Hurricane Ophelia (known as Storm Ophelia in Ireland and the United Kingdom while extratropical) was regarded as the worst storm to affect Ireland in 50 years, and was also the easternmost Atlantic major hurricane on record. The tenth and final consecutive hurricane and the sixth major hurricane of the very active 2017 Atlantic hurricane season, Ophelia had non-tropical origins from a decaying cold front on 6 October. Located within a favorable environment, the storm steadily strengthened over the next two days, drifting north and then southeastwards before becoming a hurricane on 11 October. After becoming a Category 2 hurricane and fluctuating in intensity for a day, Ophelia intensified into a major hurricane on 14 October south of the Azores, brushing the archipelago with high winds and heavy rainfall. Shortly after achieving peak intensity, Ophelia began weakening as it accelerated over progressively colder waters to its northeast towards Ireland and Great Britain. Completing an extratropical transition early on 16 October, Ophelia became the second storm of the 2017–18 European windstorm season. Early on 17 October, the cyclone crossed the North Sea and struck western Norway, with wind gusts up to in Rogaland county, before weakening during the evening of 17 October. The system then moved across Scandinavia, before dissipating over Norway on the next day.
Three deaths can be directly attributed to Ophelia, all of which occurred in Ireland. Total losses from the storm were less than initially feared, with a minimum estimate of total insured losses across Ireland and the United Kingdom of US$87.7 million.
Meteorological history
On 3 October, a broad low-pressure area had formed along a stationary front about west of the Azores. The low meandered over the north Atlantic for days. On 6 October, a large wind field had formed associated with the low. The low only developed shallow, weak convection, along with a long, curved cloud band, and a cold-core center—typical characteristics of an extratropical low. The system began to acquire subtropical characteristics on the next day, benefits from warm sea surface temperature of , thus the National Hurricane Center (NHC) noted for a high chance of tropical cyclogenesis. Although the system lost some of its organization due to dry mid-level air, it managed to develop gale-force winds and a well-defined center. Deep convection continued to develop near the center early on 9 October, and the NHC classified the system as Tropical Storm Ophelia at 06:00 UTC, about west-southwest of the Azores.
Despite moving over marginally warm waters of , the effects of cold air temperatures aloft and decreasing wind shear allowed Ophelia to gradually strengthen. At the same time, Ophelia drifted several hundred miles southwest of the Azores due to the cut off from mid-latitude westerlies. In addition, the large temperature contrast between the unusually-warm ocean surface and the extremely cold temperatures in the upper atmosphere provided instability for Ophelia's thunderstorms, which allowed the storm to continue strengthening, despite marginally warm ocean temperatures. Late on 10 October, Ophelia began to move southeastwards as it embedded in a mid-to upper-level trough. A slight degradation of the structure of the storm resulted in some weakening early on 11 October, but this was short-lived as deep convection wrapped around the entire storm. After developing a ragged eye, the NHC upgraded Ophelia to a hurricane at 18:00 UTC about south of the Azores. Upon the upgrade, Ophelia becoming the record-tying tenth consecutive hurricane to form during the 2017 hurricane season; this was the fourth such occurrence after 1878, 1886, and 1893 seasons. Afterwards, Ophelia steadily intensified as it became nearly stationary, intensifying to a Category 2 hurricane late on 12 October, as the eye became better defined. Ophelia achieved its initial peak intensity at 06:00 UTC on 13 October, with maximum sustained winds of and a central pressure of .