Dennis has become the 2nd most powerful nontropical cyclones on record in the North Atlantic, packing hurricane-force winds and turning a vast swath of seas into a churning, ship-sinking cauldron with individual waves topping 100 feet. The storm slammed into Britain just one week after deadly Storm Ciara hit with high winds and heavy precipitation, prompting flooding fears.
According to Christopher Burt, a weather historian at Weather Underground, Dennis ranks as the second-strongest storm on record for this region.
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Peak winds in Britain have hit 87 mph. More rain and wind is forecast in the United Kingdom through Monday, though the focus of the impacts will shift to northern areas, closer to the storm center. The U.K. Met Office gave the storm its name as part of its program for warning of dangerous winter storms. |
| Waves in Porthcawl Harbor, Wales. Photo: Geoff Caddick. |
Dennis is the result of the Fujiwhara effect, an atmospheric version of a corporate merger as two bomb cyclones spinning in proximity merged into one behemoth on Saturday, reaching historic proportions.
People struggle with umbrellas as they walk in Bournemouth, Britain, on Saturday. (Neil Hall/EPA-EFE/Shutterstock)Dennis' 5,000 mile long cold front may bring rain to Florida
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| Moisture associated with the trailing cold front from Superstorm Dennis will move north over Florida Feb 17-22, 2020, as a warm front and could bring some much needed showers to the parched central and southern portions of the Florida peninsula. |
A nearly 5,000-mile-long cold front connects Storm Dennis to a plume of tropical moisture, known as an atmospheric river.
On Saturday afternoon, Dennis contained a minimum central air pressure of 920 millibars after putting on an extraordinary display of rapid intensification, known as bombogenesis. This pressure reading was roughly equivalent to what would be observed in a Category 4 or 5 hurricane.
Northwest-facing beaches in France, Britain, Ireland and Scandinavia are seeing extraordinarily high surf from the combination of Dennis and another hurricane-force low pressure area that is in the process of merging with Dennis to create one massive area of spin in the North Atlantic. That previous storm, which also was a bomb cyclone based on its intensification rate, slammed Iceland with blizzard conditions and winds up to 108 mph Thursday night and Friday.
The Ocean Prediction Center reported that significant wave heights were running about 56 feet between Iceland and Britain on Saturday, which means individual wave heights possibly topped 112 feet.
Surface weather map Saturday, showing Storm Dennis in the upper right, with connecting fronts extending all the way to the southwest of Florida. (NWS Ocean Prediction Center)In Iceland, whiteout conditions were observed along with wind gusts above 100 mph as the preceding bomb cyclone passed perilously close to the island nation. The Icelandic Meteorological Office issued rare red warnings for parts of the country into early Friday, with orange warnings continuing into Friday night due to the strong winds and heavy precipitation, and travel was halted virtually everywhere.
The National Weather Service’s Ocean Prediction Center reported Friday that a satellite passing over that storm, which has since merged with Dennis, 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.
Energized by an unusually powerful jet stream 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.
Braer of 1993
Remains Atlantic's Strongest
The strongest North Atlantic storm on record was the Braer Storm of January 1993, which had a minimum central pressure of 913 millibars (not to be confused with the Storm of the Century which came in early March of 1993). Illustrating the dangers such storms pose to ships, Braer storm was named after an oil tanker that broke apart during the storms in the Shetland Islands, resulting in a large and damaging oil spill.February 14 is 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.
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 Storm Ciara into Europe last weekend accomplished this feat as well.)
A strong storm in the North Atlantic has reach 930 mb and is producing #hurricaneforce winds. This is one of the deepest lows on record in the North Atlantic over the last decade! #GOESEast#marinewxpic.twitter.com/Ytb5sRSoGW— NWS OPC (@NWSOPC) February 7, 2020
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 a what is known as a positive state, 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.
Bombogenesis of Superstorm Dennis
Satellite view of an intensifying North Atlantic bomb cyclone, named Storm Dennis by the U.K. Met Office, as of Thursday morning. (NASA Worldview)February 13 at 2:29 PM
Superstorm Dennis rapidly intensified in the North Atlantic on Feb, 13, 2020.
Seen via satellite above, the storm, which the U.K. Met Office is began referring to as Superstorm Dennis since it will sideswipe that region over the weekend, resembles a giant comma drawn across a vast area of real estate. Its clouds stretch from south of Iceland all the way into the Caribbean. At its peak, the storm extended for 5,000 miles, with a cold front’s tentacles extending from near Florida all the way near the center of the beastly storm northwest of Scotland.
As of midday Feb 13, the storm was still strengthening, undergoing a rapid intensification process known as bombogenesis. Computer models showed a rare scenario playing out, with the storm maxing out at an intensity of 915 millibars, which would be just 2 millibars shy of the all-time North Atlantic record, set by the Braer Storm of 1993. In general, the lower the air pressure, the stronger the storm.
Such an air pressure reading would be more than five standard deviations from the norm, and would place the storm in the top 10 list of the strongest North Atlantic nontropical storms on record.
The weather system was being aided by a powerhouse jet stream that is roaring across the North Atlantic, and may peaked at around 240 mph by Friday or Saturday.
The jet stream — a highway of air around 30,000 feet above the surface that helps steer storm systems — is the result of strong air pressure differences between Arctic low pressure and high pressure areas to the south. It is helping to invigorate storm systems as they move off the coast of the United States and into the North Atlantic.
According to the National Weather Service’s Ocean Prediction Center, the storm’s minimum central air pressure had plunged to 940 millibars as of 8 a.m. Eastern time. A satellite with a sensor that can detect wave heights and wind speeds at the Earth’s surface passed over the storm Thursday morning and found significant wave heights of up to 51 feet. Since that metric is defined as the average of the highest one-third of waves in a particular period, this indicates that individual waves may be about twice as tall, up to a staggering and ship-sinking 100 feet.
Rapidly intensifying storm over the North Atlantic on Thursday. (RAMMB/CIRA)The OPC’s forecast for the storm calls for it to pack sustained winds of up to 100 mph, along with “phenomenal seas” when it reaches peak intensity sometime between Friday and Saturday.
To qualify as a bomb cyclone, a nontropical storm needs to intensify by at least 24 millibars in 24 hours. This particular low pressure area saw its pressure plummet at nearly twice that rate, deepening by 46 millibars in 24 hours, with further rapid intensification in the forecast. Over a longer time period, the storm’s minimum pressure has dropped by 65 millibars in 36 hours.
European impacts x2
In the U.K., which had just experienced deadly impacts from Storm Ciara, weather forecasters issued amber warnings for heavy rain from Supertorm Dennis, noting the potential for several inches of rain to fall, along with damaging winds. Flood warnings are already in effect, given the one-two punch from Ciara.
Winds are forecast to gust past 50 to 60 mph in many areas this weekend, the Met Office said. Fortunately, the center of the storm is forecast to remain far enough to the northwest to spare even Scotland from the strongest winds, though gusts at hurricane force (74 mph) are likely there. Pounding surf and possible coastal flooding is also possible, given the huge swells generated by this weather system.
Fujiwhara Effect
Storm Dennis is actually going to merge with that other storm after doing a unique meteorological dance, known as the Fujiwhara Effect, and the impacts of this interaction could be severe in Iceland. That country’s weather agency issued orange and red alerts for sustained winds of greater than hurricane force and whiteout conditions in some locations, calling for heavy snow and sleet to fall across the entire country, with the greatest accumulations in mountain areas.
Downtown Reykjavik could see sustained winds in excess of 70 mph, the Icelandic Meteorological Office warned. A forecast note issued Thursday warns of “violent easterly winds” through Friday in southern parts of Iceland, for example. The aviation forecast for Keflavik International Airport calls for sustained winds of 71 mph with gusts to 92 mph Friday, which would be strong enough to halt all flights.