The evolution of a meteorological bomb
This was a storm of several parts, consisting of (1) an inland phase, (2) transition to a coastal or “secondary” low, (3) rapid deepening (“bombogenesis”); and (4) an intense, stationary phase off Cape Cod.
In meteorological parlance, it was a textbook “Miller Type B” nor’easter evolution, albeit in extremis.
The metamorphosis is captured in the surface weather charts for Jan. 26 and 27. On Monday morning, Jan. 26, the clipper storm that would seed the more intense nor’easter began to weaken as it crossed the Appalachians. The Washington region received a dusting of snow as the clipper’s higher upper-level energy passed to our south.
As the piece of upper-level energy moved over the Outer Banks, a new surface low spun up. This was the secondary or coastal low. The Outer Banks is a prime breeding ground for cyclones during winter. Here, the temperature contrast is at a maximum between cold air sweeping off the continental U.S., and warm Gulf Stream ocean water.
In just 12 hours, the coastal storm was on its way to becoming a respectable 991 mb low, and was entering a rapid deepening, or bomb phase. A storm is considered a bomb when it drops at least 24 millibars in 24 hours, something we call bombogenesis. A cyclone can bomb out under favorable jet stream dynamics aloft, but the critical factor is the ocean — much like a hurricane, strong surface winds circulating around the vortex extract significant amounts of heat energy from the Gulf Stream.
Bombogenesis continued through 4 a.m. on Tuesday morning, Jan. 27, with the central pressure dropping to 976 mb — a rate exceeding one millibar per hour. The low tracked due north. In the lower left panel of the figure above, note how close the isobars, or pressure lines, are on the north side of the storm, intensifying the pressure gradient there. The storm also began to occlude. Occlusion is a complex process by which the cold and warm front combine, lifting a wedge of warm ocean air close to the center of the storm.
At 10 a.m., the system was in a full-rage mode, and its rapid deepening stabilized at 975 mb. Note, however, that the storm remained stationary, due to blocking high pressure to the north. This would prove to play an important role in heightening the duration and severity of the oceanic and inland impacts.
One of the more intriguing aspects of this storm was its massive size, as seen on visible satellite imagery of the storm on Jan. 27. While the primary surface low was nestled in the Gulf of Maine, its huge radius of circulation spanned the entire East Coast. The characteristic “comma shape” is unmistakable, rendered by the cold front and adjacent warm conveyor belt.
The warm conveyor is literally a moisture pipeline feeding the northern, snow-making head of the system, with one end of the “straw” dangling in the tropics. Along this elevated corridor, fast southerly winds conveyed copious water vapor along the cold front, feeding thunderstorms. The moisture was then injected into a massive snowband locked along the New England coast. The shear scale of this precipitation machine was quite impressive!
The jet stream’s contribution
When it comes to coastal bombs, the largest atmospheric influences are located in the high altitudes, yet these features are rarely discussed. In this blizzard, we saw many textbook processes take place.
While the low-level center was transferring to the coastline, separate wave-like disturbances in the jet stream began merging. A primary spin center, part of a longwave trough, was located over the deep Southeast. A second spin center, associated with a shortwave trough, was crossing the Appalachians.
As the diagram below shows, these packets of energy were coming into phase, or co-locating. Phasing is a key process that boosts the mid-level spin energy, which in turn promotes vigorous ascent of air, causing pressure to drop.
In Figure 4 (below), we jump to jet stream level at 30,000 feet, and fast forward 12 hours to the evening of January 26. Phasing of spin centers was complete, and in this diagram we track packets of fast-moving air, called jet streaks, embedded in the cores of the primary, southern stream jet and a secondary, northern branch of the jet.
The streaks are labeled, along with their corresponding pockets of rising air, denoted by the white circles with embedded, black plus (“+”) signs. In strict meteorological jargon, we refer to these as the “left exit region of jet streak 2” and “right entrance region of jet streak 1”. Streak 2 was part of the main southern jet, and streak 1 was embedded in the northern branch.
Taken separately, each streak induced the air to rise up from below, stoking the rapid drop in surface pressure. But on Jan. 26, these pockets were in the process of phasing together, creating one large region of sustained and vigorous ascent offshore from New Jersey to Maine. It’s as if this storm won the lottery in terms of deepening potential.
These upper atmospheric interactions were a major region why the cyclone underwent bombogenesis. It’s interesting to note that the jet streaks are in part induced by the intensifying cyclone itself. Yet, once they develop, along with the characteristic “S” pattern in the jet stream, they strengthen the storm. This type of positive feedback between surface low and jet stream dynamics is called “self-development.” Again, all very textbook!
High wind, large waves
With rapid deepening at the surface comes an intensifying pressure gradient, which accelerates wind speed. The strong pressure gradient is vividly illustrated below.
The location of the surface low, with 984 mb central pressure and frontal system, are depicted south of Cape Hatteras. The dark blue region illustrates the principal, heavy snow band. Thin black lines are isobars (lines of constant pressure). There is a strong anticyclone (high pressure system) located over Nova Scotia shown by the blue “H.”
I emphasize that the coastal storms and its adjacent region of high pressure is really a coupled system. This, in terms of cold air feeding into snow production, and the establishment of an asymmetric and intense pressure gradient on the north side of the coastal. Note how the isobars are squeezed together over Maine. This gradient, combined with the Coriolis effect and surface friction, propelled an intense northeasterly wind (wind blowing from the northeast). The flow accelerates over the low-friction ocean surface and slams into the coastline.
These effects generate the high, sustained winds that nor’easters are famous for. Late on Jan. 26 and throughout Jan. 27, sustained winds hit the 40 to 50 mph range along much of the New England coast, and hurricane force gusts to near 80 mph raked Cape Cod and Nantucket.
The combination of extreme wind and heavy snowfall is what defines a blizzard (technically, the sustained wind or wind gusts must exceed 35 mph for more than 3 hours, in concert with heavy falling or blowing snow leading to low visibility).
The screaming northeasterlies were monitored by the good folks at the NOAA Ocean Prediction Center. The figure below shows a Rapidscat overpass (7 a.m. on Jan. 27). Note the large, organized wind field of this storm. Winds from the south, east and northeast broadly converge into the storm’s center (white “L”). Just to the north, approaching Cape Cod is a blast of hurricane-force wind (red arrows, inside the blue circle). A second region of hurricane winds was detected in the Gulf of Maine.
A long fetch, or distance over which winds blow from a single direction, sets up tremendous seas. Wind energy is transferred into the ocean at a rate proportional to the wind speed, and duration of wind. The duration factor was really maximized for this storm, since it remained nearly stationary for so long. Wave heights in the Gulf of Maine grew to 30 to 40 feet.
The impact of these waves cannot be underscored enough, in terms of oceanic shipping, blue water and coastal fishing, beach erosion and devastation of beachfront property. One harrowing photo from the Associated Press really stands out — a beachfront street inundated by seawater, with patches of ice floating atop the water, lapping at the roof of a submerged car.
Snow: Lots of it!
This is the aspect of nor’easters that commands the most media attention for us land-lovers. There is much to the character of snowfall in terms of impact: total accumulation, duration, snowfall rate, snow to liquid ratio, and corollary effects (low visibility, stickage, drifting). In this brief story I discuss only the snow totals, which were eminently impressive for parts of New England, as shown in the figure below:
New England has had its share of 3-plus foot snowfalls, and this storm is certainly historical. The map depicts widespread 10 inch totals from New York City, through Long Island, and across all of the coastal Northeast. The jackpot was a narrow band of 30 to 36 inch snowfall across eastern Massachusetts.
But why such a narrow band? This gets to the so-called mesoscale character of strong nor’easters. Mesoscale features are on a small space and time scale. They are the important processes and variations that span just 10 to 100 miles over three to 12 hours in duration.
It turns out that nor’easters have appreciable inner-dynamics operating at the scale of counties, or groups of counties. Our understanding of mesoscale snowbands has only been fully elucidated in the past 10 or 15 years. Both myself and Wes Junker, CWG’s winter weather expert, have explained these details in past CWG stories and forums.
Mesoscale snowbands can be tricky to predict days in advance, but the high resolution models are getting better in the 12 to 24 hour time frame. Once a band sets up, a combination of radar and modeling is used to anticipate its intensity, coverage and progress. Snow rates of two to four inches per hour, and perhaps thundersnow, characterize these bands. The bands often remain stationary, or nearly so, for six to 12 hours, and produce the most extreme gradients in snow accumulation.
A classic mesoscale snowband along the back side of the strong New England coastal set up on Tuesday morning. The band remained stationary for nearly 18 hours, since the offshore low barely moved. Once again, here we see how the duration factor combines with intense snow rates, leading to an astronomical snow accumulation
The mesoscale snowband set up 150 to 200 miles to the northwest of the low’s center. It’s a feature of flow deformation (intense stretching and confluence of airstreams), frontogenesis (increasing thermal gradient) and rising air. As noted earlier, high moisture content air feeding this “sweet spot” came from the tropics.
Taking all these details together, this powerful, and historical, Nor’easter is an entity that while crippling, inspires awe and wonder about the atmosphere.