200-year rains flood the Carolinas. What's blocking the rain from leaving?

The rain just keeps coming in coastal Carolina cities from Charleston, S.C., to Wilmington, N.C., as an atmospheric feature called a “Rex block” is holding a conveyor belt of moisture in place between two strong ridges of air pressure.

Some 22 million Americans are under a flash flood watch as the rains continue to fall, with meteorologists saying up to 15 inches could be recorded by the time the rains abate, perhaps as late as Sunday night. That would make it an event that hasn’t been seen since before the Civil War, and perhaps not even since the explorer Hernando de Soto crossed the South.

“Periods of heavy rain will affect southern South Carolina and eastern portions of Georgia," the National Weather Service warned. "There is increasing confidence that a significant heavy rainfall event will occur through Sunday with some locations seeing record rainfall amounts."

Growing flash flood danger especially in the Carolinas, but also up the East Coast, came as hurricane Joaquin, a Category 4 behemoth once predicted to make landfall in the US, faded as a threat as it is forecast to go out to sea. But the hurricane’s severe low pressure in effect has blocked, or trapped, a blanket of heavy rains against an oncoming high pressure system from the North. Strong pressure ridging associated with the feature means high winds and heavy moisture even as the system is blocked from moving away.

According to the National Oceanic and Atmospheric Administration (NOAA), the Rex block – named after Dr. Daniel Rex, who first discovered the phenomenon in the 1950s – is “characterized by a high-pressure system located immediately north of a low-pressure system,” which in this case is the bulk of hurricane Joaquin. As air flows at high altitudes between the systems in a north to south direction, there is very little eastward progression, thus creating a monotonous “block” of weather.

Bottom line: Columbia, S.C., and Charlotte, N.C., are getting soaked. Weather-watchers reported as much as 12 inches of rain falling on Wilmington, N.C., overnight into Saturday. Local authorities were reporting that multiple water rescues are taking place. North Carolina officials were reporting 17,000 power outages across the state, and police said one woman has died due to the weather after a tree fell on her car as she was driving down I-95.

The colonial-era city of Charleston appeared to be taking the brunt of the heavy rains on Saturday, as residents could be seen paddle-boarding through the city’s historically low areas.

Stormy weather beset not only the South. New Jersey officials said heavy storms blew a Jersey Shore home off its foundations. The slow-moving rain could eventually cause flooding far inland, even up into New England, where soils are already soggy from days of rain.

As rain-soaked Americans braced for potential floods over a wide area, the greatest human concern on Saturday remained on the whereabouts of the El Faro, a 735-foot cargo ship that disappeared on Thursday after getting battered by Joaquin off the Crooked Islands. The ship, carrying 28 US citizens and five Poles, was en route to Puerto Rico when the captain reported that the ship was listing, and taking on water. The Coast Guard is hunting for the ship.