Lightning strikes East Potomac Park during clearing skies on Aug. 13. Inset: The graphic shows the locations where the lightning flash struck ground (green dots) and where the lightning channel propagated in air (yellow dots). (Kevin Ambrose)

Everyone in the Lincoln Memorial thought the August summer storm was over. Tourists, who had earlier rushed up the memorial’s steps to take cover from the approaching storm, were exiting back to the Mall to resume their evening activities. Thunder and lightning had ceased, and rain had tapered to sprinkles.

Suddenly, lightning streaked across the sky and struck the ground southeast of the Washington Monument. Thunder boomed a few seconds later. It was an impressive lightning flash that occurred at 10:14 p.m. on Aug. 13.

After the lightning strike, there was no more thunder or lightning in the District. The sky cleared, and the storm was over.

But what caused the unexpected strike? Was it the last flash from a weak, departing thunderstorm, or was it positive lightning produced by a distant, powerful thunderstorm that traversed many miles across the sky to strike ground in Washington? I wanted to investigate.

This graphic shows the locations where the lightning flash struck ground (green dots) and where the lightning channel propagated in air (yellow dots). This lightning flash originated at an altitude of 3.7 miles and spanned an area of 6.25 miles, from the National Mall to East Potomac Park and Pentagon City. The data was gathered from the D.C. Lightning Mapping Array, which records radio frequency impulses produced by lightning flashes. (Mason Quick/NOAA)

Mapping the location of the lightning flash

To understand more about the unusual lightning flash, I contacted Scott Rudlosky and Mason Quick, lightning scientists at the National Oceanic and Atmospheric Administration, who can determine the size and location of D.C. area lightning flashes using data from the D.C. Lightning Mapping Array (DCLMA). I provided them with my lightning photo, the location where the photo was taken and its time stamp.

I learned that my photographed lightning was a compact flash of negative lightning produced by a rapidly dissipating storm over the District. It originated 3.7 miles above Earth and struck ground just south of the storm cell, making contact in East Potomac Park and twice near Pentagon City.

The lightning flash spanned the distance from the Mall to East Potomac Park and Pentagon City, approximately three miles in a straight line. It was a relatively small lightning flash by lightning standards.

For a size comparison, a few minutes after I photographed the lightning in Washington, a positive lightning strike from a thunderstorm near Richmond hit near Cambridge, Md., almost 200 miles away. That flash was more than 250 times larger than the photographed flash and covered approximately 1,500 square miles.

This is an animated graphic showing the position of the lightning flashes from a thunderstorm that approached the District during the evening of Aug. 13. The storm rapidly dissipated as it reached the District, and the photographed lightning in this post is represented by the last cluster of dots in this loop. The data was gathered from the D.C. Lightning Mapping Array. (Mason Quick/NOAA)

Lightning physics of a dying thunderstorm

I asked Rudlosky and Quick to explain the physics behind the rogue lightning flash. Quick explained in an email:

As far as the physics, we know the storm had undergone rapid collapse, likely compressing the distance between charge regions and enhancing the electric field enough to cause the lightning flash. The time difference between the first LMA source and the first ground contact indicates a propagation velocity of about 375,000 meters / second for the downward negative leader development. This is a typical negative leader velocity.

The flash was short, lasting only about a tenth of a second, but produced three ground contacts. All three cloud-to-ground return strokes were of negative polarity, and the cloud-to-ground flash that you photographed had a modest peak current of -17 kA. My guess is that the other two strikes were out of your field of view, to the right.

Rudlosky added, “This flash remained relatively close to the ground, which makes sense since the storm was decaying without much charge left aloft.”

This is an animated, 3-D graphic showing the position of the photographed lightning flash. The green dots are where lightning contacted the ground and the yellow dots are where the lightning channel propagated in air. (Mason Quick/NOAA)

Lightning safety

Quick also stated, “It’s important to note the lightning flash occurred about 10 minutes after the apparent end of the thunderstorm, a time period when an eager soccer coach may say that it is safe to play again. Wait 30 minutes after the last lightning.”

During my thunderstorm photo shoots, I stay inside a vehicle or building and run my camera about 30 minutes after the storm ends with the hope of photographing distant anvil crawlers or bolts from the blue. I was still under the roof of the Lincoln Memorial when I took the lightning photo.

Hopefully, this story will reinforce good lightning safety. When thunder roars, go indoors and wait 30 to 45 minutes after hearing thunder to venture back outdoors. As we have seen, even a weak and dissipating thunderstorm can produce deadly lightning.

The radar at 10:15 p.m. on Aug. 13 shows light-to-moderate rain over Washington as the thunderstorm dissipates. (Radar Scope/Matthew Cappucci) (Radar Scope / Matthew Cappucci/Radar Scope / Matthew Cappucci)

Lightning strikes East Potomac Park during the evening of Aug. 13. (Kevin Ambrose)

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