The National Weather Service determined the rather pedestrian line of the storms that zipped through the northern Mid-Atlantic on June 13 was a “low-end derecho”.  Several hours after this complex of storms exited the coast, the ocean abruptly pulled back before reversing course and crashing inland, raising the sea and the prospects that the storms may have triggered a small but rare tsunami along the East Coast.

NOAA described the sequence of events, from the perspective of an eyewitness, Brian Coen:

Around 3:30pm on Thursday June 13, 2013, Brian Coen was spear fishing near the mouth of Barnegat Inlet; just south of the submerged northern breakwater. Earlier in the day around noon, thunderstorms had moved through the area. By 3:30pm the weather was overcast with a light east wind. At approximately 3:30, the outgoing tide was amplified by strong currents which carried divers over the submerged breakwater (normally 3-4 feet deep). This strong outrush continued for 1-2 minutes and eventually the rocks in the submerged breakwater were exposed. Brian backed his boat out before being sucked over as well.

At this point, Brian noticed a large wave coming in, approximately 6 feet peak-to-trough and spanning across the inlet. The upper 2 feet of the wave was breaking. This wave occurred in conjunction with a reversal of the current such that even though the tide was going out, a strong surge was entering the inlet. This surge carried the divers back over the submerged reef and into the inlet from where they were picked up

Thirty tide gauges observed a rise in water, up to nearly a  foot in spots, from along the East Coast, Bermuda and Puerto Rico, NOAA said.

“From North Carolina up through Massachusetts, we can find that signal, even though it’s very small, which tells us there was something going on,” Mike Angove, head of NOAA’s tsunami program, told the Associated Press (AP) Tuesday.

NOAA researchers are trying to determine if it was the so-called derecho that caused the apparent tsunami or a geologic disturbance, specifically a landslide off the continental shelf, according to the AP.

Storm complexes, like derechos, can trigger tsunamis, although it is rare.  When they occur, they’re referred to as “meteotsunamis”.  The Web site has an excellent overview about these phenomena and how the form:

A meteotsunami starts with a strong atmospheric event marked by a change in air pressure, such as a fast-moving front, a squall line, or a train of gravity waves in the wake of a mountain range. Even extreme weather changes the pressure by small amounts, equivalent to a few centimeters of sea-level height. Everything depends on the speed and timing of the force, along with the shape of the water body. When those are right, waves that start out small can grow through the resonance of the water body and a pressure source whose speed matches the wave’s speed.

Next, those waves are focused as they approach shorelines of the right shape. Otherwise they simply spread away from their source and fade out. Long, narrow harbors that point toward the incoming waves are affected worst because they offer more of the reinforcing resonance.

The resource lists some notable meteotsunamis that have struck coastal areas, mostly from Japan and Spain.  But, interestingly, it makes this claim: “The great eastern U.S. derecho of 29 June 2012 raised a meteotsunami in the Chesapeake Bay that reached 40 centimeters in height.”

I did an extensive web search and couldn’t find any further documentation of a meteotsunami from the 2012 derecho. In any event, if the 2012 derecho caused a meteotsunami, it’s certainly plausible the 2013 version did as well.

Of course, a geological cause is still being evaluated.

I’ll leave it to the experts at NOAA to reach a verdict on the cause and will report back once we have an answer.