On Wednesday, a powerhouse storm system sweeping through the eastern U.S. triggered a large outbreak of severe thunderstorms from South Carolina to Massachusetts. The National Weather Service Storm Prediction Center filed reports for more than 400 instances of damaging winds, 45 large hail events, and 20 tornadoes.
Tornadoes in southeast Virginia killed four people.
A rare mix of ingredients converged to create this outburst of severe weather, more typical of May than late February.
The figure below is a composite of a very unusual late February cyclone for the Mid-Atlantic, showing a radar depiction of liquid and frozen precipitation, as well as principal fronts and the center of low pressure (red “L”). In one snapshot, one appreciates the diversity and intensity of weather hazards in both the warm sector, ahead of the cold front, and a heavy snow band on the northwest side of the storm.
Over the Mid-Atlantic, a powerful squall line extended throughout the warm sector from southern North Carolina into north-central Pennsylvannia. The number of concurrent tornado warnings (small red polygons), lined up along the squall line, is simply amazing. Through 10:30 p.m. Wednesday, the National Weather Service had issued 125 tornado warnings and 137 severe thunderstorm warnings for the East Coast!
The warm front (heavy red line) cut right through Washington, D.C. as the squall line swept across the Mid-Atlantic region. Unseasonably mild, humid and unstable air lay to its south. The warm front also served as a sharp line demarcating the rotating storms (supercells) from the severe, but non-tornadic, northern portion of the squall line, where numerous severe thunderstorm warnings (yellow polygons) were issued.
The reason for this is simple: The air mass was much more unstable south of the warm front, compared to its north (another reason, related to the wind shear, is discussed below). This is illustrated in the figure below, which shows a commonly measured value of instability, called the lifted index, at 6 p.m. Wednesday.
For a lifted index value of -4 to -5, the air mass was moderately unstable over eastern North Carolina and south-eastern Virginia. But given the time of year, these values are extraordinary.
Along and north of the warm front the instability was much weaker, and also elevated. This means the mass of buoyant air supplying thunderstorm updrafts was not connected to the surface, but sat atop cooler air north of the warm front. It’s much more difficult to sustain severe thunderstorms when the vertical drafts are not physically rooted at the surface. Lightning, however, was frequent, attesting to the significant mass of unstable air sequestered aloft.
During the winter months, severe weather occurs when very strong atmospheric dynamics overcome minimal instability. This event had powerful dynamics, in spades.
There are two factors that focused the uplift of air into a single, narrow, powerful squall line.
The first was the sharply demarcated ribbon of concentrated spin associated with the upper low center.
In the graphic above, analyzed at the 18,000 foot level, there is a large vortex (centered on the red “L”) transiting the Great Lakes region. The red colors show a measure of spin, called vorticity, associated with the leading edge of the vortex. When strong, concentrated vorticity aloft moves over the surface, it induces air to rapidly rise ahead of it. The squall line was nearly parallel to, and just ahead of, this ribbon of dynamically rising air.
The second dynamic factor was a powerful jet stream at 30,000 feet, with an embedded pocket of fast winds streaming at nearly 160 mph, called a jet streak. As the jet streak approached the Mid-Atlantic, air was forced to ascend vigorously in a quadrant termed the “left front exit region”, shown by the white ellipse (below).
The air flow in a jet streak is unbalanced, and air rises from below as a means to correct the imbalance.
Thus, while strong instability was lacking over D.C., air was still forced to rise into a narrow ribbon (e.g. the squall line) due to a conspiracy of intense dynamical processes.
Strong dynamics are a hallmark of wintertime severe thunderstorm events along the East Coast. During the summer, the opposite occurs: Strong thermal instability is the main driver of intense convective lines, with the dynamics playing a secondary role.
There was much discussion of strong wind shear during today’s event. Wind shear is a measure of how much the environmental winds change speed and/or direction with height. Wind shear is a key discriminant between non-severe and severe thunderstorms, and the nature of the shear also dictates the mode of severe convection.
Wednesday’s wind shear promoted both a discrete mode of severe storm i.e. supercells over eastern North Carolina (early afternoon), and a linear mode of intense storms, with embedded pockets of tornadic rotation, later in the afternoon (across Virginia).
The magnitude of the shear was simply astounding, approaching 105 mph over North Carolina, as shown above. To put this in perspective, supercells become likely when (given the proper thermodynamic environment) deep-layer wind shear exceeds about 50 mph – a summertime value worth getting very excited about!
The deep shear was 75-80 mph over the District during the afternoon and even higher in some areas south through the Carolinas.
Not shown here, but equally important, is a measure of the shear’s tendency to induce low level rotation, called helicity. When helicity is large, low-level rotation gets aligned with air streaming into thunderstorm updrafts.
Wednesday’s diagnosed helicity values were enormous, by any standard of measure used throughout the year. These exceptionally large helicity values (exceeding 700+ m2/s2) – which get tornado chasers quite excited – remained south of the warm front through the day. This is another reason why all the tornado warnings in the figure above were aligned SOUTH of the District, where the destructive tornadoes were focused.