Published: 2007-05-05 13:02:14
Updated: 2007-05-05 13:02:14
Posted May 5, 2007
MIKE MOSS SAYS: The term "damming" used in connection with weather forecasts in our area usually refers to a phenomenon known variously as a "looping warm front," a "wedge," or most commonly as Cold Air Damming, which goes by the acronymn CAD. This is a situation in which a surface-based, relatively shallow layer of cold, stable and usually dry air is trapped along the east side of the Appalachian mountains, with warmer and often more humid air flowing across the top of the colder air, leading to cloudy skies, periods of precipitation and occasionally wintry weather in the form of snow and sometimes significant freezing rain or sleet. The warmer air may take a considerable amount of time to "scour" the cooler air out of the area, leading to prolonged periods of active weather and chilly temperatures. Part of the reason for this is that the mountains act as an effective western barrier to the dense cold air, and also lifts warm air flowing in from the west above it. In addition, the warmer, more humid air is less dense and tends to ride up and over the colder air even if it starts to flow in from the southwest rather than the south.
These CAD epsisodes can develop in two basic ways, with a third method of onset that is a combination of the others. The first is called a "classical" CAD pattern, in which a strong high pressure center over the northeastern U.S. (often called a "parent" high) pushes cold air down the eastern seaboard. As the cold air build up east of the mountains, noticeable extension of the high pressure ridge develops and "protrudes" southward into the Carolinas, with a noticeable cross-isobaric north to northeasterly flow of cold, dry air into our region as the mountains block more westward movement of the airmass. As this air undercuts warmer, moister air clouds and precipitation may develop (this can be enhanced by low pressure nearby, often south of us at the surface and west of us in the mid- or upper atmosphere). The precipitation falls in to the dry, cold air near the surface and some fraction of it evaporates, leading to an even colder near-surface airmass and a stronger cold air "wedge."
A second way for the CAD event to develop involves cool, dry air in place east of the mountains that is not strongly associated with a parent high and sometimes not terribly distinct from surrounding airmasses. However, if the air has low dew points and a disturbance nearby leads to precipitation into this airmass, it may rapidly cool relative to surrounding air, becoming more dense and forming a high pressure ridge, after which it beahves in a similar manner to the "classical" CAD, though usually not as long lasting or intense. This type of event is called "In Situ" CAD because it forms in place without significant assistance from a parent high.
There are also situations in which there is a weak or poorly positioned high to our northeast, and moderately cool dry air already in place, neither of which might lead to a strong CAD even separately, but which combine in just the right way with precipitation in the area to create a notable wedge pattern. These are called "hybrid" damming events.
Any of the CAD events I mentioned can create difficult forecasts for cloud cover, ceiling height, precipitation type and temperature, as computer models often tend to push the cold air out of the region more rapidly than reality. Figuring out how much more rapidly is not easy, however. In addition, the cold wedge may hold its position across much of the Piedmont, while it mixes out across the southern mountains and across the coastal plain, leading to some days with very large differences in temperature from one part of our viewing area to another. On the one hand, they are a fascinating phenomenon and very interesting to deal with as a meteorologist. On the other hand, while we do quite well with some CAD episodes, there will always be a few that lead to "busted" forecasts!
You can read more about cold air damming, and see some helpful illustrations, at
This next address is for a downloadable PowerPoint presentation on Cold Air Damming...