The mega-storm that walloped the Washington, D.C., region with more than two feet of snow underwent a complex, long-duration evolution — but it had four distinct phases.

The composite panel below illustrates the surface flow patterns during four phases: 1. The primary low/overrunning phase;  2. the formation of the coastal low; 3. the intensification of the coastal low, and thundersnow; and 4. the prolonged wrap-around snow.

During all phases, the snow was manufactured in a deep, subfreezing air layer, with surface temperatures in the low to mid-20s on the northwest side of Washington. This produced a light and fluffy snow with a snow-to-liquid ratio on the order of 12-to-1 to 13-to-1 — close to the climatological value for our region.

Fortunately, this type of snow does not cling to branches and powerlines. Thus, while the snow totals rivaled or exceeded those for Snowmageddon in 2010, total power outages were capped in the few-thousands, as opposed to hundreds of thousands in 2010. During Snowmageddon, the snow was much heavier and wet, because that storm’s surface temperatures were closer to freezing and the total moisture flow into the storm was significantly larger.

Phase 1: Primary low and overrunning snow phase

The low pressure center early on Friday tracked across the lower Mississippi Valley and was already tapping a prodigious amount of Gulf moisture.  Enormous thunderstorms erupted over Gulf waters and coastal states dealt with numerous severe local storms.

Meanwhile, frigid, high pressure-air began building down the eastern slopes of the Appalachians, creating a cold-air damming setup, known as “the wedge.” This dense mass of air is a classic component of big-league, Mid-Atlantic snowstorms. It helped anchor subfreezing air in place throughout the event.

Warm, moist air streaming north into the primary low center was forced to ascent the cold wedge, creating a situation called overrunning. Overrunning was the main snowfall-generating process over D.C. for the first 6-8 hours of the event.

The wedge also established a strong thermal gradient between cold, terrain-locked air and warm Gulf Stream just offshore. Warm air rising and cold air sinking within the thermal zone helped generate kinetic energy (wind) during the next phase of the storm, initiation of the coastal low.

Phase 2: Formation of the coastal low

During the evolution of a classic Miller Type-B storm, the primary low transfers its energy and low-level spin (vorticity) across the Appalachians, to the coastline. A strong temperature gradient along the coastline helps energize this process, as well as factors at jet stream level favoring broad ascent of air. Additionally, a process called “conservation of vorticity” plays out as spinning air crosses a mountain barrier.

Thus during the mid- to late-evening on Friday, the low pressure center effectively jumped to a new location along coastal North Carolina. The new center is shown in the figure below. Also note how rapidly the primary, inland low disappeared.

Phase 3: Intensification of the coastal low and thundersnow

In the figure above, note how the precipitation shield became better organized and more intense, just to the north of the coastal low. A main driver for this expansion was the underlying sea surface. Strong winds blowing over the warm water surface began efficiently extracting heat energy from the upper ocean.

These so-called fluxes added a new energy source to the storm. And all that Atlantic moisture streaming up and into the vortex began expanding the precipitation shield on its north side, ramping up snow production.

Over Washington, several intense snowbands passed from south to north during the wee hours of the morning. Numerous cloud-to-ground lightning strikes accompanied snow rates approaching two to three inches per hour. This is intense nor’easter behavior; the heaviest snow rates typically occur as the main storm intensifies, and are generated by convective processes. The radar snapshot below shows several of these bands around 5 a.m.

Several processes contributed to convectively-generated snowbursts.

The first was a plume of unstable air, arising from those energy fluxes off the Gulf Stream. The unstable air plume ascended and arced over the D.C. region, as shown in the image below. Ignoring the hatched areas, I emphasize the heavy outlines of the convective air plume – representing 250 to 500 J/kg CAPE (convective available potential energy) of elevated instability. Based on model forecast soundings, the Capital Weather Gang’s Wes Junker predicted this outcome two days earlier.

Another factor that creates intense snow bands is something termed “flow deformation.” This is a dynamic contraction of low-level air streams, on the northwest side of the vortex, that brings warm and cold air masses into close juxtaposition. The intensifying thermal gradient is akin to creating an elevated weather front, and in fact the process is also termed “frontogenesis.”

Frontogenesis was at work throughout Friday morning, from the surface to above 10,000 feet. It helped focus and concentrate the snow-forming region into multiple, intense bands. One of these frontogenetic regions is shown in the figure below.

A mathematical function is used to create “contours” (purple lines) showing frontogenesis intensity. Note the elongated corridor of intense frontogenesis at 10,000 feet, over central Maryland, on the north side of the low. This region coincided perfectly with persistent, heavy snowbands. The physics of the “deformation zone” predict regions of efficient snow-making in nor’easters.

While there is a lot of meteorological detail presented here, these are the types of processes that concentrate snow into heavy, narrow bands, leading to rather extraordinary gradients in snow accumulation.

Phase 4: Prolonged wrap-around snow

Finally, we get to the prolonged phase of moderate-intensity snow maintained throughout the afternoon and evening. The coastal low was slowly pulling away to the east, but anchored on its back side was a region of “wrap around” snow, as shown in the weather radar depiction below.

It’s as if the whirling snow machine was never destined to stop running! Part of its stubbornness was due to the slow retreat of the large, parent vortex. But another factor was an internal air current termed the cold conveyor belt, depicted in the image below.

The cold conveyor belt, highlighted by the red hatched arrow, is a mid-level conduit that transports and gently lifts saturated air in an arc around the north side of the vortex.  The conduit wraps all the way around the backside of the storm, within the “comma head” commonly seen in satellite images.

Also shown in the water vapor image above is a very pronounced dry slot (orange-black shades). Like a mighty fist, it surged into the storm from the southwest. This is air that has likely descended from great heights i.e. the stratosphere, where the air mass contains very few water molecules.  During the late morning on Friday, a portion of that dry air invaded the south and east suburbs of D.C., shutting down or abating the snow in the wrap-around band for a few hours.

Wrap-around snow issues from fairly shallow cloud layers, but can contain weak convective elements, and banding features that locally enhance the snow rates. In this particular storm, wrap-around was responsible for heaping another 6 to 10 inches on top of an already epic dump of snow!