a new center had developed over open waters to the north of Hispaniola.[8] It was downgraded to a tropical depression early the next day, now re-emerging over open waters.[9] The storm soon developed an elongated center of circulation which indicated that the system was weakening.[10] On October 24 the low-level center had completely dissolved, and only a small area of convective activity persisted. Thus, the system had dissipated, and was a remnant low pressure system when the National Hurricane Center issued their last advisories of Alpha.[11] Shortly thereafter, the remnant low was absorbed into the circulation of Hurricane Wilma on October 25.[1]

On October 22, the National Hurricane Center issued a tropical storm warning for the island of Hispaniola and a tropical storm watch for Turks and Caicos Islands and the southeastern Bahamas. The tropical storm watch was quickly upgraded to a tropical storm warning.[1] In the Dominican Republic, authorities ordered the evacuation of at least 30,000 people living in areas where flooding was possible. On October 22 and 23, the Haitian government ordered level 1 and then level 2 alerts for the county in anticipation of Tropical Storm Alpha's rainfall.[12] Haiti was already experiencing flooding from Hurricane Wilma's passage between October 8 and 19, so the threat of more rainfall was particularly dire.[13] The country's Centre National de Meteorologie issued weather bulletins through existing media distribution channels with clear instructions on how residences should prepare for floods and landslides. The local government of Sud-Est ordered the evacuation of its capital city, Jacmel. The national government mobilized personnel and resources of the Direction of Civil Protection, the Secrtariat Permanent de Gestion des Risques et des Dsastres (SPGRD), and the National Police to pre-arranged Emergency Operation Centres.[12]

Although the storm's circulation technically made landfall near Barahona in the Dominican Republic,[1] the bulk of its impact occurred in the country's poorer neighbor, Haiti.[1][12][13] When Alpha arrived on Hispaniola it brought between 4 inches (101 mm)[14] and 7.9 in (201 mm)[1] of rain which triggered floods and landslides.[12][13] The road between Port-au-Prince and Sud was closed at Grand-Gove, Ouest.[12] In the coastal city of Logne near Port-au-Prince, bridges were closed to cars but not to pedestrians. Surging rivers east of Marigot and west of Jacmel closed roads in that region, and the main road between Jrmie and Les Cayes was also closed.[12][13] Elsewhere in the country, landslides temporarily blocked several other major roads. Gonve Island, which had been suffering a brutal drought, was stricken by floods which overwhelmed and washed away pipes of the water draining systemin Nan Bar.[12]

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