A hyperactive period of intense, extratropical cyclone development unfolded last month across the North Atlantic. A series of four winter storms, each generating hurricane force winds, developed in rapid succession. No minimum central pressure records were broken, but all four storms underwent rapid cyclogenesis or bombogenesis. One of the storms, Storm Dennis (named by the UK Met Office), attained the second lowest pressure reading behind Storm Braer in 1993. Three of these four cyclones managed to achieve remarkable pressures in the 920-930 mb range.
Below is a table summarizing what we know about each of these four storms.
Next, we describe each system using a combination of satellite and surface analyses.
The first storm, which we are calling the “Greenland Bomb,” deepened explosively to 930 mb by February 8. In the surface analysis below, the storm – designated as a hurricane-force low – is located just east of the tip of Greenland. The extreme cyclonic wind field helped propel the next storm in the series, shown over Newfoundland as a 966 mb low, into Europe.
The low reached the British Isles as a 945 mb storm, causing widespread flooding and wind damage. It was named Storm Ciara by the UK Met Office, and is shown on the synoptic surface chart below.
On 12 February 2020, a third storm was brewing in the North Atlantic Ocean that was forecast to rapidly intensify into an strong extratropical cyclone that would develop hurricane-force winds. The system’s pressure dropped from 1005 mb on 12 February at 00 UTC to 962 mb on 13 February at 00 UTC, a 43 mb decrease in pressure in just 24 hours that classified this system as a bomb cyclone. We are referring to this third cyclone as the “Iceland Bomb.”
Stratospheric air intrusions are a known contributor to rapid intensification of extratropical cyclones via the advection of potential vorticity from the stratosphere into the troposphere. In the case of the Iceland Bomb, there was a strong signal of stratospheric air in the troposphere seen in GOES-16 Airmass RGB imagery, indicated by deep red/magenta in the dry slot of the cyclone.
The wave pattern across the North Atlantic was extremely active, as seen in the Airmass RGB imagery above and Atlantic surface analysis below. A storm force low preceded the Iceland Bomb, and Storm Dennis closely following.
In the image below, the RGB imagery of the Iceland Bomb does a remarkable job of delineating “textbook” conveyor belts, or the basic 3D circulation branches of a wintertime, marine cyclone. The dry conveyor belt descends from the stratosphere into the southwestern quadrant of the storm. The warm conveyor belt – with its moisture-laden, high cloud shield originating in the warm sector – stands out in stark contrast to the cold conveyor belt. The cold conveyor belt undercuts the warm conveyor belt from the east, wrapping into the comma-head of low and middle layer clouds.
The Iceland Bomb deepened further as it moved across the Atlantic and reached its minimum central pressure of 929 mb on 14 February at 06 UTC. Hurricane-force winds were sustained for 48 hours by this extratropical cyclone, slamming Iceland with winds of historic strength and producing phenomenal wave heights up to 64 ft. Although terrain enhanced, the highest wind gust recorded in Iceland was 159 mph in Hafnarfjall.
Almost like dejavu, another cyclone closely followed the Iceland Bomb. The low pressure system, which would eventually receive the name of Storm Dennis, originated in the same location as the Iceland Bomb and took nearly the same path. The system deepened from 996 mb on 13 February at 12 UTC to 956 mb on 14 February at 12 UTC, a 40 mb drop in 24 hours, classifying this system as another bomb cyclone.
Unlike the Iceland Bomb, this cyclone moved a little more to the east, impacting Ireland and the United Kingdom with intense hurricane-force winds. A wind gust of 118 mph was recorded in the Scottish Highlands, and a gust of 107 mph was recorded in Brocken, Germany, the strongest gust recorded outside of the United Kingdom (WNEP).
Other impacts included intense waves with heights up to 52 ft, severe flooding produced by prolonged intense rainfall, and structural damage caused by falling trees. All of this occurred just a week after the region was hit by storm Ciara, which exacerbated the impacts.
Rapid intensification of Storm Dennis was aided by a stratospheric air intrusion that originated in a trough located over the eastern United States that can be seen in the Airmass RGB imagery above. Deep red/majenta coloring indicates dry upper levels and high ozone concentrations associated with stratospheric air in the troposphere. Orange coloring indicates potential vorticity in the Airmass RGB product, and it trails all the way from the trough towards Europe, feeding into storm Dennis. Also pictured above is a massive atmospheric river stretching across the entire North Atlantic that provided ample moisture for the development of Storm Dennis.
The low pressure centers from the Iceland Bomb and Storm Dennis met up just south of Iceland where they did a Fujiwara dance, rotating around each other and then merging into one low pressure center. The dance can be seen in the Atlantic surface analysis above and the Airmass RGB imagery below. The Fujiwara effect is most commonly observed when two tropical cyclones pass close to one another. The combined system continued to move northeastward and dissipated by 18 February at 00 UTC.
Broader Perspectives Of The Hyperactive Storm Period
Having bomb cyclones in February is not unusual, as the peak season for these events in the North Atlantic and North Pacific is September through April, but having FOUR bomb cyclones in rapid succession of this intensity is unique.
This winter has featured a persistent, strongly positive arctic oscillation (AO) index, as shown below. Such a strong, positive index is driven by intense low pressure over the polar North Atlantic and high pressure ridging in the subtropical Atlantic.
The extreme pressure difference or gradient, in turn, has ratcheted up the intensity of the zonally-oriented polar jet stream coursing across the Atlantic Basin – at times reaching 240 mph. The dynamics of such an intense jet are a key reason for the sequence of very intense North Atlantic cyclones.
When one compares this winter’s East Coast snow activity (thus far, one of the least snowiest winters on record for portions of the Mid Atlantic), with the winter of 2009-2010 (record-breaking seasonal snowfall), the contrast in persistent North Atlantic Oscillation phase is striking (image below). Since early December, nor’easters have failed to materialize along the typical breeding ground of Cape Hatteras.
The intensity of the Iceland Bomb and Storm Dennis was aided by a massive atmospheric river spanning the entire width of the Atlantic Ocean. The atmospheric river funneled moisture from the tropical Caribbean all the way to Europe, following along a 5,000 mile long surface cold front and an intense upper air jet, which aided Dennis in producing an impressive 6.2 inches of rain over 48 hours in Cray Reservoir in South Wales as well as high rain totals across the region (WNEP).
Shown below is the integrated water vapor (IVP) product at 12 UTC on 16 February, showing extensive plume of Caribbean moisture entering into the large circulation of Storm Dennis (top panel). Also below, the Atlantic surface analysis the day prior illustrates the lengthy cold front, bisecting the entire Atlantic basin, that helped concentrate this atmospheric river.
This stormy period will be remembered for the incredible low pressures, verified hurricane-force winds (using ASCAT), and significant wave activity. The North Atlantic has remained stormy in early March, but pressures have been higher, though hurricane-force wind events continue.
Thank you for reading!
Deirdre Dolan (U. of MD-College Park), Jeffrey Halverson (U. of MD-Baltimore County), and Michael Folmer (NWS/NCEP/OPC)