An explosive mix of atmospheric ingredients converged at an unlikely hour Thursday, spawning a destructive and deadly tornado in southeast Virginia. This article highlights some of the structural attributes of this storm, and offers an explanation of how it arose.
The damage survey
As reported in yesterday’s article on this storm, one inch hailstones accompanied destructive winds in Thursday’s Cape Charles storm. The National Weather Service forecast office in Wakefield, Va. conducted a survey of the damage area, and their storm survey confirmed a mix of tornadic, straight-line wind, and large hail damage.
The Cape Charles supercell produced an EF-1 (80-100 mph wind) tornado which initiating over the bay and then moved onto land, with a total path length of eight miles. The tornado was on the ground for 15 minutes, from 8:25 to 8:40 a.m.. Downburst winds, which caused straight-line wind damage, occurred in conjunction with this tornado, with maximum wind speeds of 65-75 mph. Golf ball-size hail mixed in with the downburst winds, with some baseball-sized stones. Large hail blown sideways by high wind created significant structural damage.
8:30 a.m. is not the time of day we expect to see a supercell thunderstorm, much less a tornado. In fact, when compared to the historical incidence of all Virginia tornadoes, early to mid-morning is the least likely time of day for tornadoes. A truly exceptional set of environmental airflow and instability characteristics has to combine in just the right way to trigger tornadic supercells so early in the morning.
A closer look at satellite and radar
The preponderance of evidence gathered on this event – ground observations, damage assessment, radar data, satellite imagery, and storm environment – consistently portrays a powerful supercell thunderstorm that formed in the early morning hours of July 24.
Several supercells developed ahead of a line of sub-severe storms, as portrayed in Figure 1. This is a visible satellite view, showing cloud top structures across the Tidewater region of Virginia. I’ve added a few annotations to guide the discussion. First, you’ll note the weak convective line, extending southwest to northeast, along a cold front that sagged south across Virginia during the early morning. Two supercells are indicated by the red arrows, and are quite distinct in this image. In fact, the anvil clouds of these storms were so tall compared to the surrounding clouds, they cast shadows onto the lower cloud layers.
The tornadic supercell that struck Cape Charles is the easternmost of the two supercells. Just to the south of these storms lies is a broad, cloud-free zone. This is important; it enabled strong, early morning heating across region, leading to rapid destabilization of the air mass feeding into the supercells. More on this below.
Figure 2 shows the regional radar snapshot at 8:00 a.m.. Note the two supercells, and possibly a third (concentrated red regions of radar with very heavy rain rates) on the southeast side of the rain band. This band lies along the cold front, shown by the solid blue line with blue triangles. The tornadic supercell is shown crossing the mouth of the Chesapeake Bay, approaching Cape Charles.
Figures 3 and 4 zoom in on this supercell, rendering the storm’s radar structure as a 3D image. By all appearances, this is a solitary, Great Plains-style storm, with a spreading anvil cloud at 40,000 feet and an overshooting cloud top penetrating to 50,000 feet. The overshooting top develops when a vigorous updraft surges into the lower stratosphere. A 10,000-foot overshoot is significant, a signature of a severe thunderstorm.
To the immediate north of the updraft lies the core of heavy rain and hail, shown by the magenta and white colors. Here you can see the entire volume of precipitation cascading through the supercell. You can see large hail embedded in the storm, which is shown in white. This is corroborated by reports of 1-inch and larger hailstones. Such large stones require a very strong updraft (think of the upward velocity required to levitate millions of these stones!).
Figure 4 shows the same supercell a few minutes later, as it was transitioning to its tornadic phase. I wish to point out two important features. The first is called an “updraft vault” – a pronounced indentation in the volumetric rain, where no rain is falling. This signifies a very intense updraft – so strong, that precipitation particles are ejected from the updraft core, before they grow large enough to fall out! Large hail was likely developing in the “overhang” region atop of the vault (whitish-gray region), then cascading to the surface on the north side of the vault.
The second key aspect is a developing hook-like appendage on the southwest flank of the updraft vault. This is a sign that a mesocyclone was strengthening. The mesocyclone is typically 3-6 miles in diameter, and pulls in rain and hail, wrapping around its back side, and forming a hook-like shape on the radar display. The hook echo is one indicator of a possible tornado; the other is a unique signature in the Doppler wind field (not shown here) measuring the actual spin of the parent mesocyclone.
All the right ingredients for supercells
As the cold front moved across the region, it forced the rising of warm, humid air ahead of it. Since this warm air mass was unstable, convective storms developed. The air mass over Tidewater was in fact strongly unstable, with surface-based values of convective available potential energy (CAPE) as large as 2,000- 2,500 Joules of energy (per kilogram of air) feeding into the storm (Figure 5).
CAPE is a measure of the energy available in the atmosphere to fuel storms, and it is computed from balloon soundings. Values of 1,000-1,500 J/kg are common in mid-summer, during the mid-late afternoon. Values exceeding 2,000 J/kg during the early morning of July 24 presented an exceptional situation. No doubt, strong early morning sun in the cloud-free zone southeast of the cold front contributed to these high values; note how well the zone of large CAPE (red shaded region) lines up with the cloud-free zone depicted in Figure 1. These large CAPE values ensured that the most vigorous updrafts would develop in the southern-most storms along the cold front.
Intense updrafts are one aspect of supercells. The other is the mesocyclone, a narrow, spinning column of air extending vertically through mid-levels of the storm, and co-located with the updraft. The “meso” provides a source of spin for the much smaller and concentrated tornado. Wind shear – the change in speed and direction with height – is essential for mesocyclone formation. For supercells, we look for 40 knots or more of speed increase through a depth of 6-8 km. Winds that also veer (change direction in a clockwise manner with altitude) promote strong, long-lived mesocyclones, and increased probability of tornado formation.
Figure 6 shows a sequence of fine-scale analyses, obtained from NOAA’s Storm Prediction Center, at 8 a.m. and 9 a.m.. These panels depict the strength of the vertical wind shear over Tidewater.
Between 8-9 a.m., a small pocket of intense wind shear, in excess of 40 knots, was in the process of developing over the lower Chesapeake Bay region. The sudden increase in shear was likely due to the approach and intensification of a shortwave trough in the jet stream flow. This is a significant amount of shear for mid-summer in the Mid Atlantic.
By comparing Figures 5 and 6, note that the strong region of instability lines up nicely with the pocket of intensifying wind shear. This combination was ideal for the sudden growth of multiple supercells ahead of the frontal zone.
Only about 25%-30% of supercells – even those with strong mesocyclones – produce a tornado. Tornadogenesis is in fact a rare and delicate process. To elucidate why a tornado developed over Cape Charles requires a very fine-scale analysis, one that is not supported by available observations. Further understanding is hampered by our lack of knowledge on the precise mechanisms involved in tornadogenesis. The final step often involves a sudden strengthening of the mesocyclone at low levels, associated with the formation of a special type of spiral downdraft. The downburst winds proximal to the tornado may have been a manifestation of this spiral downdraft.
It’s possible that differences in surface friction between the bay and adjoining Delmarva Peninsula may have helped to concentrate and amplify low-level spin beneath the supercell’s mesocyclone –but to say that, we would be entering the realm of speculation.