Wednesday, December 26, 2012

December 26, 2012: Cold Air Damming Bringing Frozen Precipitation This Morning

Overview of the Storm

This winter storm, called Euclid by The Weather Channel, is creating its heaviest snows and high winds across the Ohio Valley, where blizzard conditions prevail this morning along the back, cold side of this mid-latitude cyclone.  The center of the storm is over the Tennessee Valley, and the system is moving toward the northeast.  These enormous, intense, winter cyclones have "cold side impacts" and "warm side impacts" - a case of a storm with a split personality!  The band of heaviest snow usually develops 100-200 miles to the northwest of the storm center.  But in the southeast segment, called the "warm sector", the combination of warm, humid air and vigorous uplift often creates pockets of severe local storms, including thunderstorms and tornadoes.   Winter Storm Euclid has created both a blizzard on its northwest side, and tornadoes in its warm sector.   Here is the radar snapshot of this impressive storm from earlier this morning, clearly showing the cold and warm side precipitation features:

Surface weather in Winter Storm Euclid.  Adapted from WeatherTap
In this diagram, you can see the center of the cyclone, marked with a large, red "L".  To its north and west is the heavy snow band.  To the southeast, over the Carolinas, is a warm air mass containing bands of thunderstorms - some of which may produce another round of severe weather later today.
Note the orange-colored wedge of ice over the Appalachians - this is the region of overlap between warm and cold air masses - with the warm air sliding over dense, cold air trapped in mountain valleys.   The heavy magenta line roughly bisects the storm into its warm-side and cold-side.

Baltimore Impacts:  Why Geography Matters


Closer inspection of the morning radar shows a curious wedge of ice and snow over central and western MD, NOVA and the WV panhandle - extending along the spine of the Appalachians in Virginia:

Snow and ice wedge over the Mid Atlantic.  Adapted from WeatherTap
"The Wedge" as meteorologists refer to this setup, is classic Mid Atlantic, wintertime meteorology.   The Appalachians frequently trap cold, sub-freezing air flowing in from the northeast.  A shallow, frigid layer called Appalachian Cold Air Damming abuts the Appalachians over the Piedmont, and also gets trapped within the long, linear valleys of the mountains (such as the Shenandoah Valley).   This cold air is overrun above by warm, humid air flowing off the Atlantic.  The vertical layering of air masses provides a recipe for cloud layers containing various forms of frozen precipitation - freezing rain, sleet, snow.  Thus far, we have experienced all of these on this "multiple choice" winter day.

Cold air damming develops when a ridge of high pressure (caused by dense, chilly dammed against the mountains) sets up across the Mid Atlantic.   The pressure ridge east of the mountains is shown on this morning's surface map.  Solid lines are isobars (lines of constant pressure):

Surface map showing pressure pattern within Winter Storm Euclid.  Adapted from NOAA

The pressure ridge tightens the pressure gradient, accelerating a chilly, northeasterly breeze across the Piedmont.  This brings in more cold air at the surface, reinforcing the cold air wedge.

Let's see what this chilly air current does to the temperature field.   The morning surface temperature map (below) shows the cold air wedge in the pattern of isotherms (lines of constant temperature).   Red isotherms are temperatures above freezing;  blue are those below freezing;  the purple isotherm is the critical freezing line:

Surface temperature map showing critical freezing line (purple isotherm) and cold air wedge.  Adapted from NOAA
You can see the wedge-shaped pattern of sub-freezing air sitting over the Maryland Piedmont and mountains.   As is quite typical, the surface freezing line trends SW to NE, and this morning it bisects the major cities Washington and Baltimore.

Finally, the map below indicates that this morning's cold wedge extends upward through 5,000 feet.  This is a deep, sub-freezing air layer and it guarantees that snow will fall:

5,000 foot temperature map showing deep cold air wedge.  Adapted from NOAA
In fact, compare the location of this deep, cold wedge with the region of radar-observed snow, and you see excellent correspondance:

Location of snow wedge.  Adapted from WeatherTap.

You don't see correspondance with the ice storm extending south along the mountains.  This is because sleet is generated from a much shallower layer of cold air damming...temperatures at 5,000 feet are above freezing, meaning precipitation starts in the clouds as rain.  As it falls into a sub-freezing air layer perhaps only 2,000 feet thick (or less), it quickly freezes into sleet grains.

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