Southeastern U.S. Ice Storm Potential Discussion

After a very warm November across most of the United States, temperatures have gone in the opposite direction this month thus far, bringing below average temperatures to a large area. While the cold doesn’t look to be quite as expansive by the end of the month, most of us are going to have to deal with two to three more intrusions of Arctic air over the next ten days. An interesting pattern will begin to evolve by the middle of this week, which could set the stages for a potential ice storm across parts of the Southeast due to cold air damming (CAD) and a noteworthy winter storm that will trek from the Pacific Northwest to the Great Plains and eventually into the Great Lakes and interior Northeast regions. It wouldn’t even be out of the question for a changeover of frozen precipitation to occur on the backend of this system for parts of the Ohio Valley and Tennessee Valley later in the weekend, but we still have time to iron out details like that.

As I mentioned on social media yesterday and alluded to on Friday, the pattern by the end of the week into early next weekend is looking more favorable for an icing event to unfold, which would primarily impact a region stretching along the east side of the Appalachians. However, due to the progressive nature of the overall pattern, exact timing will be key, which makes this a relatively low confidence forecast. By the end of this article, I will have detailed the regions that should keep an eye on this possibility, but first, I want to discuss why the pattern could support such an event, along with what has to happen for an ice storm to occur.

The Pre-Existing Pattern:

An upper-level trough is going to begin digging into the Northern Plains early this week and will generally progress eastward as the week progresses. This trough will likely be centered over the Great Lakes/Ohio Valley region by Thursday, and an associated surface low pressure system will be near the Hudson Bay. You might be wondering what in the world this has to do with a potential ice storm this weekend east of the Appalachians, but it’s important because a cold front associated with this low pressure system is going to sweep across much of the U.S. this week, eventually making it to the Gulf Coast towards the end of the week. This will help to establish a cold and very dry air mass over the eastern two-thirds of the nation.

Surface high pressure is going to build into the central U.S. starting around mid-week, and that high is going to move eastward to the East Coast (centered over the northeastern U.S.) by the end of the week into early next weekend. This is a fairly classic way to get some form of a cold air damming scenario to set up east of the Appalachians. To put it simply, winds blowing from the northeast transports colder air southwestward, which gets jammed up against the mountains. The Appalachians act as a barrier, and the cold air basically gets stuck (jammed up against the mountains).

Take a quick look at what the latest GFS model (Figure 1) is showing for surface dew points early Friday afternoon. This is incredibly dry air with dew points down to minus 10 into South Carolina and northeast Georgia. It may seem counter-intuitive that such dry air would be necessary to set the stages for an extended wintry precipitation event, but it is necessary in some cases, which I’ll explain why in a bit.

Figure 1: Projected Surface Dew Points On Friday Afternoon, December 16th

Figure 2 is a vertical profile of temperature (red line) and dew point (green line) over Greenville, SC from the latest GFS for Friday afternoon. I hope that this doesn’t seem overly complicated, but notice how the temperature and dew point lines are very much spread apart from the bottom of the image up to a certain point. This shows just how dry the atmosphere is from the surface to about a mile above the surface.

Figure 2: A Skew-T Profile Generated By The Latest GFS Model For Friday Afternoon, December 16th In Greenville, SC

Please bear with me. This is about to make sense in a minute, I promise.

The Low Pressure System That Could Bring The Necessary Precipitation:

The storm system that is going to be responsible for this possible madness in parts of the Southeast this weekend is going to move into the Pacific Northwest later in the week, and an upper-level trough off the West Coast will move inland, which will help to maintain a surface low pressure system as it makes its way to the Great Plains. The low pressure system is going to then move in a northeast direction from the Great Plains to the Great Lakes/Northeast region along a developing jet streak. Since this system will be taking such a track, a lot of warm air from the Gulf of Mexico is going to be transported over a large section of the Southeast and even get transported as far north as Tennessee Valley and over parts of the Ohio Valley this weekend.

Let’s Make Sense Out Of All Of This:

As all of that warmer air gets transported over the Southeast, it’s not simply going to push the cold air out of the way that is wedged east of the Appalachians. It’s going to just go over it. With time, temperatures are going to warm to above freezing above the surface and will begin to mix with and erode away that cold and dry air mass. However, with such a setup, that can take quite a while, and forecast models handle such scenarios horribly.

Also, remember how I kept going on and on about how dry the air mass will be beforehand? As precipitation begins to fall through that dry air column, a process known as evaporational cooling will occur. Although this would initially prevent precipitation from reaching the ground, this would actually lead to further cooling, even if the surface high pressure to the north began to move off the East Coast.

Who Needs To Keep An Eye Out For A Potential Ice Storm:

The biggest question mark right now is. . .how soon will precipitation begin developing and moving into the Southeast? If it moves in much later on Saturday into Sunday morning, that’s going to give more time for that warm, moist air from the Gulf of Mexico to erode away that cold air mass. That will also give more time for the surface high pressure to the Northeast to move off the East Coast, which is what will be responsible for the cold air wedging. If the precipitation begins to move in Friday evening into Saturday morning, then we’re going to have a bigger ice storm potential on our hands. Keep in mind though that forecast models often overestimate how quickly the colder air will move out.

Regions that could be impacted by this potential winter event will be northeast Georgia, Upstate and northern South Carolina, central and eastern parts of North Carolina (inland zones), a large region in Virginia and maybe a bit farther north from there. From that point, temperatures should be cold enough throughout the atmosphere to support more of a mixed bag of frozen precipitation farther north before making the change to freezing rain. Regions that should avoid icy weather includes locations like northwest Georgia, most of eastern Tennessee (Chattanooga for example), all of Alabama, etc. Some of the models do show a changeover to some frozen precipitation for parts of the Tennessee Valley and extending into the Ohio Valley late weekend, but if that occurs, that would be due to Canadian high pressure behind the system transporting cold air quickly enough southeastward to result a changeover. That’s highly uncertain at this point and is something that will have to be addressed in a couple of days.

If precipitation were to start early, regions farther south into Georgia and South Carolina would have to be monitored and included in the “threat zone”

Bullet-Point Summary:

• A pre-existing cold air mass will initially be in place across the eastern U.S. with cold air damming setting up east of the Appalachians.
• A storm system will help transport warm, moist air into the Southeastern U.S. and northward. However, this warm air will initially go over the colder air before mixing to erode away the cold air mass, which will take time.
• However, the combination of cold air damming and evaporational cooling could result in an icing event that could impact northeast Georgia, Upstate and northern South Carolina, central and eastern parts of North Carolina, a large region in Virginia, and maybe a bit farther north from there.
• The quicker precipitation moves into the area, the greater the chance this has to be an impactful event. This might make it necessary to expand the threat zone a little farther south into Georgia and South Carolina, if precipitation moves in too quickly. It would also mean that the onset of precipitation might involve sleet initially before transitioning to freezing rain.
• If precipitation moves in much later, this decreases the ice storm threat quite a bit, but some icing issues could still occur. Again, forecast models often overestimate how quickly cold air will move out with such setups. This means that you should not rely on mobile weather apps during this period, as they may have temperatures warmer for Saturday than what might occur.
• Regions that will avoid an ice storm include regions like northwest Georgia, most of eastern Tennessee (Chattanooga for example), all of Alabama, etc. However, some forecast models have rain changing over to frozen precipitation late weekend as cold air rushes to the southeast. That is highly uncertain at this time and will need to be addressed in a couple of days.
• Christopher Nunley should be doing an article tomorrow on the winter weather threat across the Great Plains, which I didn’t address here.
• As far as asking questions on how much wintry precipitation you’ll get in your backyard, I have NO clue. Right now, we’re trying to simply pinpoint locations that might get any amount of frozen precipitation late week into next weekend.