Update Monday afternoon:
A Flash Flood Watch continues through early Tuesday morning for the South Carolina LowCountry. As of 2:50 pm, radar data showed several bands of heavy rain rotating through the Beaufort and Charleston areas, where the National Weather Service says an additional 2 to 4 inches of rain is possible.
Heavy rain is overspreading the South Carolina #LowCountry this afternoon, where a Flash Flood Watch continues through tonight. See how much more rain may fall at https://t.co/6gulZfIeY2 #SCETVwx pic.twitter.com/nFgEzM9QOL
— SCETV (@SCETV) September 20, 2021
Original story posted Sunday evening:
Tropical moisture and a slow-moving front will team up to produce at least two days of unsettled weather across the Palmetto State. Periods of heavy rain could cause flash flooding in the LowCountry starting Monday afternoon, followed by a potential flood risk in parts of the Upstate and PeeDee regions Tuesday.
A fetch of tropical moisture is forecast to intersect a decaying frontal boundary near the South Carolina-Georgia border Monday. As this boundary slowly lifts north, bands of showers and thunderstorms are expected to move inland from the Atlantic Ocean, with an enhancement to the rainfall expected immediately along the coast near Beaufort and Charleston. The flood risk will be elevated in these areas Monday night and Tuesday as a southeasterly wind continues to support rounds of rain over the same areas. A similar situation may unfold Tuesday or Tuesday night farther north as the bands of heavy rain shift into the Grand Strand and PeeDee regions.
A second area of enhanced rainfall is possible across the Upstate region as the remnant moisture from Nicholas moves through the southern Appalachians Monday and Tuesday. Nicholas was at one point a hurricane that hit southeast Texas last week, but is now just an axis of moisture and energy crawling through the Tennessee Valley. The National Weather Service has expressed a lower degree of confidence in the flood potential from the decaying system, but says periods of heavy rain will be possible along and northwest of Interstate 85.
An approaching cold front is also likely to produce numerous showers and thunderstorms across all of South Carolina Wednesday, some of which could be strong. The flood risk could continue or be exacerbated in some areas where these storms hit similar regions that experienced heavy rain Monday or Tuesday. The front is expected to bring noticeably cooler and drier air into South Carolina following the official first day of autumn Wednesday, with potentially below-normal temperatures arriving by Friday.