Cold air damming affects weather patterns near mountain ranges by creating colder temperatures and more precipitation than normal. Major areas affected by cold air damming include the Appalachian Mountains in the east, the Rocky Mountains and the Cascades in the west. However, the Appalachian experiences this two to three times per month beginning in October and lasting through April. Temperature differences of up to 36F can occur. Damming occurs when high pressure parks itself near a mountain range.
Three types of weather related conditions are the wedge, blocking and seeping. Freezing rain and sleet develop from the wedge. This occurs when warmer air sits over the cooler air. The thicker the layer of cold air, the more likely it is that sleet will form.
Blocking occurs when high pressure gets in front of another storm and combines. This causes the storm to stop, making a storm stay in place for much longer than forecast. It can lead to flooding. Seeping brings an increase to precipitation and even snow if the system is strong enough. It makes rainy conditions much more wintry.
Cold air damming makes forecasting difficult because no one knows when this will occur. Computer models still are not sophisticated enough to address this condition. Winds play a big part in damming for if the conditions are right, the high pressure with not climb above the heights of the mountains. This is classical damming.
Hybrid damming and in situ damming along with classical damming are primary processes of this weather pattern.
The tops of mountain ranges generally produce warm air. This warm air works like the cover on a pot keeping the high pressure in place. If the air on the top of the mountain cools, then the cold air dam will not last long. Pressure amounts called signatures include 850 mb, 500 mb, and 300 mb. Each has a particular condition associated with it. Those with 500 mb are classical cold air damming events. Those with 300 mb encourage the development of cold air dams where the polar jet is above the surface high. The dams with 850 mb take place over the mid-Atlantic states as far south as Alabama. Canadian highs make these dams.
Upslope and cool-air pooling are commonly confused phenomenon which have some of the same characteristics but not the most significant ones. A dam must three specific characteristics: a mountain range; a parent high; and damming signatures.
Excellent examples of these conditions include the most recent blizzards of 2006, 2003, and 1978.