An extremely ominous shelf cloud approaches Rehoboth Beach, Del., on Aug. 19. (Don Burdick)

Most people flock to Rehoboth Beach, Del., in the summertime for a merry atmosphere and tranquil skies. Don Burdick’s photo from Rehoboth Beach looks like something from another planet.

“It was really spectacular to see that come in,” said Burdick, who was visiting the beach with friends last week. “We had just spent $42 on an umbrella and chairs, and then had to pack up a half-hour later.”

Burdick and his friends hurriedly sought shelter at the Green Turtle restaurant. “I love it there. … You can look out and have a view of everything. It’s like being on a cruise,” he joked. And a view they had!

The picture, snapped last Monday, is eerie. It shows an unusually well-pronounced “shelf cloud” portending an imminent storm — one that brought 75 mph winds just offshore. Half-dollar-size hail was reported just north of Indian River Bay. You can see the hail core on the left in Burdick’s photo.

Shelf clouds often give rise to some of the most ominous sky scenes you’ll see. And while they may look scary, they’re seldom cause for alarm. Instead, they mark the beginning of a storm’s transition to becoming outflow dominant.

Storms are like giant engines. They ingest air, use it as “fuel” and expel their exhaust. When a storm “exhales,” that gust of cool, dense air hits the ground and fans out. If a storm spits out more air than it’s taking in, the storm starts to decay — a surge of crisp rain-cooled air surging out ahead of the storm. That’s called an outflow boundary. And outflow boundaries often make for the gnarliest shelf clouds.


An outflow boundary forms along the leading edge of severe thunderstorms in Delaware on Aug. 19. (GR2 Analyst/Matthew Cappucci)

Shelf clouds can stretch for tens or even hundreds of miles. And they generally sweep in ahead of any rain or lightning — though the lightning threat can extend far beyond the storm. If you see a shelf cloud, it’s time to dash. After all, they’re the leading edge of strong to damaging wind gusts. Erratic wind shifts along outflow boundaries can sometimes spin up waterspouts just offshore near bodies of water.

The outflow boundary is easily visible on radar as a thin blue line racing out before the storm. The farther ahead of a storm the outflow runs, the weaker a storm gets. But a dying storm can often produce the strongest wind gusts as it drops its rain and precipitation, yielding a final burst of wind.


An outflow boundary can be seen on radar racing head of severe thunderstorms in Delaware on Aug. 19. (GR2 Analyst/Matthew Cappucci)

Burdick’s photo shows some elegant meteorology at work. On the right, the shelf hangs low toward the water’s surface. Over land, ragged appendages dangle ahead of the towering storms.

A core of impressive rain and hail can be seen on the left, also marked on the radar shot. A clump of air spilled forward by this precipitation core lurched out ahead of the main line, kinking the outflow boundary and causing the turbulence visible toward the middle and left of the image. It’s possible the tube-like phenomenon is the start of a roll cloud, akin to a detached horizontal vortex of outflow than can roll like a rolling pin miles ahead of the storm. It forms between the cool, sinking air associated with the outflow and warm, humid air rising quickly upward into the storm.

“The inflow was insane,” Burdick said. “Then I saw what almost looked to be a waterspout or something. Something was really stirring up the water only four or five feet wide.”

Burdick’s description is spot on. A collapsing storm to the west intersected with the outflow from the storm to Burdick’s north, resulting in several small eddies near the surface. It’s possible that one or two of these got stretched upward just ahead of the scary cloud in the intense inflow, forming tiny dust-devil like whirlwinds over the water that mimicked a small waterspout.

As the outflow passed overhead, beachgoers were caught off guard as a battalion of beach umbrellas came somersaulting by like a march of clumsy projectiles.

“The lifeguards were heroic in trying to get everyone off that beach,” Burdick said. “There was one guy who really knew how serious it was. He was running at top speed to grab an umbrella before it made it down to the boardwalk. It could have impaled somebody.”

The gathering of objects along an outflow boundary is the same reason we can sometimes “see” it on radar. Insects, dust, errant rain drops and other items frequently get caught up in the breeze up at the height of the radar beam. The rapid change in temperature — often on the order of fifteen degrees or more — across the front also means a change in the refractive index of air, influencing how the radar beam passes through it.

With storms in the forecast toward midweek, more epic shelf clouds could be in the offing. Next time you see one, just remember — it’s not Armageddon. Instead, it probably comes from an outflow boundary.

In the meantime, Burdick said the photo has been getting a lot of buzz — both around the country and close to home.

“My husband said it was something that belongs in a textbook,” Burdick said. His husband happens to be a climate scientist at the National Oceanic and Atmospheric Administration.

Here are some more photos of this shelf cloud from social media:

Striking cloud formation in Rehoboth Beach DE this afternoon; the bottom portion looked just like a waterfall. Unsurprisingly, a fast and ferocious storm arrived 10 minutes later.

Posted by Mary Beth Ruppert on Monday, August 19, 2019