By Friday, a drier airmass should begin entering the Blue Ridge and Upstate. Rain chances in those areas should decline, however in the Pee Dee and Low Country thunderstorms will stay elevated into the start of the weekend.
Strong thunderstorms are expected across portions of the Palmetto State on Wednesday and Thursday. This storm threat will come as a result of a cold front entering the Mid Atlantic from the north.
On Tuesday afternoon, South Carolina was sandwiched between two surface weather systems: A stationary front and coastal low to the south and a cold front to the northwest. Circulation around the former feature was driving an excessively humid airmass from the Gulf of Mexico into the Low Country, Midlands, and Pee Dee, contributing to heat indices in the mid-90s. At the same time, the latter cold front was entering the Tennessee Valley from the Ohio River Valley.
By late afternoon Wednesday, the cold front is expected to approach the Blue Ridge and Upstate from the northwest. As this front arrives, it is modeled to push a pulse of shortwave energy through the state. That feature should interact with the warm and anomalously humid airmass that is present over the state, and broken lines of thunderstorms are expected to develop and push from the Upstate southeastward through the Midlands and to the I-95 corridor by the late evening. A second shortwave is expected to push through the Palmetto state on Thursday, this time initiating storms in the morning with a southwest to northeast forward motion.
Conditions will be in place to support potentially long-lived squall lines of thunderstorms. Rain rates in excess of two inches an hour will be possible at times and there will be a potential for flash flooding, especially in urban and poorly drained areas. In addition, strong downburst winds could occur in front of and within the lines of thunderstorms.