Cold Air Damming along the Appalachian Mountains
For a simple example of cold air damming click (here).
For a more detailed example click (here).
Cold air damming is a critical process for the production of snow, sleet and freezing rain along and east of the mountains of the Middle Atlantic and Southeast U.S. The process helps supply cold air in the first few hundred to few thousand feet of air above the ground. Cold air damming is called such because cold air is pushed up (or "dammed") against the east slopes of the mountains and thus block from moving west. Many times however the cold air can spill into some of the valleys, such as in Asheville, NC or Roanoke, VA. On the map below, black dots denote the locations of Roanoke, VA, Asheville, NC, and Charlotte, NC.
Map created using nationalatlas.gov
Typically the surface high pressure system supplying the cold air is centered over the Northeast or northern Middle Atlantic states. The winds around the base of the high pressure system push the dense low level cold air west into the Appalachian Mountains. The air then has no place to move but south and southwest along the foothills, piedmont, and coastal plain all located east of the mountains. As noted by Bailey, Lackmann, Hartfield, and Keeter in a climatological study (see reference below), cold air damming is identifiable on surface maps as a narrow "U-shaped" pressure pattern in the isobar analysis. Winds at the surface are from the north-northeast or northeast over the coastal plain and piedmont, then veer from the east or even southeast into the mountains. The winds in the piedmont tend to cut across the isobars, that is bisect the bowl shaped part of the U in an isobar, at least partially due to the barrier effect of the mountains.
Sometimes the high pressure system to the north or northeast is ill-defined or not present. In these cases, a ridge of high pressure is "implied" to the west of a low pressure trough located near the coast or a trough associated with a coastal warm front. An approaching trough or storm system to the west of the mountains will further define the ridge. Cold air that already exists is held in place rather than strong cold advection from the north.
The western piedmont and foothills of South Carolina, North Carolina, and Virginia are particularly prone to major ice storms compared to most of the country. A few of the cities included in this "Piedmont Ice Storm Zone" are Greenville, SC, Charlotte, NC, Hickory, NC, Winston-Salem, NC, and Danville, VA. Just as a side note, there are some valleys of interior New York state and Pennsylvania that may be just as much or more vulnerable.
Asheville, NC in a valley sandwiched between the Blue Ridge mountain range to the northeast and the Smokey mountain range to the south and west, is not as prone as the Piedmont to the east but still gets its fair share of cold air damming induced ice events. What is interesting about Asheville is that the official observation at the airport south of town frequently shows a persistent south wind during these events. The topography appears to affect the wind direction in this case. Also, since the airport is located on the west side of the high pressure ridge axis, a southeast wind (I.E. containing a south component) would not be unreasonable. The coastal plain, east of the piedmont is also affected at times but marine influence keeps the air warmer if the winds have enough of an east component off the Atlantic Ocean.
Contribution to Winter Precipitation
Cold air damming aids in the formation of sleet and freezing rain by helping to sustain cold air near the surface when the cold air otherwise would not have been there. If the vertical temperature profile of the troposphere is cold enough to support snow except in the lowest several hundred to several thousand feet, cold air damming can supply colder air near the surface. The snow can then reach the ground without melting.
In either case of ice or snow, the air need not be "cold" in the sense of being below freezing. If the air is very dry, then melting and evaporating of precipitation can cool the temperature below freezing. In this type of scenario, any location that remains underneath the leading edge of precipitation for an extended period of time is in danger (or pleasure) of being nailed with a major ice storm or snowstorm. This is a forecaster's nightmare as the narrow zone of heavy ice or snow accumulations will be difficult to pinpoint and may only be as wide as a few tens of miles.
Steve Keighton, Science and Operations Officer at the National Weather Service in Blacksburg, VA presents a brief but thorough discussion of precipitation type forecast problems associated with cold air damming. See references at the end of this page.
Influence on Precipitation Production
Cold air damming can contribute to at least light precipitation production as warmer, higher dew point air from the southwest, south, or event east is lifted over the dense colder air. In this case, the cold air acts as an "effective terrain" such that the lighter warm air must to some extent rise up over the colder air.
Another lifting mechanism called "frontogenetical forcing" also contributes to precipitation formation in some circumstances. Frontogenesis is the process of increasing the temperature (or density) gradient by pushing opposing warm and cold air masses together. Air is forced upward toward the cold air since the boundary slopes toward the cold air. The process also increases the steepness of the boundary causing the air to rise in a more abrupt fashion. The strength of the lift is influenced by additional factors such as the stability of the air above the frontogenetic zone. If the air above is more stable it will have resistance to rising. Lowering the stability lowers the resistance and results in stronger upward motion. As far as cold air damming goes, frontogenetical forcing can occur anywhere on the periphery of the cold dome, especially as warm air moving north ahead of a developing surface low pushes against the southward advance of the cold air.
Upslope that occurs as air flows into the mountains also provides lift for additional precipitation production if the air is already near saturated.
Duration and Strength of Cold Air Damming Events
The duration of an event depends in part on the strength and depth of the cold air, how efficiently the cold air is being maintained, and the various mechanisms acting to warm the air. Melting and evaporation of precipitation helps to further cool the air. These processes are critical in situations where the air is marginally cold enough to support snow or freezing precipitation. Adiabatic cooling occurs due to the upslope that occurs from the lower piedmont into the western piedmont, foothills, and mountains. In cases associated with extensive cloud cover, the clouds protect the cold air by reducing the heating from the sun's radiation.
The development of a coastal warm front and its associated trough of low pressure help to exaggerate the sharpness of the high pressure ridge to the west. The shaper ridge shape and the fact that winds blow from higher to lower pressure forces the wind into a more northerly direction to help funnel more cold air from the north. At the least, warm air near the surface off the Atlantic Ocean has a more difficult time advancing west. A coastal front frequently develops along the coast of the Southeast and Middle Atlantic states of the U.S. as a low pressure system moves northeast from near the Gulf Coast. In this case the front appears on a surface weather map as an extension of a warm front that extends east from the low pressure system and then the coastal front portion turns north up the coastline. A coastal warm front can certainly still form without the approach of a low pressure system.
Coastal warm fronts sometimes moves west with time if east to southeast winds increase ahead of a low pressure system. If the front passes a particular location then the cold air damming event is over. To the west the event continues. Warm air impinging from any direction, east, south, or west can force the retreat of the cold air and end the event. Warm air aloft can also mix with air at the top of the cold dome and erode the cold air from top to bottom. Heavy rain falling through warm air aloft will transfer heat into the cold air and warm it.
Examples of Cold Air Damming
Simple Example: January 25 to 27, 2004
Detailed Example: December 14 to 16, 2007
Cold Air Damming discussion from "theweatherprediction.com" website - Article written by Robert Meyer, website created by Jeff Haby
Cold Air Damming in the Appalachian Mountains - Alec S. Bogdanoff
A Comprehensive Climatology of Appalachian Cold Air Damming - Christopher M. Bailey and Gary M. Lackmann, North Carolina State University, Raleigh, NC; Gail Hartfield and Kermit Keeter, National Weather Service, Raleigh, NC