Hurricane Leslie (2018)

Map plotting the track and the intensity of the storm, according to the Saffir–Simpson scale

On 19 September, the National Hurricane Center (NHC) began forecasting the development of a non-tropical low pressure system within the next several days far to the southwest of the Azores, with the possibility of the low gradually acquiring tropical or subtropical characteristics.[5] Three days later, an extratropical cyclone formed along a frontal boundary about 1,300 km (805 mi) south-southwest of the Azores,[6] which was associated with part of the remnants of Hurricane Florence.[7][8] The low then acquired gale-force winds as it meandered over the northern Atlantic.[9] On 23 September, the low lost its frontal features and the outer cloud bands became increasingly well-defined, and at 12:00 UTC, the NHC designated the system as Subtropical Storm Leslie.[6]

Leslie as a hurricane-force extratropical cyclone on 27 September

Leslie continued its erratic movement as the steering current remained weak.[6] The future development of Leslie was uncertain, due to the possibility of an approaching, larger low pressure system becoming dominant and absorbing Leslie, although some global forecast models maintained Leslie as the dominant system following the merger.[10] Late on 24 September, Leslie embedded in an area with dry air and moderate wind shear, causing its cloud pattern to become ragged and less organised,[11] and Leslie weakened to a subtropical depression early on 25 September.[6]

Twelve hours later, Leslie transited back to an extratropical low.[6] Microwave data showed that the system had a surface circulation elongated around an intruding baroclinic zone.[12] Late on 25 September, the NHC forecasted that Leslie would regain subtropical characteristics in a couple of days.[13] Soon afterward, Leslie merged with a frontal system and subsequently strengthened.[6] Early on 27 September, Leslie attained hurricane-force winds, likely due to baroclinic process.[14] Despite weakened to storm-force strength later that day, Leslie began to show some subtropical characteristics, and its shower activity gradually became organised.[15] On the next day, Leslie lost much of its baroclinic field, and its convection became more organised.[16] Therefore, the NHC once again classified Leslie as a subtropical storm and resumed issuing advisories at 12:00 UTC.[6]

Hurricane Leslie near its initial peak intensity on 3 October

Soon after being re-classified, Leslie moved to the due west under the interactions with the subtropical ridge to the west and a large deep-layer low that formed to the east.[16] On 29 September, the storm turned to the southwest as it embedded to the deep-layer low, and Leslie's deep convection became more concentrated near the center. This signified its transition into a tropical cyclone.[17] Later that day, Leslie developed an anti-cyclonic outflow to its northeast and southeast, and it developed a warm core structure.[18] At 18:00 UTC, Leslie became fully tropical, and it transited to a tropical storm about 1,850 km (1,150 mi) west-southwest of the Azores.[6]

On the next day, Leslie moved slowly to the southwest over the northern Atlantic due to two mid-level ridges, without much change in intensity.[19] Starting on 1 October, Leslie began to intensify, while the NHC noted an increase in convective banding in the north and northwest of Leslie.[20] Leslie continued to steadily intensify over the next days. On 3 October, Leslie developed a ragged eye,[21] and it intensified to a hurricane at 06:00 UTC.[6] At the same time, Leslie became stationary due to very weak steering current.[21] Twelve hours later, Leslie achieved its initial peak intensity with maximum sustained winds of 140 km/h (85 mph) and a minimum central pressure of 970 hPa (28.64 inHg).[6] Later that day, Leslie began to accelerate northward, under the influence of a shortwave trough to the northwest and a mid-level ridge to the southeast.[22]

The hurricane began to weaken on 4 October as it moved over cooler waters.[23] Leslie weakened to a tropical storm at 18:00 UTC.[6] Its structure continued to degrade, with the NHC noted that the system lacked an inner wind core.[24] Late on 5 October, Leslie turned to the southeast, as it encountered the southern edge of a mid-latitude westerlies.[25] Despite moving over cool waters with just 24 °C (75 °F), Leslie maintained its intensity throughout 7 October.[26] Nonetheless, the storm bottomed out with winds of 85 km/h (50 mph) early on 8 October.[6]

Hurricane Leslie approaching the Iberian Peninsula on 13 October

Later that day, Leslie began to intensify again, as satellite images showed that the storm developed a small central dense overcast with the cloud pattern became more symmetric.[27] Meanwhile, Leslie turned to the south-southeast, as the storm was steered by a mid-level ridge.[28] Over the next day, Leslie continued to intensify under favourable environment. Microwave data revealed that the storm developed a small mid-level eye,[29] and Leslie re-attained hurricane status early on 10 October.[6] Leslie then turned southward, under the influence of a mid- to upper-level trough.[30] Later that day, Leslie turned sharply to east-northeast, due to a mid-latitude trough.[31] The hurricane continued its intensification trend as it moved over an area with low wind shear.[32] Leslie reached its peak intensity with maximum sustained winds of 150 km/h (90 mph) and a minimum central pressure of 968 hPa (28.59 inHg), early on 12 October, about 1,060 km (660 mi) south-southwest of the Azores.[6] At the same time, the small eye of the hurricane was apparent on satellite images.[33]

Leslie began to weaken later that day while accelerating to east-northeast, due to the combination of increasing wind shear and cooler waters.[34] At 06:00 UTC on 13 October, Leslie made its closest approach to Madeira Island, passing about 320 km (200 mi) north-northwest of the island.[6] Soon afterward, Leslie began its extratropical transition while continued racing east-northeastward, after fully embedded in the mid-latitude westerlies.[35] The weakening trend of the hurricane slowed down despite unfavourable environment, possibly because of baroclinic forcing.[36] At 18:00 UTC, Leslie dropped below hurricane strength and completely lost all the tropical characteristics simultaneously, at about 190 km (120 mi) west-northwest of Lisbon, Portugal.[37] Three hours later, the NHC issued the final advisory on Leslie as it approached the western Portuguese coast.[38] Leslie's extratropical remnant made landfall in Figueira da Foz at 21:10 UTC, causing damage throughout the central part of the country.[39][6] Afterward, Leslie's center became ill-defined while moving northeastward, over the Bay of Biscay.[6] The low reached western France by 15 October.[40][41] Early on 16 October, Leslie's remnant was absorbed into Hurricane Michael's extratropical remnant, which was situated to the west, following a brief Fujiwhara interaction.[42][43]

From late September through early October, Leslie brought high surf to the East Coast of the United States, inducing the highest swell observed in some locations for years. Leslie also generated the single-longest period of tropical swells observed in the Outer Banks in the last 20 years, producing surf at chest-height or higher. The highest surf was observed on 26–28 September, when Leslie was a powerful extratropical cyclone with hurricane-force winds.[44]

Late on 11 October, the Government of Portugal issued a tropical storm watch for Madeira Island,[45] which was changed to a tropical storm warning six hours later.[46] It was the first known tropical storm warning issued for that island in recorded history.[1] Madeira officials closed beaches and parks.[47] The threat of the storm caused eight airlines to cancel flights into Madeira. More than 180 sports matches on the island were canceled, more than half of which affecting the Madeira Football Association.[48] The warnings were discontinued on 13 October as Leslie moved away from the islands.[49]

In advance of Leslie, IPMA issued red warnings for high winds or dangerous coastal conditions for 13 out of its 18 districts, including the capital Lisbon.[50] Several events were canceled or postponed, including the Revenge of the 90s party scheduled for the night of 13 October in Lisbon being postponed to the 20th. A Mafalda Veiga concert in Campo Pequeno, the play "Baixa Terapia" at the Tivoli BBVA, and the evening session of the French Film Festival were also canceled.[51] In addition, the final match of the Rink Hockey Female European Championship, played on 14 October in Mealhada, was suspended with less than two minutes left.[52]

At 21:10 UTC on 13 October, Leslie made landfall in Figueira da Foz as a storm-force extratropical cyclone with winds of 110 km/h (70 mph). It was the first cyclone of tropical origin to hit the Iberian Peninsula since Vince in 2005, and the strongest cyclone to hit the peninsula since 1842. Winds up to 175 km/h (109 mph) were recorded in Figueira da Foz, which has been attributed to a sting jet.[53] Heavy rains and strong waves affected the entire country, leaving 324,000 homes without power, more than 60 people needed to be evacuated. At least 1,000 trees were uprooted in the coastal areas.[54] The storm caused 2 deaths, and slightly injured 28 others.[2] Estimated damage nationwide by Aon exceeded €100 million (US$115.5 million), including an insurance loss of €60 million (US$69.3 million).[55]

Winds of up to 96 km/h (60 mph) were reported near Zamora. Villardeciervos also recorded winds of 87 km/h (54 mph); strong winds caused trees to be uprooted. AEMET issued the yellow level warnings due to the risk of storms related to Leslie, advising citizens to beware objects that may fall on public roads. There are no reports of fatalities due to Leslie in Spain.[56]

The moisture of Leslie's dissipating extratropical remnant fed a quasi-stationary cold front over southwestern France, generating heavy thunderstorms and leading to flash flooding in that area.[57] Carcassonne received 160–180 mm (6.3–7.1 in) of rainfall within five hours; water level in the city rose 8 m (26 ft) during that period. 15 people died because of the flash flood, mainly in the town of Villegailhenc, Aude.[58] This was partially due to the fact that the Aude River rose to a height of 7 m (23 ft), its highest level since 1891.[3] Wind gusts of 111 km/h (69 mph) and wave heights of 7.8 m (26 ft) were recorded in Sète.[59] Damage in Aude were calculated at €220 million (US$254 million).[60]