Cold air damming occurs when a cold low level air mass is locked into place due to topography. One place where this commonly happens is around here, the Mid Atlantic. High pressure to the north pumps cold air into the Mid Atlantic Region. This air jams up against the Appalachian mountains, creating an extension of the high pressure along the Appalachian mountains .When a storm system arrives from the west, warm air overruns the cold, producing precip. Initially, the cold and dry air mass would cause any precipitation to evaporate, lowering the temperature of the air column through evaporative cooling, but eventually, precip will reach the ground. If the airmass is cold enough, the precip would be frozen.
Below is a graphic drawn from a model run depiction of a winter storm scenario, which demonstrates the cold air damming. The black arrows denote air flow from the arctic high to our north and the yellow lines show the isobars, which denotes a constant air pressure. Notice how a tongue of high pressure extends into the Mid Atlantic. Since the cold wedge is driven up against the mountains, precipitation over West Virginia is in the form of rain as the warm air advects (travels) northwards with the low and overruns the cold air. The mountains jut up into the warm air layer and as a result, the precipitation is rain.
The precipitation should start out as a little snow. As the warm air intrudes into the cold airmass at upper levels, the snow melts then refreezes forming sleet. As the warm air increasingly erodes the cold air, only a surface layer of cold air remains because warm air is less dense so it rises. Eventually, the cold air loses as a new low also forms to the east of the high pressure tongue. As a result, the cold northerly flow is cut off and replaced with warmer southerly and easterly flow.
The rate at which the cold air erodes is a major issue when forecasting. A difference of a degree or two could result in vastly different scenarios. Computer models tend not to do well with CAD events due to their low resolution.
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