A fast-moving line of showers was developing over the region at the time, and radar detected a lightning strike in that area. Chris Vagasky, a lightning expert at Vaisala, a company that operates a national lightning detection network, tweeted to us that it sensed “a very large +CG [positive cloud to ground] stroke near Dulles” just before 10:45 a.m.
Vagasky later tweeted that an average lightning bolt produces 30,000 amps of current. “This was estimated to be 442,000 Amps,” he wrote.
Additionally, multiple eyewitnesses in the vicinity posted that they saw lightning strike before the resounding rumble.
“I saw a bright flash before the loud thunder, followed by a brief downpour. It was lightning. In S. Herndon,” tweeted @jif101.
“Saw flash, heard clap here in Broadlands. Approx 7 sec gap so w/in 3 miles,” tweeted @agbast.
“Shook the house in Herndon. Small lightning flash and a few seconds later,” tweeted @capsforthewin.
It is unusual for such a localized lightning strike to induce thunder that resonates over such distances. On our Twitter feed, we received reports of the probable thunder clap as far away as the District and College Park, Md.
However, just the right atmospheric conditions were in place for the clap to travel far. The weather balloon launched at Dulles Airport at 8 a.m. Tuesday showed the presence of a very strong low-level inversion, or a layer where the temperature was warming with altitude, acting like a lid. “That reflects sound back towards ground and allows it to propagate long distances,” noted Capital Weather Gang’s Matthew Cappucci, in a tweet.
At its source, the noise was deafening.
“I live in Reston and my ears are still ringing,” tweeted @designergirl 45 minutes after the blast. “Thought it was an explosion due to the noise and shaking.”
A Fairfax fire department spokeswoman said it had not received reports of any issues that might cause a boom.
If it were not for the lightning detection network and eyewitness reports, a meteor would be a plausible alternative natural explanation. On Sept. 17, a meteor exploding in the sky was the source of a boom heard far and wide in western Virginia and the West Virginia panhandle.
A cold front approaching the Washington region triggered Tuesday morning’s boisterous thundershower. The same front could generate more vigorous storms late Tuesday afternoon and evening, which could produce gusty winds, heavy rain and even small hail. The National Weather Service’s Storm Prediction Center has placed the region in a Level 2 out of 5 risk zone for severe thunderstorms.
Justin Jouvenal contributed to this report.