The Ideal "Wedge" set-up for the East Coast. Appalachian Mtns and High pressure. Image Credit: worldatlas.com (arrows and letters added by me)
Known areas of cold air damming around the world. Image Credit: meted.ucar.edu
Known areas of cold air damming around the world. Image Credit: meted.ucar.edu

Welcome to the second edition of Meteorology 101! In this edition, we’ll be talking about a phenomenon known as Cold Air Damming. We’ll examine what causes these kinds of events to occur and what kind of weather they bring to North Georgia and the Athens area. If you’ve been following our daily weather posts over the past year, you’ve seen us mention a “wedge of cool air,” or “The Wedge,” every so often.  As you can see from the map above, Cold Air Damming (sometimes abbreviated as CAD) occurs in many different locations across the world. To put it simply for now,  Cold Air Damming occurs when cold air becomes entrenched along the eastern slopes of mountain ranges.

Cold Air Damming on the East Coast:

Let’s see, I mentioned something about mountain ranges being important in this process. Do we have one of those around here? Yes we do! The Appalachian Mountains are integral in the cold air damming process on the east coast. What else do we need? We need some mechanism to push the air up against the Appalachians. That’s where our high pressure system comes in. Ideally, the area of high pressure will be located somewhere around New England. The location and strength of this area of high pressure will determine the duration and strength of the wedge of cool air.

The Ideal "Wedge" set-up for the East Coast. Appalachian Mtns and High pressure. Image Credit: worldatlas.com (arrows and letters added by me)
The Ideal “Wedge” set-up for the East Coast. Appalachian Mtns and High pressure. Image Credit: worldatlas.com (arrows and letters added by me)

As high pressure settles into the Northeast US, winds will begin flowing in a clockwise direction. Winds will then flow out over the Atlantic Ocean, bringing a moist flow ashore. As these winds begin turning to the northwest, to complete their journey around the high pressure center, they encounter the Appalachian Mountains. The Appalachian Mountains act as a barrier, and when the east winds encounter this barrier, the flow becomes impeded resulting in cool air spilling down the leeward side of the Appalachian Mountains. Warmer air closer to the coast creates a nice boundary where the cool air can funnel down into the Piedmont. The cool air slowly spills south, and eventually enters Northeast Georgia and the Athens area. See the diagram below for a visual depiction!

Great diagram depicting how the cool air is forced south down the Appalachian Mountains. Image Credit: http://wnct.images.worldnow.com/
Great diagram depicting how the cool air is forced south down the Appalachian Mountains. Image Credit: http://wnct.images.worldnow.com/

How Can You Identify a Cold Air Damming Event?

An easy, though not surefire way to identify whether or not “The Wedge” will develop is to look at a surface weather map where isobars are plotted. Isobars are lines of constant pressure. If you look at a surface weather map and observe a high pressure center near New England, you want to also look down near Virginia, North Carolina, South Carolina, and Georgia. If you see isobars that form a “U” shaped pattern, it’s a good indication that Cold Air Damming is occurring. Here’s an example of a surface weather map in which The Wedge is present.

August 17, 2013, Wedge event. Remember the "cold snap" we had in August? Image Credit: NOAA. (Annotations by me).
August 17, 2013, Wedge event. Remember the “cold snap” we had in August? Image Credit: NOAA. (Annotations by me).

Remember the cold snap we had in the middle of August 2013? High temperatures in Athens only reached 68°F and 66°F on August 16th and 17th, respectively. This was the result of a Cold Air Damming event. On August 17th, an 1028 mb area of high pressure was located near New York State with an area of low pressure along the Mid-Atlantic coast. This set-up allowed a shallow layer of cool air to seep down into North Georgia, and damp, dreary conditions were also present.

So in review, look for an area of high pressure around New England, and then look to see if the isobars are in a “U” formation around the Piedmont.

What Kinds of Weather can The Wedge bring?:

Depending on the time of year, the weather that Cold Air Damming events cause can be drastically different. Most CAD events occur in the fall and winter months, though given the right setup, they can occur during summer months as well. Summer Wedge events are typically not as strong as their winter counterparts, but they can provide a break from the humid and hot weather we see in Georgia. However, in the winter months, Wedge events can bring some dicey weather to North Georgia. If precipitation is present during a Wedge event in the winter, a myriad of precipitation types can occur. Snow, sleet, freezing rain, or just plain rain can affect some regions, especially North Georgia. The next edition of Meteorology 101 will break down precipitation types, so no worries if you’re not familiar with those. We’ll break them down in more detail soon!

Many of Georgia’s ice storms are the result of Cold Air Damming events. When temperatures are freezing at the surface, rain that falls will freeze on contact to objects near the ground, causing a multitude of problems.

Radar Image from an icing event on December, 15, 2005. The pink shading indicates an area of mixed precipitation. Image Credit: weatherunderground.com
Radar Image from an icing event on December, 15, 2005. The pink shading indicates an area of mixed precipitation. Image Credit: weatherunderground.com

A Cold Air Damming event accompanied by an area of low pressure in the Southeast US brought freezing rain to Athens, Georgia, on December, 15, 2005.

Wrapping up The Wedge:

In summary, The Wedge is another name for Cold Air Damming. It is shallow, cool air that “wedges” down the eastern side of the Appalachian mountains and builds across the Carolina’s and into Georgia.  We’ve reviewed the necessary ingredients we need to have to see a Wedge (a mountain range and high pressure), and the process which causes cool air to ooze its way down the spine of the Appalachians. You can also now identify whether a Wedge is happening by looking at a surface weather map, and you have an idea of the types of weather a Wedge event can bring to North Georgia and the Athens area. Now you’ll know what we mean if you see us forecast something like: “Tomorrow we’ll see temperatures in the mid 40s as a Wedge builds in due an area of high pressure in the Northeast US. Expect gusty northeast winds as well with a chance of drizzle and showers.” Wedges can present major forecasting problems because our weather models have a hard time detecting them. Sometimes, our forecasts can be wrong because our models don’t estimate the intensity of the Wedge. If it builds in, it could easily lower our temperatures by 10-20 degrees below average. Then again, if it goes away fast, we could see temperatures jump 10-20 degrees as the shallow layer of cool air retreats. Tricky stuff!

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