Cold-air damming is a weather phenomenon that happens quiet often in Central Pennsylvania. It plays a major role in our local weather. It is when cold/cool moist air dams up against a mountain slope and settles in the valley. This phenomenon happens east of the Appalachians, Rockies and Cascades.
When the winds have a easterly flow cold/cool moist air from the Atlantic Ocean is moved inland. The cooler air is more dense (heavier) and has a very hard time going over the mountains. The cool moist air settles in the valleys east of Interstate-99. The eastern counties (Centre, Blair, Huntingdon, and Bedford) will be cooler and cloudier compared to the western counties. There could also be drizzle, sprinkles and showers over the eastern counties. While, the western counties are dry. Cold-air damming may also lead to freezing rain and ice during the winter months. A shallow pocket of cold air (air below 32 degrees) settles in the deeper valleys while the air aloft is warmer (air above 32 degrees). This is a classic setup for icy and slipper roads when rain is passing over the region.
High pressure north-northeast over Pennsylvania is what can cause cold-air damming to occur. The winds around a high are clockwise. This setup brings cool moist air from the Atlantic ocean and bring its into Central PA.