Editor focusing on extreme weather, climate change, science and the environment.
February 16 at 4:00 PM
Storm Dennis, the second-strongest nontropical storm on record in the North Atlantic Ocean, caused widespread flooding across parts of the United Kingdom on Sunday, along with winds exceeding hurricane force.
The storm, which is producing waves up to 80 feet tall west of the United Kingdom, dumped more than five inches of rain in South Wales, almost an inch more than the area typically receives for the entire month of February. The resulting flooding has prompted numerous evacuations and even cut off some communities.
The U.K. Met Office, which named the storm, issued its first “red” warning for heavy rainfall since 2015, its highest warning category. The country’s Environment Agency issued a record number of flood warnings, 594, for a single day, according to John Curtin, executive director of flood and coastal risk management at the Environment Agency.
While the rainfall totals were noteworthy, they were not unprecedented. However, coming just one week after another severe bomb cyclone, known in the United Kingdom as Storm Ciara, the ground was already saturated when this one arrived. This caused many rivers, creeks and streams to overflow their banks and even triggered landslides.
Video showed a landslide moving down a mountain in Tylorstown, South Wales, on Sunday morning. Several severe flood warnings were issued, meaning the conditions posed life-threatening danger. Gwent County police said residents of Skenfrith, Monmouthshire, were advised to evacuate because of the flooding.
The Environment Agency has predicted the River Ouse in York could rival record levels seen in 2000.
Forecasts call for more high winds and showers Monday as the storm center slowly spins to the northeast of the region. Water levels in many rivers are not expected to crest until Monday or Tuesday, which will prolong the flood risks.
The storm is being blamed for at least two deaths in the United Kingdom after two bodies were pulled from rough seas in separate searches along England’s southeastern coast. In addition, a man in his 60s fell into the River Tawe in South Wales on Sunday morning, though that death has not been tied to the storm, according to the BBC.
In addition to heavy rain, the storm also produced wind comparable to a Category 1 hurricane, which is noteworthy considering its center was just south of Iceland and the strongest winds were roiling the seas hundreds of miles northwest of Ireland. Nevertheless, a wind gust to 91 mph was recorded at Aberdaron in North Wales on Saturday, with several gusts to 70 mph or greater noted elsewhere.
Storm Dennis, seen near peak intensity as it spun south of Iceland on Saturday. (NASA)
Dennis resulted from the rare merger of two unusually intense bomb cyclones, which on Saturday recorded a minimum central air pressure of 920 millibars. According to a list maintained by Christopher Burt, a weather historian at Weather Underground, Dennis ranks as the second-strongest storm on record for this region. (The top spot is still occupied by the Braer Storm of 1993.)
The storm maintained an intensity of 930 millibars into Sunday, which is a strength that few nontropical storms ever reach, let alone maintain for extended periods. The air pressure in the center of Dennis plunged by 56 millibars in 24 hours, between 4 p.m. Eastern time Friday and the same time Saturday, according to the U.S. National Weather Service’s Ocean Prediction Center. This drop was more than twice the intensification rate required to be considered a bomb cyclone.
In general, the lower the pressure, the stronger the storm. The 920-millibar reading was roughly equivalent to what would be observed in a Category 4 or 5 hurricane.
Before it merged with an intense low-pressure area swinging northeast of Newfoundland on Friday night and Saturday morning, the first bomb cyclone in this series dealt Iceland a severe blow, with blizzard conditions and winds up to 108 mph Thursday night and Friday. The Ocean Prediction Center reported Friday that a satellite passing over that storm detected a significant wave height of 64 feet west of Ireland. This means individual waves in that area were potentially as high as 128 feet.
At its peak, Dennis produced individual waves at least 112 feet tall, along with sustained winds of hurricane force. It occupied an extraordinary stretch of real estate, extending from south of Iceland southeast into Britain and southwest across the Atlantic and into the Gulf of Mexico.
Powerful waves break on the shoreline of Porthleven, in southwestern England, on Sunday. (Alastair Grant/AP)
A nearly 5,000-mile-long cold front connected Dennis to a plume of tropical moisture, known as an atmospheric river. On Sunday, that connection was more abbreviated but still evident based on satellite imagery.
Dennis is not an isolated event but rather part of a string of bomb cyclones that have prowled the waters of the North Atlantic in recent weeks.
Energized by an unusually powerful jet stream — a highway of air at about 30,000 feet that is driven by the thermal contrasts between air masses — these weather systems have been developing rapidly and reaching extraordinary intensities in a region already known for strong winter storms. Winds in the core of the jet stream reached 240 mph late Friday, which led transatlantic flights to see ground speeds exceeding 800 mph, though the record for fastest transatlantic passenger flight, first broken Sunday, held.
Friday marked the date of the climatological peak for bomb cyclones in the North Atlantic, given the typical intensity of the jet stream and intense air mass differences that tend to move over moisture-rich waters. What’s been especially noteworthy about the winter’s weather, however, is the frequency and intensity of the storms spawned here.
A bomb cyclone machine
Very few of these storms typically see their minimum air pressure drop to 930 millibars or lower; yet this has now happened three times in the past 10 days, with Dennis ranking as the most intense of the three storms. (The low-pressure area that helped propel Ciara into Europe last weekend accomplished this feat as well.)
The strong near-zonal — or straight west-to-east — jet stream is characteristic of periods when a weather pattern above the North Atlantic, known as the Arctic oscillation (AO), is in what is known as a positive phase, with low pressure predominating near Greenland and a ridge of high pressure to its south.
The AO is one of the main reasons winter has been absent in much of the eastern United States and parts of Europe, and it’s helping to turn the North Atlantic into a virtual bomb cyclone express lane.
There are some signs that Dennis may have been sufficiently large and intense to cause this persistent weather pattern to change. Computer models on Sunday signaled that cooler-than-average conditions might arrive in the eastern United States for early March, potentially giving rise to some snow events after all.