A massive winter storm will affect nearly half of the U.S. population before it is over. The sprawling storm will produce hazardous conditions from the southern Rockies to Washington, D.C.
On the northern flank of the storm, snow will be the primary hazard, but the southern part of the storm will feature ice. Meteorologically speaking, there is a delicate dance between moisture, cold air and other atmospheric processes that determines whether a region gets rain, ice, sleet or snow. Within this storm, a phenomenon that meteorologists call “the wedge” is shaping up to be a key player for some southern states. I’ll explain.
The National Weather Service has issued a vast array of watches and warnings across the U.S. as of Friday morning. Much of the messaging along the southern swath of the storm is because of the potential for freezing rain or sleet. In the graphic above, parts of Georgia and the Carolinas even have explicit ice storm warnings posted. Here in Georgia, it is the first ice storm warning in over a decade.
Before I dig into the role of cold air damming and “the wedge,” it is instructive to review the graphic below. It is a good primer on why some places get different types of precipitation. For snow, the temperature must be below the freezing point from cloud base to the surface. That will certainly be the case on the northern side of this storm. However, there are points southward in which warmer air above the surface will limit snow and favor sleet or freezing rain.
At this point, I should remind you that most of our precipitation in the U.S., even rainfall in the summertime, begins as ice in the clouds. With a storm like the one being monitored for this weekend, a thin, elevated warm layer can produce sleet. In fact, there is a likelihood that sleet will mix in at points along the southern precipitation region. In the Washington, D.C., and Baltimore areas, the sleet mix might keep snow totals down from maximum potential, but a significant snowfall is still likely for that region.
A deeper warm layer will cause melting. If the warm layer extends to the ground, that leads to “good ole rain.” Later in this storm, warm air will poke its way into parts of the Southeast to produce rain. However, there are many situations in which there is a shallow layer of sub-freezing air at the surface. In parts of the Southeast this weekend, that will be the case. When melted ice falls through the warm layer, it hits the surface as a liquid then refreezes within the sub-freezing air at the surface. That’s the distinguishing factor between freezing rain and sleet (ice pellets). Remember, sleet is solid pieces of ice that bounce.
We can now talk about the importance of the wedge for this storm. “When conditions are right, a strong flow of cool, high pressure air flows south along the east side of the Appalachian Mountains into Georgia, bringing air that is cooler, usually drier, and higher pressure than the air it replaces,” my colleague Pam Knox wrote in 2019 in her outstanding blog.
The formal meteorological term is “cold air damming.” The National Weather Service glossary defines CAD as, “The phenomenon in which a low-level cold air mass is trapped topographically. Often, this cold air is entrenched on the east side of mountainous terrain.” It is often called a wedge because of the shape of the pressure lines and temperature contours on a map.
Let’s examine the weather scenario forecasted for Sunday morning (graphic below). Do you see the “wedge-looking feature” in the orange and black lines protruding southwestward into the Carolinas and Georgia? That’s cold-air damming against the Appalachian Mountains. Why does it happen?
It can be a bit complex, but I found a website at North Carolina State University that presents it at an appropriate level for the public. “Winds circulate in a clockwise direction around high pressure in the northern hemisphere, so in this type of pattern, cold air funnels in from the north or northeast and pushes up against the Appalachian Mountains,” it explained.
The website went on to say, “These wedge patterns can be notoriously stubborn, especially if the parent high to our north is nearly stationary, since the winds around that high reinforce the cold air near the ground.” Wedge events are often associated with low clouds, which limits solar heating. Additionally, if there is precipitation falling into the wedge, evaporation can further lower its temperature.
The wedge is a critical player for the ice storm in the Carolinas and Georgia this weekend. Brad Johnson, a meteorologist and professor at Florida State University, he texted, “What a phenomenon the wedge is. In its absence North Georgia and Charlotte might’ve been spared.” He’s right. The cold air source for this region is the wedge.
John Knox, at the University of Georgia, is one of the foremost experts on the wedge. “The wedge is usually stronger than the models think and erodes more slowly,” he wrote in an email. Based on studies from his research group, the wedge usually penetrates south of Athens and can reach the Atlanta area and beyond. “I would not be surprised if the Ice Storm Warning is expanded,” he added.
Because of the regional impacts of the wedge, Knox and colleagues recently proposed a comprehensive field campaign called “WedgeEx” to study the phenomenon, but it was not funded. “Forecasting the wedge is difficult, both in terms of intensity and duration, but there hasn’t been enough research done to gather detailed information on the cloud microphysics of these situations to improve the forecasting of it,” he wrote.
Knox concluded with some big-picture context about the rest of the storm. “In terms of areal extent, I’m thinking this is the biggest snowmaker event since the Storm of the Century in 1993,” he said. “The potential for a foot of snow nearly continuously from Oklahoma to Maine is astounding.”
I had similar thoughts in one of my articles earlier in the week.
