Persistent heavy rainfall this week, which has led to 3 to 7 inches of rain as of Wednesday morning based on radar data and cooperative observers over the Upstate and Midlands, is likely to lead to river flooding in the coming days statewide.
The prevailing wind flow around a nearly stationary upper-level low over eastern Tennessee is transporting rich moisture from the Gulf of Mexico and Atlantic Oceans and depositing it over the state. The upper low is forecast to move slowly northward away from the state on Friday. Before it does, another 2 to 5 inches of rain is likely to fall over the Upstate, Midlands, and Pee Dee regions Wednesday into Thursday based on the latest projections from high-resolution model simulations. Additional heavy rain on already moist soils will continue the threat of flash flooding, and for this reason, the National Weather Service has continued its Flash Flood Watches for northern South Carolina through Thursday morning.
Flash flooding is the primary statewide concern; however, a few strong thunderstorms may produce damaging gusts or a brief tornado over the Lowcountry, Pee Dee, and Grand Strand regions Wednesday afternoon into Thursday. The Storm Prediction Center has placed these areas in a “marginal risk” (level 1 out of 5) for severe thunderstorms during this time.
Once the heaviest rains exit on Friday, additional flooding is forecast on rivers statewide through the weekend thanks to runoff. As of Wednesday morning, the Congaree River at Carolina Eastman (near Columbia) was experiencing minor flooding. It is forecast to reach moderate flood stage on Thursday morning and peak within 1.5 feet of major flood stage Saturday morning.
River Flood Threat: The Congaree River at Carolina Eastman (near #Columbia) is forecast to reach moderate flood stage tonight. It's expected to peak within ~1.5 feet of major flood stage Saturday morning. Forecast may change based on upstream rain. #scwx #SCETVwx pic.twitter.com/pqrZCy61pW
— Ray Hawthorne (@ray_hawthorne) May 20, 2020
Several additional rivers are forecast to receive minor flooding between Thursday and the weekend; these include the Saluda River, the Wateree River, the Pee Dee River, and the Enoree River in the Midlands. Farther downstream, The Great Pee Dee and Santee Rivers, as well as the Black Creek near Quimby, are also under Flood Warnings because of expected flooding Thursday into Friday.