Cold Air Damming Makes Freezing Rain Likely Thursday
It looks like things could get fairly nasty Wednesday Night and into Thursday for much of the Atlanta area north of Interstate 20, and especially for the northeast counties, including Gwinnett. An incoming low pressure system in the Gulf of Mexico will bring precipitation to Georgia beginning Wednesday morning. Meanwhile, the high currently over the eastern part of Kentucky will move northeast, running into the Appalachians, and causing a ‘wedge’, or cold air damming to affect northeast Georgia.
What happens is that the colder air gets trapped on the western edge of the mountains, and the approaching lower pressure, warmer air slides above the colder, heavier air. You can tell when we’re in a CAD situation when the wind comes from the east, and it doesn’t warm up much during the day.
The presence of warm air above cooler closer to the ground tends to favor the formation of sleet or freezing rain, since the warm air layer will melt any snow as it passes through, while the cold air close to the ground will either re-freeze the precipitation, or cause freezing when the water hits the surface.
In the last day, forecasters have increased the total amount of expected precipitation from the storm to almost an inch, while dropping the low temperature Wednesday night, especially in the northeast counties most affected by the wedge.
Here’s what to expect tomorrow in the Gwinnett area:
7 AM - The leading edge of the precipitation moves to the Atlanta area, but nothing may hit the ground because the air is so dry. Look for dew points to increase in the morning, and the temperature to rise above early morning below freezing lows.
11 AM - Light rain begins, with temperatures above freezing, so no problems.
7 PM - After sunset, temperatures begin to cool, and the rain increases in intensity. We may see a little sleet in the early evening.
11 PM - Temperatures drop below freezing. Continued rain and sleet, with some freezing rain.
4 AM - The intensity of the rain begins to slow, but because temperatures have been below freezing, we’re probably seeing freezing rain at this point.
11 AM - Temperatures rise above freezing, as most of the rain has passed from the area.
Of course, this is just my best guess, and things could change. The further south and west you are, the better the chances that this is just a rain event. In any case, Thursday morning could be nasty.
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