That line of powerful thunderstorms not only knocked out power to more than 30,000 customers in the Philadelphia area, but it stirred up the Atlantic Ocean sufficiently Tuesday night to generate a "meteotsunami."

"It's not very common," said Sarah Johnson, a meteorologist at the National Weather Service in Mount Holly, "and it's still not a well-understood phenomenon."

So just what are these things?

Unlike the disastrous seismic-driven tsunamis, these are waves set off by sudden changes in water levels wrought by radical changes in air pressure — or the weight of air — over the ocean.

In Tuesday's swiftly moving squall line, air rose rapidly, said Jim Eberwine, erstwhile marine forecaster at the Mount Holly office who now lives at the Shore, and that's why the storms were so strong.

Thunderstorms occur when rapidly rising air cools and condenses.

The updrafts lower the weight of the air at the surface, and this can agitate the ocean surface dramatically. The updrafts Tuesday were prodigious, Eberwine said, with cloud tops reaching above 60,000 feet from New England to the Mason-Dixon line. Such cloud heights are more typical of Tornado Alley, he said.

Near the Atlantic City surf, the water level suddenly dropped 1.3 feet between 9:45 and 10:15 p.m., Johnson said, and then jumped back up several inches, similar to a plunging action.

The weather service knew it was coming since it had been alerted by the National Tsunami Warning Center, which had detected a disturbance about 130 miles offshore, Johnson said.

At 8:54 p.m., the weather service issued a statement saying, "Air pressure sensor and tidal-gauge readings in and near the coastal waters indicate that a weather-generated tsunami has been triggered by the line of thunderstorms as it moved over the ocean."

The net result was hardly catastrophic, Johnson and Eberwine said, and it probably had little effect on any flooding. And unless your local media outlet informed you that the Shore had experienced a meteotsunami, you wouldn't have felt a thing.

Tide levels were elevated anyway because of the astronomical effect of the new moon, Eberwine said. Said Johnson, "You wouldn't have noticed it unless you were at the Shore or in a boat."

But a similar event occurring on a summer afternoon when bathers and boaters were in the water would pose a serious hazard since it creates dangerous currents.

In 2013, a tsunami-like wave swept three people off a jetty on Barnegat Inlet.

No new tsunamis or meteotsunamis are in the forecast, but the weather service says that by the end of the week, from 2 to 5 inches of rain could have fallen on the region. Expect flood watches.