Cold-Air Damming occurs when cold air is trapped on the east side of the Appalachian Mountains in Central Pennsylvania. The easterly surface winds push cold air against the east side of the mountain. Since cold air is more dense than warm air it has a hard time climbing over the mountain. The cold air then ends up pooling in the valleys. This keeps the valleys colder than the higher terrain.
Cold-Air Damming plays a significant role in Central Pennsylvania when it comes to what kind of precipitation we get. It can determine if we get sleet, freezing rain or rain. This time of the year precipitation normally start out as snow but as the snow falls it falls through warm air (air above 32 degrees) melts and becomes rain. Depending on how deep that cold pool of air is will determine the type of precipitation. If the cold pool is deep enough the rain will refreeze and turn into an ice pellet becoming (Sleet). If the cold pool is shallow the rain will continue to fall as rain but freeze on contact becoming (freezing rain).
Eventually the warm air aloft will mix with the colder air in the valleys and that will slowly warm the air up.