An increasingly dynamic and potentially impactful weather pattern is unfolding across the Southeast and Mid-Atlantic as we move through Sunday and into Monday, with conditions becoming favorable for a widespread severe weather event across portions of the Carolinas and surrounding states.
The setup is being driven by a powerful upper-level shortwave trough that is amplifying across the central United States while moving eastward toward the Appalachian region.
At the surface, a deepening area of low pressure is tracking from the Midwest toward the Great Lakes while a strong cold front surges eastward across the Mississippi and Tennessee Valley.
Strong southerly winds in the lower atmosphere will transport increasing moisture northward in a process known as moisture advection allowing dewpoints to climb into the 60s across much of the Southeast and even reaching as far north as portions of the Mid-Atlantic.
As this warm and increasingly humid air mass spreads northward, instability will gradually build across the region.
Forecast guidance suggests that modest to moderate CAPE values will develop across much of the Southeast and Mid-Atlantic, generally ranging from a few hundred to perhaps over 1,000 joules per kilogram in some areas. While these values are not exceptionally high for severe weather events, they will be more than sufficient when combined with extremely strong wind shear across the region.
One of the most notable aspects of this setup is the strength of the low-level jet. Forecasts indicate that winds at approximately 850 millibars, which is about 5,000 feet above the ground, could increase to between 50 and 75 knots across parts of the Southeast and Mid-Atlantic. These strong winds will dramatically increase wind shear across the region and provide the necessary energy to support rotating thunderstorms.
(Sun 10 pm)
(Mon 12 pm)
As the cold front pushes eastward, thunderstorms are expected to increase in coverage and intensity across the Mississippi Valley and Tennessee Valley before spreading toward the Carolinas late Sunday night into Monday. Early storms may initially form along the cold front or within the warm sector ahead of it. Some of these storms could briefly become supercells (a type of thunderstorm characterized by a rotating updraft called a mesocyclone).
Supercells are capable of producing all types of severe weather including large hail, damaging winds, and tornadoes. However, the strong forcing along the advancing cold front will likely favor the development of a more linear storm mode as the event progresses. When storms develop along a strong cold front with intense lifting, they often organize into a squall line, which is a long line of thunderstorms capable of producing widespread damaging winds.
Forecast models suggest that a robust squall line could develop and extend from the Carolinas northward toward the Mid-Atlantic by Monday afternoon. This type of storm structure is commonly referred to as a QLCS, or Quasi-Linear Convective System. (A QLCS is essentially a long line of thunderstorms that can produce powerful straight-line wind gusts along its leading edge.)
Straight-line winds occur when strong downdrafts within thunderstorms push air outward along the ground, creating damaging wind gusts that can topple trees and power lines. Embedded within this line of storms, smaller circulations can develop that occasionally produce brief tornadoes. These are commonly referred to as QLCS tornadoes. They can still be damaging and occur with little to no warning.
Timing again plays a major role in determining the magnitude of the severe weather threat across the Carolinas tomorrow. Current guidance suggests that a line of storms moves in between 5 AM and 11 AM across the mountains and western Piedmont. The storms are then expected to continue eastward across central portions of the state from late morning into early afternoon before reaching the coastal plain later in the day.
Mon 9 am
Mon 1 pm
Some areas across the western Carolinas, including the foothills, may experience the front before peak daytime heating occurs, which could limit instability. However, strong wind shear and forcing will likely compensate for this and still support severe storms.
Areas farther east, including the Charlotte metro and portions of the Highway 16 corridor, may see storms move through closer to peak heating which could allow slightly greater instability to develop.
Behind the departing system, attention will shift toward the development of northwest flow snow across the higher elevations of the North Carolina mountains.
As temperatures fall steadily Monday afternoon and evening, rain showers across the mountains and potentially the Northern Foothills will transition to snow, particularly along the North Carolina and Tennessee border where elevations are higher and temperatures cool more quickly.
As the storm system moves away from the region, a much colder and drier air mass will settle across the Southeast through the middle of the week. With high pressure in place, overnight temperatures Monday night through Wednesday night are expected to drop near or below freezing across many areas.
These colder temperatures could pose a concern for early spring vegetation, especially sensitive plants that may have begun blooming during the recent stretch of unusually warm weather. Frost and freezing conditions during these nights could damage vulnerable plants if precautions are not taken.