We've had two weeks worth of 90-degree heat in September, but there's some relief from the heat coming!
It comes on the heels of a pattern that's somewhat unique to our area. It's called "cold air damming" or "the wedge."
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What is cold air damming?
Without the presence of the Appalachian Mountains, this pattern wouldn't even be a thing.
Dr. Gary Lackmann devotes an entire chapter to this in his textbook, which we covered extensively my senior year at NC State.
It develops when there's an easterly component to the wind.
In a more classic setup (like the one we see this week), the wind is coming in out of the northeast. That's cooler air that's being transported down into the Carolinas.
Air rises along the mountains in response to this (technical term is ageostrophic motion).
We, then, get a layer of warm, moist air above 3,000 feet. This layer of warmth traps our cool, stable air at the surface.
This will lead us to be cooler than surrounding areas like Georgia, Tennessee and the coasts of South and North Carolina.
High temperatures within the "CAD" region Wednesday and Thursday will be 6 to 12 degrees below average for late September.
How often does this happen?
The most recent study I could find is from 2016, but its results confirm what was previously studied.
Cold air damming is most frequently seen in the fall and winter months, and it is pretty rare in the summer.
September is one of the months in which CAD happens most frequently in the Eastern U.S. During the winter, it is oftentimes a catalyst for winter weather.
How long has it been since we've been this cool?
If you look at our extended forecast, you'll see a few afternoons below 70 degrees and a few morning lows below 55 degrees.
It's been nearly four months since we've seen afternoons in the 60s, and it's been about three-and-a-half months since we've seen mornings below 55 degrees in the Triangle.
