2A basic rule of thermodynamics is that cold air is denser than warm air, and one of the more significant manifestations of this rule occurs in the winter, with cold air damming. There are other examples of this same phenomena in the High Plains (inversions) and out west, but the consequences of this process are more evident along the Appalachians, so we will discuss that region here.
Cold air damming along the east coast shows up most evidently when cold high pressure is being supplanted by warmer air. Southwesterly, warm flow moving from the Gulf and Lower Mississippi Valley rides over the Appalachians and then can’t descend into the lee of the Appalachians. Warm air can sneak around the cold pool and up the coast, and is lifting northward more rapidly west of the mountains, so it is not uncommon to see surface analyses that look like this:
or temperature analyses that look like this
While there is the immediate impact, in that temperatures fail to warm as swiftly as residents of locations like Roanoke or even Washington DC might appreciate, the occurrence of damming also leads to another threat: Freezing precipitation. Warmed moisture falling through a colder surface layer leads to a higher probability of freezing rain, sleet, or, depending on the depth of the cold air, snow.