Airmass damming

The concept of air damming is an interesting one. Traditionally this is expected with cold air, and namely in the mid Atlantic region of the US. It essentially entails a high pressure area moving over a mountainous area introduces cold air and traps it, while forcing it further southward. Cold air cannot rise easily and is stuck near the surface, so when a force (high pressure area) is compressing it, it must take the path of least resistance (further south, along the mountains). Heres a quick and easy diagram.

This is usually present in autumn or winter. Interestingly this mechanism is also present right now with the smoke outbreak, which is why states like South Carolina and Virginia will see extremely poor AQI values. Here is an image from the 12z HRRR (high resolution rapid refresh).