An epic thunderstorm produced strobe light-like lightning as it barreled through the D.C. suburbs Tuesday night. A seemingly continuous barrage of lightning lit up the skies with a constant flicker, the light show visible more than 30 miles downwind of the storm. But as soon as it got to the District, the storm abruptly fell apart — and the dazzling display was no more.
The storms, which developed around sunset over the Appalachians, evolved into a broken line and moved east as the evening wore on. These storms were feeding off a hint of instability that built up once sunshine broke out amid late-day clearing earlier in the day. The storms were also invigorated by an upper-level disturbance arriving overhead.
At 8:58 p.m., the National Weather Service issued a severe thunderstorm warning for the cities of Fairfax, Manassas Park and Manassas.
Freezing levels were around 11,000 feet, though the storm towered to nearly 50,000 feet high. You can see the storm’s top here:
The high freezing levels meant hail was not favored, and radar data suggests there was not much in the way of large hail being formed in the storm. However, there was probably a high concentration of graupel, small ice-like particles, above roughly 18,000 feet. Ice like this tends to take on a positive charge. The rain below, meanwhile, was negatively charged.
A high density of both rain and ice particles of differing charges leads to a highly charged storm.
There was ample humidity in the lower atmosphere to fuel storm growth. But higher up, it was a different story. Weather balloon data recorded around 8 p.m., just before storms approached the area, showed a layer of dry air between 12,000 feet and 24,000 feet, and a layer of very dry air upward of 32,000 feet.
That dry air at the mid levels did not inhibit storm development, but it did play a key role in lightning processes. That is because the air’s dielectric strength increases as humidity goes up.
What that means is it is easier for a spark to jump in dry air than moist air. The presence of dry air at the mid levels and around the periphery of the storm’s updraft allowed more discharges in this layer and over long distances — partly explaining why some of the bolts leaped considerable distances.
Another thing that helped? Additional storm activity both earlier on and nearby had sent a sheet of downwind anvil-shaped clouds billowing in advance of the storm. You can see that here:
That carried a positive charge a decent distance from the updraft, producing bolts farther away and allowing “anvil crawlers,” which is a particular form of lightning, to ripple along the underside of the high clouds. In addition, the clouds provided a source for the diffuse light from each flash to reflect off, so those of us in the District could see the flashes even when the storm was 15 or more miles away.
When the storm was over Manassas Park and Bull Run, it peaked in terms of lightning flash density. This can be seen with data from the GOES-16 weather satellite, hovering 22,236 miles above us. At least 40 strikes occurred in a two-kilometer grid box over five minutes. Imagine eight or more lightning strikes — some in the cloud and some hitting the ground — within a mile of you every minute, in addition to all the lightning flashes originating outside that radius. About 20 percent of lightning strikes actually hit the ground.
As soon as the storm neared the Potomac River, it collapsed. Watch the lightning activity suddenly disappear in the moments leading up to its demise:
So if you were in the District awaiting a decent fireworks show, better luck next time.