A potentially significant severe weather setup is beginning to take shape across the Carolinas and much of the Mid-Atlantic as we head into Monday as a powerful spring storm system organizes across the eastern United States. We are increasingly confident that widespread severe thunderstorms will develop across the Southeast and Mid-Atlantic as a large and deepening upper-level trough moves eastward out of the Mississippi, Ohio, and Tennessee Valleys and crosses the Appalachian Mountains. (An upper-level trough is an elongated area of lower pressure and colder air aloft that promotes rising motion in the atmosphere, which is a key ingredient for thunderstorm development.)
As this trough strengthens and ejects northeastward, a powerful ribbon of winds in the mid and upper levels of the atmosphere will overspread the region, providing the dynamic support necessary for organized severe storms to develop and intensify.
At the surface, a strengthening low pressure system is expected to track from the Great Lakes into southern Quebec while a deep pre-frontal trough extends southward through Pennsylvania, Virginia, and the Carolinas. (A surface low refers to an area of lower atmospheric pressure where air converges and rises, often serving as the organizing center for storm systems. A pre-frontal trough is a zone of shifting winds and lower pressure that forms ahead of an approaching cold front and can act as a focus for thunderstorm initiation.)
Ahead of this system, strong southerly winds will transport warm, humid air northward into the region in a process known as moisture advection, which simply means moisture being transported into an area by the wind.
Dewpoints are expected to climb into the upper 50s to lower 60s by Sunday night into Monday, indicating a much more humid and unstable air mass moving into the Carolinas and surrounding areas. (Dewpoint is a measure of atmospheric moisture and higher values generally support stronger thunderstorm development.)
As this moisture increases and wind fields strengthen throughout the atmosphere Sunday Night into Monday, conditions will become increasingly favorable for severe thunderstorms despite only modest levels of instability.
Thunderstorms are expected to develop along and ahead of a powerful cold front extending from the Ohio Valley southward toward the Florida Panhandle early Monday. (A cold front is the leading edge of a cooler and denser air mass that forces warm air ahead of it to rise rapidly, often triggering thunderstorms.)
Storms may already be ongoing along portions of the front Monday morning, but additional development is expected as the atmosphere gradually destabilizes through the day. (In meteorological terms, destabilization refers to the atmosphere becoming more favorable for rising air parcels, which is necessary for thunderstorm formation.)
We believe that the air mass ahead of the cold front across the Carolinas will remain largely uncapped. (A cap refers to a layer of warm air aloft that suppresses thunderstorm development by preventing rising air from reaching the levels necessary to form storms. When the atmosphere is uncapped, storms can develop more easily if other forcing mechanisms are present.)
This means that even relatively small disturbances or lifting mechanisms could trigger thunderstorms earlier in the day before the main squall line arrives. A few of these early storms could become supercells, which are thunderstorms characterized by a persistent rotating updraft known as a mesocyclone. Supercells are particularly concerning because they are capable of producing large hail, damaging winds, and tornadoes.
As the day progresses and the cold front moves through the foothills and closer to the Central and Eastern regions of the Carolinas and Virginia, the greatest severe weather threat will likely occur during the afternoon when daytime heating combines with falling surface pressure to increase instability across the region.
(Instability is often measured using a parameter called CAPE, or Convective Available Potential Energy. CAPE represents the amount of energy available for thunderstorms to grow vertically, with higher values indicating stronger potential storm updrafts.)
Forecast guidance suggests that pockets of surface-based CAPE could exceed 1000 J/kg in parts of the Carolinas and Virginia. While this level of instability is considered moderate, the presence of very strong wind shear will compensate and allow storms to organize effectively.
(Wind shear refers to a change in wind speed or wind direction with height in the atmosphere. Strong wind shear helps thunderstorms maintain structure and can allow them to rotate.)
In this case, we are monitoring very strong low-level wind fields that could support rotating storms. One key parameter is storm-relative helicity, often abbreviated as SRH.
(Storm-relative helicity measures the potential for air to rotate within thunderstorms, particularly in the lowest kilometer of the atmosphere.)
Forecast values between 300 and 500 meters squared per second squared are being indicated by several models. For context, values above about 150 are often considered supportive of tornado-producing storms, meaning this environment could be quite favorable for rotating supercells capable of producing tornadoes.
During the afternoon hours, a few discrete supercells may develop ahead of the main squall line. ((Discrete storms refer to individual thunderstorms that form independently rather than merging into a continuous line. These storms often have the greatest tornado potential because they can fully ingest warm, unstable air without interference from neighboring storms.))
Eventually, however, storms are expected to consolidate into a squall line along the advancing cold front. (A squall line is a long line of thunderstorms that typically produces widespread damaging wind gusts as it moves across an area.)
Within this line, embedded circulations may still develop, which can occasionally spin up brief tornadoes even within the larger line of storms. These brief spin ups on Monday could be high end tornadoes along and east of Highway 18 due to the amount of wind energy and wind shear available.
The primary hazard with this system is expected to be widespread damaging straight-line winds. (Straight-line winds occur when strong downdrafts within thunderstorms spread outward along the ground, producing powerful gusts capable of downing trees and power lines.)
However, the strong wind shear and favorable helicity values mean that tornadoes remain a secondary but important concern. Depending on how much the atmosphere destabilizes Monday afternoon, a few stronger tornadoes cannot be ruled out, especially across portions of the Carolinas and Virginia where the overlap of instability and wind shear may be greatest.
One of the greatest uncertainties with this setup will be the exact timing of the cold front and the thunderstorms associated with it. If the line of storms moves through earlier in the day before peak heating occurs, instability may remain somewhat limited which could reduce the overall severity of the storms. However, if storms linger into the afternoon or if additional development occurs ahead of the main line, the severe threat could increase substantially and quickly.
The orientation of the deep-layer shear vectors relative to the cold front will also play a role in determining the tornado risk. (Deep-layer shear vectors describe the direction and strength of winds throughout a large portion of the atmosphere.) If these winds are oriented more perpendicular to the front, storms may be more likely to rotate and produce tornadoes. If they are more parallel to the front, storms may form primarily in a linear mode, which tends to favor damaging winds rather than tornadoes.
So we will be monitoring wind vectors for areas where winds “back” into the prefrontal boundary. (Backing winds are winds that develop in smaller areas along a gust front creating winds perpendicular to the advancing frontal winds)
Regardless of the exact details, the overall atmospheric setup strongly suggests that a line of strong to severe thunderstorms will sweep across the area late Sunday night through Monday afternoon. Residents across the Carolinas and surrounding areas should begin preparing now by reviewing severe weather safety plans and ensuring they have multiple reliable ways to receive warnings should they be issued.
Once the severe weather threat moves east Monday evening, the focus will quickly shift to a dramatic change in the weather pattern behind the departing cold front. Much colder air will surge into the southern Appalachian region as strong northwest winds develop.
This pattern will create the potential for northwest flow snow across the higher elevations of the North Carolina mountains. (Northwest flow snow occurs when cold air moving from the northwest is forced upward as it encounters the Appalachian Mountains. As the air rises over the terrain, it cools and condenses, producing clouds and snow showers on the windward side of the mountains.)
As temperatures drop below freezing Monday afternoon and evening, rain showers across the mountains will quickly transition to snow, particularly along the North Carolina and Tennessee border where elevation plays a major role in snowfall potential. Elevation influences temperature, with higher elevations generally being colder and therefore more favorable for snow rather than rain.
Moisture wrapping around the backside of the departing storm system combined with strong upslope flow will allow snow showers to develop Monday afternoon and possibly continue into early Tuesday morning. A few snow showers may break containment and move into the adjacent foothills Monday Night.
At this time, snowfall amounts appear relatively light and the event is expected to remain below advisory levels for most locations. Forecast guidance suggests only a modest chance of snowfall totals exceeding two inches along the higher elevations near the North Carolina and Tennessee border, with no accumulation across the adjacent foothills even if snow showers break mountain containment. However, even light accumulations could lead to slick spots on mountain roads as temperatures fall below freezing Monday night.
In addition to the snow showers, gusty winds will develop both ahead of and behind the cold front from Sunday night through Tuesday as the pressure gradient strengthens around the departing storm system. ((The pressure gradient refers to the difference in atmospheric pressure between two areas, and when this difference becomes large, winds increase as the atmosphere attempts to equalize the pressure imbalance.))
In the mountains, wind gusts could become strong enough to require wind advisories, particularly Monday into Monday night as the cold air pours into the region.
Hard freezes are likely to occur on Monday and Tuesday nights. As temperatures plummet into the upper teens, this freeze could devastate crops that emerged early due to our earlier warm spell.
Overall, the upcoming weather pattern represents a classic early spring transition across the Carolinas and southern Appalachians. A powerful storm system will bring the potential for severe thunderstorms capable of producing widespread damaging winds and isolated tornadoes before a rapid surge of colder air ushers in northwest flow snow showers across the mountains shortly afterward. This combination of severe weather followed by snow is not uncommon during early spring when strong temperature contrasts and dynamic storm systems move through the eastern United States.
Stay safe. Stay planful. And stay weather aware Monday.