January 21, 2010 - The Scoop on Cold Air Damming
What is Cold Air Damming and why does it occur here? Read all about it Mallory's Weather Blog!
Sometimes I think it's easier when we have a huge snowstorm to forecast. At least we always know the precipitation type: snow. Whether it's 6” or 26”, it's all snow. It's cold air damming events - like the one we will be experiencing Thursday night and Friday - that make us meteorologists go gray.
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Cold Air Damming (abbreviated as CAD) is one of those phenomena that needs the perfect combination of atmospheric elements to occur. It's usually the reason for prolonged cold, fog, and yes, wintry weather in Valley.
CAD is known as a mesoscale weather phenomena. Mesoscale is a fancy term for small scale. Other mesoscale phenomena include lake effect snow, sea breezes, and thunderstorms. Damming occurs in few places in United States, but mainly along the eastern slopes of the Appalachians and Rockies.
Sometimes CAD sets up and it's no big deal. The temperature may be a little bit cooler and some extra clouds will float around the Valley. It gets a “bad rap” when another system happens to be affecting the Valley during wintertime.
High pressure over southeastern Canada or New England starts off the CAD process. Clockwise winds around high pressure bring northeast or east winds to our area. The eastern wind component pushes up against the east side of the Appalachians. Without strong momentum in the atmosphere, the cold air remains trapped on the east side of the mountain range.
On the Weather page of www.whsv.com, look at the state temperature map during January 21st and 22nd - notice the cooler Valley. There may not be a drastic difference in temperature from Shenandoah Regional to Richmond, but it may be just enough for the Valley to see ice and Richmond to see rain.
If the surface layer is below freezing, then why don't we see snow? It's simple. The cold air is trapped in the lowest 5,000 feet of the atmosphere. When we have low pressure moving in from the west, southerly winds ahead of the cold front begin to overrun the shallow cold air (refer to the Jan 20th blog for more on overrunning) already in the Valley. This sets us up for mixed precipitation events. Precipitation will fall through the warm layer aloft (usually above 5,000 ft), melt, and then fall through the shallow cold air mess. The depth of the cold air mass determines whether we see sleet or freezing rain.
What are those exact specifics? I'll have those details in Friday's blog.