In the midst of a sweltering heat wave, Sunday came and went for most people with nary a raindrop during the daylight hours. Around sunset, several thunderstorm cells erupted over the Blue Ridge Mountains and coalesced into an intense line of storms.
As that storm complex slid off the high terrain, it generated a spectacular light show as it approached Frederick, Montgomery, Carroll and Howard counties. With each strobe-like flicker, heaped cloud formations in the foreground of the storm complex became starkly contrasted against taller thunderstorm clouds to the rear. At these distances and altitudes, most of the light show was produced by intracloud lightning flashes.
Karen Yep, viewing the lightning near Olney, commented on Facebook that it was “near constant” for an entire hour.
It “seemed like a once in a lifetime experience,” Yep wrote. “Never seen anything like it in my 38 years.”
“Right up there among the most intense lightning producers I have seen in the East,” added Kevin Folk.
The highly electrified storm complex was the only game in town (within the greater Washington-Baltimore region), as the lightning discharge summary below illustrates. This map is courtesy of an experimental, high-resolution network called the DC LMA (Lightning Mapping Array).
The network data indicates more than 70,000 point sources or discharges fired off in the one hour between 8 and 9 p.m. local time. This is the period during which the storms reached maturity, triggering several severe thunderstorm warnings. Doppler radar estimated wind speeds up to 80 mph just a few thousand feet off the ground north of Poolesville, where there was also a report of utility poles and wires down.
The storm emitted most of its lightning discharges in the upper atmosphere, at and above 15 km (9 miles; note altitude scale along top of figure), as shown in the graphic above. This indicates exceptionally deep and vigorous cloud updrafts were present and driving the electrical activity to tremendous heights.
Why would such powerful updrafts follow the setting sun? After all, the sun heats up the ground and thus destabilizes the atmosphere, most strongly in the late afternoon. The evening appearance of strong to severe thunderstorms testifies to lots of stored, buoyant energy in the atmosphere, which can linger for hours after the sun has set. It’s like a battery that’s been fully recharged in the afternoon, then starts to dwindle after dark.
The measure of available buoyant energy, per the evening Dulles weather balloon, was more than 1,800 Joules (J) per kilogram (kg) of air. During the midafternoon, the value was more than 3,000 J/kg. (Imagine how many billions of kilograms of air a thunderstorm complex processes over several hours, and you start to get a sense of just how much energy is involved here.)
Careful review of the Doppler radar at Dulles revealed that the storm complex likely became organized around a small, mid-level vortex that was moving across the Blue Ridge, from the northwest (figure below). This type of circulation, visible at intermediate levels in the Doppler data, helped these storms ingest unstable and buoyant air from the south and east, sustaining and intensifying the storm’s cells. This is important; there was insufficient vertical wind shear (increase in winds with altitude) in the evening to organize storms, and the vortex provided an organizing and focusing mechanism.
Why all the talk about buoyant energy? It’s important when it comes to generation of lightning. The formation of lightning in clouds is based on a hypothesis called charge induction and separation. Trillions of liquid cloud droplets and ice crystals jostling in a turbulent, intense updraft cause enormous static electrical charges to build. The lighter (and more buoyant) ice particles transport charge of one sign to the upper cloud reaches, while the heavier liquid drops settle near cloud base and concentrate charge of the opposite sign there. The potential difference between these two strongly polarized regions creates enormous voltage, internal discharges and flow of current.
Last night, the Dulles Airport weather balloon showed buoyant energy was concentrated in the critical layer high up in the atmosphere, where temperatures were between 14 and minus-22 degrees, to support charge induction.
The result? Very frequent electrical discharges within the cloud, illuminating the night sky with a measure of shock and awe.