The tranquil pattern that dominated U.S. weather through the Fall was broken abruptly during the first week of October by a catastrophic flood event in the Southeast, South Carolina being the hardest hit. Like many extreme weather events, it can be blamed on a highly improbable combination of separate meteorological factors, any one of which by itself probably wouldn’t have been noteworthy at all.
Hurricane Joaquin developed about midway between Bermuda and the Bahamas late on Sunday, 27-Sept, and drifted southwest into the Bahamas while steadily gaining strength, eventually becoming a Category 4 storm. For a time there was a real threat of US landfall, and many residents along the Eastern Seaboard breathed a sigh of relief when the cyclone eventually turned out to sea, lashing Bermuda with tropical storm force winds on Sunday, 4-October on its way into the open Northern Atlantic.
However, the sense of relief proved premature. A strong non tropical disturbance, cut off from the main jet stream, settled across the Southeast and began to steal tropical moisture from Joaquin skirting by just offshore. This rich moisture tongue ran up and over a stationary front hung up along the coastal Carolinas, allowing every bit of the moisture to be wrung out, like hands white with strained effort, wrings the water out of a sponge. Vast areas of South Carolina from the central Midlands down to the Lowcountry saw more than 12″ (300 mm) of total rainfall, one site just north of Mt. Pleasant, SC tallying 26.88″ (682.8 mm). All told, more than 11 trillion gallons of water fell over the Carolinas in a four day period, enough almost to fill Lake Okeechobee, the massive “eye” of south Florida, to the brim. All time records for daily rainfall were broken in Columbia and Charleston, and Charleston saw its October monthly rainfall record surpassed in just three days. In the Lowcountry the flooding was exacerbated by the “king tide”, an unusual surge of the astronomical tide that was the strongest in 26 years.
The prodigious amounts of water quickly overwhelmed most SC watersheds. Gills Creek, which runs through the state capital of Columbia, surged in a matter of hours to 17.08′, almost double the previous all-time record stage of 9.4′, before the gauge washed away. Flow rates for the Congaree River at Columbia spiked at four times the previous record maximum for the day, and for a brief time, the Congaree was moving more water downstream than was the Mississippi River at St. Louis. The rapid rise in the Congaree and the adjacent Columbia Canal caused a breach, threatening the city’s water supply and precipitating a boil advisory. At least 19 dams failed across SC, forcing thousands of residents to evacuate, a process complicated by the dozens of bridges and hundreds of roadways that were significantly damaged and impassable. This historic flood will take months to clean up, and may end up as the most destructive natural disaster in South Carolina’s living memory.