An extremely powerful Nor’easter

The storm is remarkable not only for its late winter appearance in the Mid Atlantic, but its tremendous energy and reach. Emerging off coastal Carolina early Tuesday morning and moving north along the Delmarva, the area of low pressure underwent an explosive period of development as it moved into New England.

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When the pressure drop is sustained at an average rate of 1 millibar per hour, over a 24 hour period, forecasters term such a storm a “meteorological bomb.” On Tuesday afternoon, automated weather stations along eastern Massachusetts recorded pressure drops of nearly 20 millibars in three hours – that’s an astounding rate of more than 8 millibars per hour. One site dropped nearly 50 millibars in 48 hours. These are rates typically only observed in rapidly intensifying hurricanes. Talk about creating a hole in the atmosphere!

The figure below shows multiple snapshots of the storm, as it transformed from a weak coastal system into a full-fledged, super-torqued maelstrom off New England. The evolution shows both the lines of equal pressure known as isobars and weather radar depiction. The metamorphosis over 12 hours is astounding.

Note an increase in the number of isobars around the low’s center, and increased crowding of isobars, especially on the north side. Over the warm western Atlantic, the storm began to rapidly intensify – with central pressure dropping to below 980 millibars (perhaps as low as 977-978 millibars). Along the New England coast, measured gusts reached 60-70 mph combined with snow rates of 4 to 5 inches per hour.

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The radar, being limited to the coast, does not show the full extent of the precipitation, but very intense precipitation persisted along the northern quadrant of the storm. Here, a warm ribbon of humid air – termed the warm conveyor belt – lifted prodigious quantities of water vapor into the cold, snow- and ice- manufacturing zone of the clouds.

A semi-stationary snow band developed about 200 miles northwest of the center by 2 p.m. (shown in the last frame). This type of band develops where the airflow is becoming alternatively stretched and squeezed (the so-called deformation zone) and is a staple of wintertime Nor’easters. Snow rates in this band were broadly two to three inches per hour across New England. A time-lapse video of accumulating snow in the Adirondacks revealed a snow rate of five to six inches per hour, which is truly astounding.

To appreciate the full scope of the storm, the image below compares two satellite snapshots – one at 3 a.m. and the other at 9 a.m. The imagery reveals the water vapor concentration in the mid- and upper- atmosphere, and false coloring is used to enhance the contrast between very humid air (blue and green) and dry air (orange and red).

The curved comma-shape delineates the warm conveyor of mild, humid air surging northward, then hooking westward into the “snow zone”. Between 3 and 9 a.m., there is tremendous expansion of the saturated (green) zone in the comma head – as vast quantities of conveyor-belt laden moisture condensed into heavy rain, and froze into sleet and snow.

As in all strong coastal systems, upper air dynamics played a key role in the storm’s rapid genesis and intensification. In the next image, two interacting packets of upper-level energy, called shortwave troughs, are shown approaching one another and phasing at 8 a.m. During this time, the coastal storm had begun its period of rapid intensification. The phasing created a much deeper, larger, single trough that in turn lead to more vigorous ascent on its “downwind” side (boxed region along the Mid Atlantic and New England coast).

A messy “multiple choice” storm over Washington

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On Monday night, the storm’s formative phase was anything but typical, in terms of classic development of a large snow shield that enveloped the metro region. The “snow zone” became stratified according to temperature, with a zone of heavy sleet transitioning to rain on the south and east edges. This transition zone set up roughly parallel, and along, the I-95 corridor.

At times, sleet and snow mixed together across the transition – with heavy, wet snow prevailing close to the rain-snow line, and a drier, fluffier snow to the north and west. Freezing rain mixed in at times where the region of sleet abutted cold rain. The entire transition from snow-ice-rain was dynamic, advancing northward and westward throughout the early morning.

Blame this wintry mix on an invasion of mild, oceanic air several thousand feet above the surface. The graphic below illustrates the process. Contrary to the best model guidance, the coastal low tracked further westward, over the mouth of the Chesapeake Bay. Such an “inside track” enabled a surge of mild Atlantic air to penetrate further inland. The mild air layer is wedge-shaped in cross section, overriding an inverted, wedge-shaped mass of subfreezing air near the surface.

Imagine now this 3D “layer cake” of air temperature oriented northwest to southeast, from Frederick, Md. to the Eastern Shore, respectively. A line depicting the location of this atmospheric cross-section is shown on the final image in this story (a map of our region’s snow accumulation).

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The diagonal interface between the warm and cold wedges is where interesting things happen. Rain that forms in the cloud layer falls through varying thickness of subfreezing air. A deep cold layer, near the northwestern (back) edge of the cold wedge, allows drops time to refreeze into ice grains (sleet). Where the cold layer is very shallow, near the southeastern (front) portion of the wedge, rain survives to the surface, then freezes on contact with the ground. This gives rise to the dreaded freezing rain.

Further to the northwest, near Frederick, the cold air mass was so deep that it extended all the way into the cloud layer. Snowflakes forming in the clouds, remained frozen as snow all the way to the ground. Sleet, mixing with snow and freezing rain, prevailed along the middle of the transect, over the I-95 urban corridor. Further to the southeast, milder air with temps above freezing extended all the way from cloud base to the ground – meaning plain rain formed in the clouds, and did not refreeze during its descent and impact.

One additional surprise was much colder air than predicted, at the surface, from the northern neck of Virginia into parts of Southern Maryland, during the early morning. This reinforcement of the cold-air “wedge” in low levels was a type of cold air damming, induced by the coastal low over North Carolina. Cold air damming sets up when a cold, northerly flow gets trapped/funneled southward by the Appalachians; the dense air pools close to the ground over the Piedmont and along the lee slopes of the mountains.

In the following graphic, you can see the flow of cold drawn southward on the east side of the mountains. Low pressure over North Carolina established a cold inflow from the north, and the 3,000-4,000 mountain barrier channeled the flow southward. This process was not well-predicted by the models, and led to a lighter, fluffier snow (12:1 ratio) on the north side of the rain-snow line. It also prolonged our period of sleet through the morning hours.

Finally, we get to the snow that fell. The bands of snow depth are remarkably parallel, and show the typical orientation during mixed-phase type winter storms. The zone of two to three inches and zones to the southeast reflect prolonged mixing by sleet, with lesser amounts of freezing rain and just plain rain, far to the southeast. The snow jackpot was in Frederick County, with orographic enhancement along the Catoctins and South Mountain.