Hurricane Leslie (known as Storm Leslie or Cyclone Leslie while extratropical) was the strongest post-tropical cyclone to strike the Iberian Peninsula since 1842. A large, long-lived, and very erratic tropical cyclone, Leslie was the twelfth named storm and sixth hurricane of the 2018 Atlantic hurricane season. The storm had a non-tropical origin, developing from an extratropical cyclone that was situated over the northern Atlantic on 22 September. The low quickly acquired subtropical characteristics and was classified as Subtropical Storm Leslie on the following day. The cyclone meandered over the northern Atlantic and gradually weakened, before merging with a frontal system on 25 September, which later intensified into a powerful hurricane-force extratropical low over the northern Atlantic.

While Leslie began to weaken late on 27 September, the low began to re-acquire subtropical characteristics, and by 28 September, Leslie had completed the transition to a subtropical storm once again. Leslie became fully tropical and gradually intensified, becoming a Category 1 hurricane early on 3 October, and initially peaked with 1-minute sustained winds of later that day. Leslie gradually weakened, falling to tropical storm intensity late on 4 October. The cyclone continued to slowly weaken before beginning to re-intensify on 8 October. Two days later, Leslie reached hurricane status for the second time. Leslie continued to slowly strengthen, reaching peak intensity with sustained winds of and a minimum central pressure of , early on 12 October. Leslie then began to gradually weaken later that day, while accelerating towards the northeast and passed far south of the Azores. On 13 October, Leslie passed north of Madeira, before transitioning into an extratropical cyclone just off the Portuguese coast later that day. Leslie's remnants made landfall in central Portugal a few hours later. The low continued moving northeastward while rapidly weakening, passing over the Bay of Biscay, before dissipating by 16 October over Spain.

The storm was responsible for 17 deaths in mainland Europe, including 2 direct deaths in Portugal and 15 indirect deaths in France. In November 2018, Aon estimated that Leslie's damage total exceeded €424 million (US$500 million). Hurricane Leslie prompted the issuance of tropical storm watches and warnings for Madeira Island. Leslie brought wind gusts up to to Portugal, felling thousands of trees, causing the collapse of hundreds of structures, and damaging hundreds of buildings, signs, and pieces of equipment. Over 300,000 citizens were left without power. Damage across the country was estimated to be €120 million (US$145 million). City, municipal, and the national government provided funds to repair damage to buildings as well as the local forests, and also launch cleanup efforts. Leslie brought strong winds and torrential rainfall to Spain, most notably the Catalonia region, causing a river to spill its banks, nearly four dozen landslides to occur, and damaging several structures and vehicles. The storm disrupted transportation and caused 14,000 power outages throughout the country. Leslie and a cold front, the latter of which was almost stationary, combined to cause record-breaking flooding in the Aude, France. The flooding in the department was considered to be the worst since 1891. Floodwaters damaged buildings, roads, and vehicles throughout several towns and cities, causing €220 million (US$254 million) in damage. Nearly 1,000 people were evacuated when a dam overflowed in Pezens Municipality, and over 8,000 people lost power nationwide.

Meteorological history Origins, formation, and tropical transition

The United States-based National Hurricane Center (NHC) began forecasting on 19 September 2018 that an extratropical low would form in a few days between the Azores and Bermuda. This system developed along the boundary of a front early on 22 September, approximately west-southwest of Flores Island. This system was associated with the southern portion of the remnants of Hurricane Florence, which had previously split into two storms on 18 September. Amid favorable environmental conditions, the system separated from the front, and its banding features became better established, signaling the formation of Subtropical Storm Leslie by 12:00 UTC on 23 September. Around that time, Leslie was within an upper-level low and the former's gale-force winds were displaced from the center. The subtropical cyclone was located within an area of minimal steering currents, causing the storm's movement to be erratic. Moderate vertical wind shear and dry air hindered the development of thunderstorm activity or convection outside the southeastern portion of the system, as it tracked generally southwestward. The cyclone turned towards the south and later the east on 24 September. During that time, Leslie's cloud pattern became irregular, with several cloud swirls existing within a broadening circulation. Leslie weakened into a subtropical depression around 00:00 UTC on 25 September, due to the continuing wind shear and dry air. The storm began a southeastward motion later in the day, followed by a turn back to the east. Leslie became an extratropical cyclone around 12:00 UTC, after its circulation expanded along a baroclinic zone. At that time, an expansive stratocumulus cloud shield was becoming established and cold, stable air was entering the region.

During the next couple of days, Leslie's track consisted of an incomplete counterclockwise loop. At the same time, the baroclinic processes strengthened the extratropical cyclone, leading to hurricane-force winds of by 00:00 UTC on 27 September. A weakening trend commenced during the middle of the day and persisted until 28 September. However, as Leslie tracked westward, it began to reacquire tropical characteristics and was declared a subtropical storm once more around 12:00 UTC. The storm had reestablished bands of convection near its center and its wind field had shrunk considerably. While Leslie had transitioned from a cold core to a warm core system, it was still located within a sizable low-pressure system. Over the next couple of days, the cyclone experienced minimal change in strength as it tracked in a southwesterly direction around the western edge of an extensive cyclonic circulation. Thunderstorm activity developed near Leslie's center and anticyclonic outflow materialized to the northeast and southeast of the system. As a result, Leslie became a tropical storm by 18:00 UTC on 29 September, while it was located approximately west-southwest of Flores Island in the Azores.

Initial peak intensity and weakening

Located within a region of cool sea surface temperatures and strong northwesterly wind shear, the tropical storm intensified little during the next couple of days. Banding features began to increase on 1 October, as the wind shear subsided. Leslie continued a west-southwestward to southwestward motion over the next day or so, while located in weak steering currents and positioned between high-pressure systems that were located to the west and northeast. As Leslie began to strengthen more quickly on 2 October, convection increased both in intensity and organization. Microwave imagery displayed that Leslie had developed an eye at its low levels. The storm intensified into a hurricane around 06:00 UTC on 3 October. Meanwhile, Leslie became stationary, with its southerly movement ending. Leslie reached its initial peak at 18:00 UTC as a Category 1 hurricane, with 1-minute maximum sustained winds of . At the time, the storm had a large, irregular eye, with sporadic dry air entrainment. The storm began to track northward between a shortwave trough, which was located to the northeast, and a mid-level ridge, which was located to Leslie's southeast.

Moving over cooler, sea surface temperatures, Leslie began to weaken slowly on 4 October. The cyclone weakened to a tropical storm around 18:00 UTC. By 5 October, Leslie's inner core had dissipated, and the storm possessed only small bands of convection on the outer limits of its circulation. Convection refired over the storm's center later that day, although the maximum winds were located away. Leslie turned towards the east from late 5 October to 6 October, under the influence of mid-latitude westerly flow. Although the tropical storm traversed a region of even colder, sea surface temperatures during the next couple of days and was located in a region of moderate wind shear, the storm changed little in strength. Leslie's convection became more organized on 6 October, as banding features became more defined and a mid-level eye formed to the southeast of the storm's low-level circulation center. Leslie turned towards the east-southeast on 7 October, still under the influence of the westerly flow. Leslie bottomed out as an tropical storm at 00:00 UTC, as it passed over an area of cool sea surface temperatures.

Peak intensity, extratropical transition, and demise

Leslie began to restrengthen later on 8 October, as an inner core began to develop and convection became more intense. While the storm continued to track in a southeastward direction, traversing an environment of warmer sea surface temperatures and lower wind shear, a mid-level eye feature began to develop. Turning towards the south, Leslie restrengthened into a hurricane around 06:00 UTC on 10 October. Around that time, the storm's strongest convection was concentrated towards the north, and an irregular eye had emerged on satellite imagery. The cyclone began to proceed east-northeastward early on 11 October, as strengthening continued. Leslie peaked at 00:00 UTC on 12 October as a Category 1 hurricane, with maximum sustained winds of and a minimum central pressure of , while located approximately south-southwest of Flores Island. The storm had a small, cloud-filled eye around that time.

Leslie began to slowly weaken once more later that day, as sea surface temperatures decreased to and wind shear increased. The mid-latitude westerly flow began to rapidly accelerate Leslie towards the east-northeast, with the cyclone passing north-northwest of Madeira Island around 06:00 UTC on 13 October. Shortly after, Leslie began to transition into an extratropical cyclone, as colder air entered from the south and west and the system's mid-level eye decayed. The cyclone's convection continued to disintegrate as a result of even colder sea surface temperatures and strong wind shear. The NHC issued its final advisory on Leslie around 21:00 UTC on 13 October, after the storm became fully extratropical. The cyclone moved ashore near Figueira da Foz at 21:00 UTC, with winds of . Leslie weakened as it traversed the Bay of Biscay on 14 October, reaching western France by 15 October. An almost stationary, convective cold front over southern France was assisted by Leslie. The latter helped to spawn a surface low over the Mediterranean Sea which increased convection near the cold front, and it provided unstable air parcels, reduced evaporation, and added moisture to the mid-levels of the airmass over France. Leslie's remnant was absorbed into Hurricane Michael's extratropical remnant by 16 October, following a brief Fujiwhara interaction.

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