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It's a term that's been tossed around the weather center recently - cold air damming. We're very prone to this weather phenomenon in the Northern Shenandoah Valley thanks to our location between the ridges. Cold air damming is when low level cold air is trapped by terrain (like mountains.) It is common and most strong in the winter months, but it can happen all year round. It occurs along the eastern slopes of mountain ranges.
A typical set-up for cold air damming here would be an area of high pressure over New England and low pressure to our south. The flow around high pressure is clockwise, and the flow around low pressure is counterclockwise. Both of those combined in the right location promote on-shore (or easterly) flow. That means the air blows over a relatively cool Atlantic ocean and deposits that cool, damp air right here in the Valley. In the winter time, the flow can trap a shallow cold air mass in the Valley, leading to freezing rain and sleet situations.
Cold air damming brings us some not-so-nice weather conditions. We have widespread cloudiness, persistent cold temperatures, and sometimes precipitation.
With the most recent winter storm we received a mixed bag of snow, sleet, and freezing rain. This was due to cold air damming. High pressure was over New England, and low pressure formed over the Southeast. A wedge of cold air got trapped in the Valley, while warmer air from the low was transported north, over top of the cold air. As the precipitation started to fall, it melted from snow to rain. When those raindrops encounter the cooler layer of air, they froze into sleet.
A cold air damming event will continue until either warm air takes over in all levels of the atmosphere or when a cold front pushes through and shifts the wind from an easterly flow to a westerly flow.
Finally, weather models often underestimate the effects of cold air damming, so it can make for a tricky forecast.
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