A strong cold air damming event took place early this week, leading to dangerous ice accumulations from freezing rain and sleet in areas from Pennsylvania to North Carolina. Cold air damming is one of the most interesting weather events in the United States, and can be one of the hardest to accurately predict.
Cold air damming is a phenomenon that occurs when strong winds from the north/northeast allow cold, dense air to push up against the Appalachian Mountains. As cold air is denser than warm air, the cold air stays close to the surface and can't ride up and over the mountains. Since it can't go up, it spreads out and floods the Piedmont region with a relatively shallow layer of cold air that's capped above by warmer air. The region of cold air that gets dammed by the mountains is often called a "cold air wedge."
Aside from looking at a temperature analysis like the one posted above, this cold air damming is best seen by looking at a model-generated upper-air observation for Greensboro, NC yesterday afternoon.
The diagonal blue dotted lines that stretch from bottom-left to top-right are temperatures in Celsius (where 0 is freezing).
The red line shows the temperature of the atmosphere as you increase with altitude (shown in thousands of feet by the horizontal gray lines), and the green line shows the dew point as you increase with altitude.
You can see that air temperatures cool from ground up to around 2,000 feet above ground level, at which point they sharply begin to warm up between 2,000 and 3,000 feet. This dramatic warming of the atmosphere is called an inversion. The model sounding shows that the cold air wedge in Greensboro was about 2,000 feet thick yesterday afternoon.
This persistent cold air damming allowed for sleet and freezing rain to cake areas with ice, but especially hit the North Carolina Piedmont hard. Some areas from Charlotte to Greensboro recorded over one quarter of an inch of ice along with sleet, leading to slick roads and adding to the list of snow days some school systems will have to make up.
Cold air damming events are responsible for some of the worst ice storms ever recorded in the southeastern United States, including one that occurred in December 2002 that left over an inch of ice accretion from freezing rain near Raleigh, NC.