Update: The Tornado Watch for Horry and Georgetown counties expired at 6:45 p.m.
Between a sea breeze coming in off the ocean and a warm front inland, the atmosphere is unstable over the coastal counties. Storms may develop and produce damaging winds and hail. Because there is plenty of shear in the atmosphere, a tornado is not out of the question.
Once you move west of Horry and Georgetown counties, the atmosphere is cooler and more stable so severe weather in unlikely.
Track the rain here.