Storm Dennis

Not to be confused with Hurricane Dennis.

Storm Dennis
(Winter Storm Mabel)

Storm Dennis at its near-record peak intensity on 15 February, spanning most of the North Atlantic
Type	Extratropical cyclone European windstorm Bomb cyclone
Formed	9 February 2020 (2020-02-09)
Dissipated	Currently active
Highest gust	140 mph (230 km/h)
Lowest pressure	920 mb (27.17 inHg)
Fatalities	6 fatalities
Power outages	26,000+
Areas affected	United States, Mexico, Canada, United Kingdom, Ireland, Iceland, Norway, Sweden, Netherlands
Part of the 2019–20 North American winter and the 2019–20 European windstorm season

Storm Dennis is a weakening European windstorm which, in mid-February 2020, became one of the most intense extratropical cyclones ever recorded, reaching a minimum central pressure of 920 millibars (27 inHg). The thirteenth named storm of the 2019–20 European windstorm season, Dennis struck the United Kingdom and Ireland at peak intensity less than a week after Storm Ciara, exacerbating the impacts from that storm.

The system was first noted on 9 February by The Weather Channel, who 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 – by now known as Storm Dennis, officially named by the Met Office on 11 February – emerged into the north Atlantic. Dennis subsequently underwent explosive intensification 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 six fatalities have been recorded from Storm Dennis as of 18 February: one in the United States and five 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

Map plotting the track of the storm according to the Ocean Prediction Center

The system which would become Storm Dennis was first noted as a small cut-off low pressure area crossing the coast of California from the Pacific Ocean on 9 February, at which point it was unofficially named Winter Storm Mabel by The Weather Channel based on predictions of the system bringing severe winter weather to portions of the United States over the coming days. Mabel moved across Texas on 11 February as a slowly developing low with embedded thunderstorms, bringing high winds and blizzard conditions to portions of the Midwest the following day as slow development continued.

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.

Weather warnings in the United Kingdom

Weather warnings in the United Kingdom are issued by the Met Office.

Warning severity	Event	Date	Areas affected
Red	Rain	15 February	South Wales
Amber	Rain	16 February	South Wales, Welsh Mountains, Central Northern England, parts of South East England, parts of Cornwall
Amber	Rain	15 February	South Wales, Welsh Mountains, Central Northern England, Southern Scotland, parts of Cornwall
Yellow	Wind	17 February	North Wales, Northern England, Northern Ireland, Scotland
Yellow	Wind	16 February	All areas
Yellow	Rain and wind	15 February	England, Wales, Northern Ireland, Southern and Central Scotland

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 States, Canada & Mexico

The cut-off low pressure system that would eventually become Storm Dennis, named Winter Storm Mabel by The Weather Channel, moved off the Pacific and made landfall in southern California on 9 February, bringing squally rain showers to California, Arizona and New Mexico as it moved eastwards over the following two days. On 11 February, a slowly-developing Mabel brought rare thundersnow along the Mexico–United States border, with appreciable accumulations of snow on the ground being reported as far south as El Paso, Texas; parts of neighbouring New Mexico; and even higher ground in northern Mexico. This band of thunderstorms brought heavier snowfall further north as they became more organised, with up to 3 in (7.6 cm) of snow across the Texas Panhandle, including in the cities of Hereford and Pampa.

Mabel became increasingly organised as it moved north-eastwards across the central United States; by 12 February, the storm had brought up to 4 in (10 cm) of snow to the Albuquerque, New Mexico metro area; up to 7 in (18 cm) of snow near Prescott, Arizona; and up to 12 in (30 cm) of snow in the mountains of Colorado. Wind speeds also began to increase on 12 February, bringing sustained blizzard conditions to areas of North Dakota including Fargo, Grand Forks and the Red River Valley area. Blizzard conditions were also reported in South Dakota and Minnesota as winds gusted up to 50 mph (80 km/h), with Interstate 29 being closed in both directions from South Dakota to the Canadian border as a result. In Kansas City, Missouri, overnight temperatures dropped to as low as 3 °F (−16 °C).

Near Wichita, Kansas, a First Student school bus (of the International 3300 model) carrying fifth-grade students on a school trip lost control during heavy snowfall on the morning of 12 February and crashed off the Kansas Turnpike, resulting in 26 injuries; 17 passengers required hospitalisation. Further traffic accidents were reported in Iowa, where a state trooper was seriously injured after being struck by a truck while dealing with a previous road traffic accident on Interstate 80. In Omaha, Nebraska, an elderly man was found dead, having succumbed to apparent hypothermia. Hundreds of schools were closed across the Midwest on 12 February, including 170 schools in Iowa, while 6,000 homes were left without power in Kansas.

Brief snowfall related to Mabel, enhanced by the lake effect, passed through the Great Lakes region later on 13 February, as the centre of the storm accelerated rapidly towards the east and out into the Atlantic; however, the fast motion of the storm meant that impacts further east were minimal. However, snowfall in the Chicago area was still heavy enough to severely disrupt flights at O'Hare International Airport; in total across the United States, more than 5,700 flight delays and 600 cancellations were recorded on 13 February alone as a result of adverse weather conditions. Heavy rainfall was reported from Florida northwards to Boston, Massachusetts, causing isolated incidents of flooding.

United Kingdom

Satellite image of south-central England and south-east Wales showing flooding, 17 February 2020

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. Four people were killed and one woman was missing, later found dead, in Wales.

Ireland

Met Éireann issued a total of six yellow and orange wind and rain warnings spanning from Saturday morning until the early hours of Monday. At the peak, a status orange wind warning was active between 10am and 10pm Sunday for nine counties: Donegal, Leitrim, Sligo, Mayo, Galway, Clare, Limerick, Kerry and Cork. The yellow warnings were extended until 3am Monday.

As a result of the storm, an abandoned vessel named MV Alta was washed up high on rocks in Ballycotton, County Cork. The vessel had spent over a year drifting at sea since October 2018 after its crew were rescued by a USMC rescue team when the ship became disabled en route to Haiti from Greece. An Irish Coast Guard Rescue 117 helicopter was dispatched to the site, which is a Special Area of Conservation. It reported nobody onboard and that it showed no immediate environmental impact to the area.

Sweden

Wave in Lysekil, Sweden, during Storm Dennis

On 16 February 2020, winds of around 30 m/s (110 km/h; 67 mph) were recorded on the west coast of Sweden. Class-2-warnings were issued by the Swedish Meteorological and Hydrological Institute. Flooding occurred in areas along the west coast, most notably in Borås. The Uddevalla Bridge was closed, trains on the Gothenburg–Borås and Stenungsund–Uddevalla lines and several ferries on the west coast were cancelled. Trees were felled by the storm, destroying power lines and about 17,000 customers lost power.

On 17 February 2020, new class-1 and 2 warnings were issued for the west coast of Sweden. Flooding of roads and areas close to bodies of water continued and increased, especially around Uddevalla and Älvsborg.

The Netherlands

On 15 February the KNMI issued a yellow warning for the whole country for potential gusts of up to 100 km/h (28 m/s; 62 mph) The peak of the storm hit the Netherlands in the morning with gusts of up to 120 km/h (33 m/s; 75 mph). The storm was stronger then expected and caused a lot of damage, because of its strength and because some trees and roofs had been weakened by Storm Ciara a week before.

Aftermath

Political controversy

There was criticism in the United Kingdom of the government's response to the flooding from Storms Ciara and Dennis. Councillor Tim Swift, the leader of Calderdale Metropolitan Borough Council in West Yorkshire which includes the Calder Valley region severely impacted by flooding over the two storms, praised local efforts to reduce the impact of flooding immediately beforehand and respond to flooding as it subsequently happened. However, according to Swift, austerity measures had hampered the local council's ability to respond to flooding events and construct flood defence measures in a timely manner, claiming that reduced funding from central government had "let down" people in areas vulnerable to flooding; new flood defences in Mytholmroyd were under construction at the time but had not yet been completed following repeated delays, resulting in the flooding of the town for the third time in eight years.

Prime Minister Boris Johnson was branded "a disgrace" by Labour's shadow environment secretary Luke Pollard for being absent from flooded areas and for his failure to convene an emergency Cobra meeting in response to the crisis.

See also

• Braer Storm
• 2019 England floods

External links

• NOAA Satellite Liaison Blog
• CMISS blog article
• German Weather Service article (in German)
• MeteoFrance article (in French)