Hurricane Leslie was the strongest cyclone of tropical origin 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.

The 2018 Atlantic hurricane season was the third in a consecutive series of above-average and damaging Atlantic hurricane seasons, featuring 15 named storms, 8 hurricanes, and 2 major hurricanes, which caused a total of over $50 billion in damages and at least 172 deaths. More than 98% of the total damage was caused by two hurricanes. The season officially began on June 1, 2018, and ended on November 30, 2018. These dates historically describe the period in each year when most tropical cyclones form in the Atlantic basin and are adopted by convention. However, subtropical or tropical cyclogenesis is possible at any time of the year, as demonstrated by the formation of Tropical Storm Alberto on May 25, making this the fourth consecutive year in which a storm developed before the official start of the season. The season concluded with Oscar transitioning into an extratropical cyclone on October 31, almost a month before the official end.