The Washington, D.C. metro region was visited by two early-December winter precipitation events (December 8-9, December 10).   In this article, we briefly examine the anatomy of each, explaining what happened, and why each was a very different type of storm.

December 8-9 “multiple choice” storm

This is not the type of multiple choice exam I would give my meteorology students, nonetheless, it has all the elements of the classic D.C. wintery mix (a.k.a. “multiple choice day”):  Snow, sleet, freezing rain, cold rain.

The first key element was a pressure and temperature pattern called Appalachian Cold Air Damming (CAD).   This develops when a persistent wedge of high pressure builds southward along the lee slopes of the Blue Ridge, funneling cold air over th re region from the north.  The cold air is dense and heavy, and locks in tight against the mountain slopes and valleys.

The two figures below illustrate the cold air wedge entrenched against the Appalachians on the morning of Sunday, December 8.   Figure 1 shows the pressure ridge, with high pressure nosing down the Piedmont from northeast to southwest.  This pattern parallels the strike (trend) of the Appalachians, northeast to southwest.   Superimposed is the distribution and intensity of precipitation obtained from weather radar.  Moderate snow was falling across northern Virginia and central Maryland.

Figure 2 (below) reveals the telltale thermal signature of a textbook CAD event.   The image shows the pattern of isotherms (lines of constant temperature) close to the surface.    That finger of sub-freezing temperatures nosing down the Piedmont is unmistakable.  It’s a frigid -8° C along the Mason-Dixon Line and -6° C over northern Virginia, while temperatures of up to +8° C lie along the North Carolina Outer Banks.   The layer is quite cold but fairly shallow.  It is not as well defined at 5,000 ft and above.   CAD is a strong expression of our region’s geography; the core of coldest air does not extend above the altitude of the Appalachian ridgeline to its west.

While CAD became established in the lowest 5,000 feet, the high pressure system aloft began retreating to the northeast.  Winds shifted from northeasterly near the surface to southwesterly aloft, bringing in a flow of significantly warmer air from the Gulf of Mexico – especially between 5,000-10,000 ft.

The process by which relatively mild, humid air glides over the cold wedge is called overrunning.    During this second key process, moisture is lifted, the air cools and precipitation develops in a widespread, stratiform shield over the cold dome.

A third important setup for the wintery mix was the layering of air with very different temperatures.   This is what determines precipitation type and transitions among types.    On December 8-9, this process unfolded in classic textbook fashion.

I’ve taken data from balloon soundings released at Dulles to illustrate this thermal “layer cake” and how dramatically it evolved over a 24-hour period.  The sequence is shown in Figure 3 below.

The diagram shows four different snapshots of the vertical temperature structure, from the surface to 18,000 ft.    Different horizontal levels are shown on each diagram.  The change in temperature with altitude is shown by the solid red line.  Next to it is a diagonal, dashed blue line, which I’ve labeled “0° C”.  This is the magic line that discriminates between liquid precipitation – to the right of the diagonal – and frozen forms of precipitation – to the left of the diagonal.   Temperature values of the 0°, -10° and -20° C lines are shown along the bottom of each chart.

(If the use of diagonals to show how temperature changes with altitude seems a bit odd, well – it’s meteorological convention.   It does take a little getting used to).

Let’s step through this figure panel by panel.

7 AM Sunday 12/8.   The CAD wedge is firmly in place, having developed from northerly winds overnight.  The sounding reflects a very deep, subfreezing air layer, -5° to -6° C, from the surface to well above 10,000 ft. Overrunning has started; precipitation formed as snow in the cloud layer, remained as snow all the way to the ground. The snow arrived fast and furious, thanks to an energetic upper-level wave in the jet stream that got the air rising vigorously.

1 PM Sunday 12/8.   A special balloon release at this time shows some significant changes.   There is continued cooling below 5,000 ft as the northerly wind funnels down the mountains.  But above 5,000 ft, 5° C of warming has commenced, as mild air moves in on southwesterly breezes.   Precipitation started forming as rain in the cloud layer.  But the rain re-froze into ice pellets while falling through several thousand feet of cold wedge.

7 PM Sunday 12/8.   The strong warm surge above 5,000 feet continued.  In the past 12 hours, temperature in the elevated warm layer increased by 8° to 9° C.   But the cold wedge remained stubborn.   However, its depth diminished significantly, and it was being “eroded” from south to north by a mild flow impinging from the south.   Rain now survived as liquid essentially down to the surface.  But there, temperatures four degrees below freezing ensured the rain froze on contact, forming an icy glaze.  This is how freezing rain differs from sleet.

7 AM Monday 12/9.    The surge of moisture overrunning the cold dome has ended, but the “layer cake” continued to undergo further change.   The nose of warm air aloft descended to below 5,000 ft.    Just a thin veneer of subfreezing air clung to the surface.   Light freezing drizzle continued to glaze over Dulles.  Throughout the morning, the northwestward push of milder air at the surface continued, slowly eliminating the CAD from east to west.  But afternoon high temperatures failed to push higher than about 33-34 °C at Dulles.

If there’s one lesson to be learned from all this, think STUBBORN.   The CAD, once established, is notoriously hard to scour out.   This is a fundamental problem with the prediction models.  They consistently erode the cold air too quickly.   As a result, the ice often hangs on longer, and transitions among multiple choice precipitation types are slower to occur.

December 10 southern clipper

This second storm wasn’t the proverbial “polar express” but a briskly moving, weak area of low pressure that passed to the south of the DC region.    The figure below shows this weak impulse and associated bands of precipitation (rain, sleet and snow) as it skirted by at 7 a.m. on December 10:

Temperatures were close to freezing in the snow-making layer between 5,000-10,000 ft.  Near the surface, temperatures hovered around freezing.  This rendered a  wet type of snow that quickly accumulated on trees.

Thankfully, the system moved rapidly, and the main slug of moisture passed south of our region.  And while computer models were suggesting a persistent, heavy snow band would set up across the region, this did not entirely materialize.

Related: A half-right, half-wrong snow forecast, and how we can improve

All of these factors kept accumulation totals down across the immediate Washington metro.  However, snow rates at times exceeded an inch per hour.   There were a few dynamic processes that made this system an efficient snow producer:

1.    Vigorous ascent of air was promoted by an upper-level wave disturbance in the jet stream, coupled with a very fast (200 mph) pocket of wind called a jet streak.  When we talk about a system’s “dynamics” being favorable for snow formation, these are the types of processes that enhance the rate at which moisture converges and ascends into a storm system;

2.     A broad, moderate patch of snow was did congeal to north and west of the low’s center, oriented southwest to northeast.  This streamed over the Metro region for several hours.  Such a region of stratiform snow develops from overrunning – the process by which warm, humid streaming northward and westward is lifted over colder, denser air to the north of the storm.

However, there was another subtle process at play over our region, termed frontogenesis.   This was the creation of a warm front between two dissimilar air masses.    The convergence of wind at some level can bring together warm and cold air masses.  Along the narrow band where this thermal contrast is maximized, a front develops.  Fronts are regions where uplift of air and formation of precipitation become concentrated.

In the figure below,  the purple contours indicate the strength of the frontogenesis processes.   You will note the bulls-eye located over central Maryland, where moderately heavy snow was falling.   This dynamic region matches perfectly the orientation and dimension of the snow shield.

Wrap Up:  Comparing the two systems

In summary, D.C.’s snowy “Act Two” was quite a different snow-producing creature than the prolonged, wintry mess of Sunday-Monday.    The Sunday storm was produced by a very stubborn feature emplaced by terrain called cold air damming.   Waves of heavy precipitation developed with the passing of successive upper-air disturbances during the 24-hour period.

Tuesday’s system was highly progressive and dynamically-charged, a single fast-moving low pressure center, with no hint of cold air damming.    Nor was it a coastal low or Nor’easter.    The storm was in and out in the space of six hours, basically a fast “ripple” in the jet stream flow aloft.