Storm Dennis was a European windstorm which, in February 2020, became one of the most intense extratropical cyclones ever recorded, reaching a minimum central pressure of 920 millibars (27.17 inches of mercury). The thirteenth named storm of the 2019–20 European windstorm season, Dennis affected the Republic of Ireland and the United Kingdom less than a week after Storm Ciara, exacerbating the impacts from that storm amidst ongoing flooding in the latter country. A precursor low over North America was named by The Weather Channel, which unofficially named it Mabel, moving eastwards across the southern United States. After bringing blizzard conditions to the Midwest and heavy snowfall to New England, the cyclone emerged into the north Atlantic, where it redeveloped into Storm Dennis, officially named by the Met Office on 11 February – Dennis subsequently underwent explosive cyclogenesis on 13 February, reaching its near-record low pressure south of Iceland the following day. Destructive winds and heavy rainfall moved south into the British Isles over the weekend of 15–16 February as Dennis passed north of Scotland; the storm subsequently began to weaken, making landfall in Norway the following day. At least five fatalities have been recorded from Storm Dennis as of 18 February in the United Kingdom. Heavy rainfall caused severe flooding in Wales and southern England, with many rivers reaching their highest levels ever recorded. Further flooding was also reported in areas of northern England that had been inundated by Storm Ciara the previous weekend.
Meteorological history
Alongside their meteorological companions in the United States, the Met Office in the United Kingdom also began issuing warnings for the storm well in advance, based on strong certainty in forecasts across weather models – Storm Dennis was officially named on 11 February, while it was still located over the southern United States and before any meaningful intensification had yet taken place; at the same time, severe weather warnings were issued across the United Kingdom for the coming weekend. The cyclone moved across the Great Lakes and into New England before exiting into the north Atlantic by midday on 13 February. After drifting northeastwards parallel to the coast of Atlantic Canada, Dennis accelerated into the open Atlantic on 14 February, undergoing explosive intensification at the same time. The central pressure of Dennis dropped by 84 millibars over 54 hours, an incredibly fast rate for an extratropical cyclone. By 15 February, as Dennis stalled to the south of Iceland, it had a near-record central low pressure of 920 mb (27 inHg) and winds gusting up to 140 mph (230 km/h) offshore. After performing a tight anticyclonic loop off the coast of Iceland under the influence of the Fujiwhara effect from a second, weaker low to the west, Dennis accelerated briskly southeastwards and began to weaken, with the centre passing north of Scotland on 16 February before making landfall in Norway on 17 February.
Preparations
Weather warnings in the United Kingdom Weather warnings in the United Kingdom are issued by the Met Office.
A red weather warning for rain, the highest level, was issued for parts of South Wales on 15 February as a result of persistent heavy rainfall across already-saturated river valleys; it was the first red warning of any kind issued by the Met Office since Storm Emma in 2018, and the first red warning issued specifically for rain since December 2015. The warning covered the Welsh capital, Cardiff, and eight surrounding council areas: Blaenau Gwent, Bridgend, Caerphilly, Merthyr Tydfil, Neath Port Talbot, southern Powys, Rhondda Cynon Taf and Torfaen.
Impact
United Kingdom
In the UK, two people were killed on 15 February: a man was found dead hours after falling overboard from a tanker and a teenager died after entering the sea in Kent. On 16 February, a man was found dead in a flooded river near Trebanos, Wales. The army was deployed to assist in rescue efforts. The UK Met Office issued a red weather warning, meaning "danger to life", for prolonged periods of heavy rain in south Wales on 16 February until 11:00 GMT, covering nine council areas including Neath Port Talbot. A fourth person, a woman, went missing and was later found dead, in Wales. A fifth person, a woman, died when she was swept away by floodwater near Tenbury Wells in Worcestershire. High winds caused disruption to ferry services across Scotland and England. The Caledonian MacBrayne ferry MV Caledonian Isles was filmed lurching violently from side to side while attempting to dock at Ardrossan on a sailing from the Isle of Arran; the ferry crew were praised after eventually bringing the ship into port successfully in the difficult conditions. The ship subsequently made a delayed and "choppy", but otherwise uneventful, return journey to Arran. Red Funnel services from Southampton to East Cowes on the Isle of Wight were adversely affected on 15 February, forcing passengers to stay onboard one of the ROPAX vessels after sea conditions became too treacherous both for the vessel to disembark or proceed onward. Passengers were stuck on the ferry for 14 hours and the journey (which normally takes approximately an hour) wasn't completed until 9:15 am the following morning. In Northern Ireland, high winds caused some disruption. The Foyle Bridge in Derry was closed to high-sided vehicles, and a temporary speed restriction was imposed to all other vehicles. On Gilnahirk Road in east Belfast, numerous shop fronts collapsed onto the pavement below as a result of high winds; there were no injuries.
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