A possible tornado was located near Islandton, about six miles of Walterboro at 5:21 p.m., according to the National Weather Service.
Central Colleton and Dorchester counties were under a tornado warning until 6 p.m.
The tornado was traveling east about 35 mph.
A tornado watch was in effect for the rest of the Lowcountry until 8 p.m.
The National Weather Service issued the watch for Charleston, Berkeley, Dorchester, Colleton, Hampton, Beaufort, Jasper and Allendale counties.
The National Weather Service said conditions will start to deteriorate around 1 p.m. and last until 7 p.m. Doug Berry, a forecaster at the service's Charleston office, said the main threat for the tri-county area will start at 3 p.m.
The Storm Prediction Center, which forecasts severe weather, pinned the risk of tornadoes at 10 percent and damaging straight-line winds at 45 percent.
Berry said the conditions are not typical for this time of year. High temperatures — 80 degrees is expected in the Charleston area today — are amplifying the threat.
"It's just a matter of how unstable the atmosphere gets," Berry said. "The more the sun is able to be out today, the better chance we'll have for severe weather."
Southeasterly winds of 15-25 mph will gust to 45 mph, the weather service said. A wind advisory was in effect for the Lowcountry while a tornado watch was declared for upstate South Carolina.
"For the most part, it's a wind threat," Berry said. "We suspect it to diminish pretty rapidly once the front passes" between 5 and 7 p.m.
The high temperature Saturday is forecast to be about 60 degrees.