Relentless rain unleashed catastrophic flooding across South Carolina this past weekend. Officials say it was one of the worst disasters in the state’s history.

According to the NWS, an estimated 5.8 trillion gallons of water fell in the Palmetto state in just four days with some communities receiving more than 20 inches of rain. This storm total exceeds that of any tropical cyclone on record that has impacted South Carolina.

Receiving this massive amount of precipitation in such a short period of time overwhelmed rivers and streams, and even caused a number of dams to breach. The floodwaters inundated homes, businesses, and shut down major roadways, including parts of Interstate-95. Local officials say the storm also caused water mains to break, leaving more than 40,000 people without drinkable water. To date, seventeen weather-related deaths have been reported across the state.

This type of rainfall is considered a one in thousand year event in South Carolina. That does not mean it can only happen once every thousand years. It refers to the recurrence interval – a statistical calculation that means an event has a one in one thousand chance (0.1%) of happening in any given year in a given location.

The cause of this widespread and destructive flooding was the unique convergence of three different weather systems that essentially set up an atmospheric river – a fire hose of moisture – aimed directly at South Carolina. The first was Hurricane Joaquin, which sat over the Bahamas for days and pumped huge amounts of moisture into the atmosphere. The second was an upper level low-pressure area over the southeastern US and Gulf of Mexico. It helped pull some of Joaquin’s moisture westward toward the US coastline. And lastly, there was a stalled frontal boundary along the coast. When the warm saturated air encountered the cooler air along the front, it was forced to rise and cool. Since cool air holds less moisture than warm air, the moisture was wrung out of the atmosphere in the form of intense rain over the same area for days.

Although the rain has now cleared, South Carolina is not out of the woods just yet. As swollen rivers make their way to the Atlantic, more flooding is expected in the state’s coastal low country.

Credit: NOAA/NWS Columbia