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Twin bomb cyclones to merge into one of strongest-ever storms in North Atlantic, hammering Iceland and threatening Britain as Storm Dennis

The Washington Post logo The Washington Post 2/15/2020 Andrew Freedman
A computer model projection of surface air pressure and wind speeds on Feb. 16 associated with Storm Dennis in the North Atlantic Ocean. (Earth.nullschool.net) © Earth.nullschool.net/Earth.nullschool.net A computer model projection of surface air pressure and wind speeds on Feb. 16 associated with Storm Dennis in the North Atlantic Ocean. (Earth.nullschool.net)

A potentially unprecedented scenario is unfolding in the North Atlantic on Friday, as a bomb cyclone batters Iceland with hurricane-force winds and blizzard conditions, just as another bomb cyclone, known as Storm Dennis, rapidly intensifies behind it. Ultimately, the two storms will enter into a complex dance around each other before combining into one atmospheric behemoth, with a minimum central air pressure that could rank among the top three most intense storms ever observed in the stormy North Atlantic Basin.

Already on Friday morning, the first bomb cyclone, which has a minimum central air pressure of 929 millibars, roughly equivalent to a Category 4 hurricane, is slamming Iceland with blizzard conditions, mixed precipitation and heavy rain. Winds have been clocked at about 70 mph in the capital city of Reykjavik, and 90 mph at the international airport in Keflavik. The forecast called for gusts of higher than 120 mph in a few locations, particularly on the western coast of the island, though conditions were improving early Friday afternoon local time.

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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 not recommended anywhere. 

The National Weather Service’s Ocean Prediction Center reported that a satellite passing over the 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! Significant wave height is defined as the average of the highest one-third of waves in a particular period, and tends to run about half the height of single waves.

Buttressing the satellite observation, a buoy northwest of Ireland recorded a significant wave height of 41.3 feet Friday morning, which means individual waves were about twice as high, or nearly 80 feet, in that location.

This storm, and the one to follow it that are combining to form Storm Dennis, will turn the entire North Atlantic into a wave machine, with battering waves reaching Western Europe.

This storm battering Iceland underwent the process of bombogenesis, or rapid intensification, with its air pressure plummeting more than 24 millibars in 24 hours. Over the course of 48 hours, its pressure dropped by 67 millibars as it moved from near Nova Scotia early Thursday morning to near Iceland early Friday morning.

Energized by an unusually powerful jet stream — a highway of air at about 30,000 feet that is powered by the thermal contrasts between air masses — these weather systems are both developing rapidly and reaching extraordinary intensities in a region already known for strong winter storms. Winds in the core of the jet stream are forecast to be as strong as 240 mph late Friday, which could lead to another record for the fastest transatlantic flight, first broken Sunday.

The low-pressure area intensifying southwest of Iceland, which innocuously passed off the East Coast of the United States on Thursday night, is projected to deepen even further than the current bomb cyclone battering Iceland.

Ultimately, though, these two storms will merge after doing a unique meteorological dance, resulting from what’s known as the Fujiwhara effect, when one storm orbits another. This will result in one giant swirl centered south of Iceland on Saturday, with related energy and its associated wind and heavy precipitation swiping across northwest Europe.

Amber weather warnings are in effect this weekend in Britain, where winds up to 70 mph are anticipated in Northern Ireland, Scotland and parts of northern England and north Wales. In addition, heavy rains could lead to flooding in the wake of deadly Storm Ciara this past week. The Met Office has named this system Storm Dennis, and computer models as well as meteorologists at the Ocean Prediction Center forecast its air pressure to plummet to between 916 millibars and 924 millibars. This range would qualify it among the top five strongest North Atlantic storms ever observed.

The circulation of the first bomb cyclone that will combine to form the larger Storm Dennis, with Iceland seen just to the left of the center of the swirl. (CIRA/RAMMB) © CIRA/RAMMB/CIRA/RAMMB The circulation of the first bomb cyclone that will combine to form the larger Storm Dennis, with Iceland seen just to the left of the center of the swirl. (CIRA/RAMMB)

“Storm Dennis will bring another very unsettled spell of weather this weekend with a risk of flooding, particularly in parts of England and Wales and also southern Scotland, where snowmelt will add to the flood risk,” Met Office chief meteorologist Steve Willington said in a statement. “Following Storm Ciara last weekend and further spells of rain this week, the ground is already saturated in places. With Storm Dennis bringing further heavy and persistent rain over the weekend, there is a risk of significant impacts from flooding, including damage to property and a danger to life from fast flowing floodwater.”

High winds and showers are expected to continue across Britain into Monday. Fortunately, the core of the storm will remain north of Europe, sparing the continent its full fury.

The strongest North Atlantic storm on record was the Braer Storm in 1993, which had a minimum central pressure of 913 millibars. Illustrating the dangers such storms pose to ships, this 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.

The third extraordinarily intense North Atlantic bomb cyclone in 10 days

This is the peak time of year 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 in the North Atlantic.

Very few of these storms typically see their minimum air pressure drop to 930 millibars or lower; yet assuming Storm Dennis does so, this will have happened three times in the past 10 days. (The low-pressure area that helped propel Storm 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 a what is known as a positive state, with low pressure predominating near Greenland and a ridge of high pressure to its south in the northeastern Atlantic. On Monday, the AO set a daily record for its most positive reading since such record-keeping began.

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.

In addition to the deaths and damage from Ciara, the winter’s North Atlantic storms have also affected North America. Last month, for example, Newfoundland and Labrador were buried by one of their worst blizzards on record, when a storm underwent rapid intensification and piled snow up to the second and third stories of buildings in downtown St. John’s.

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