Most of the rain is forecast to fall in as little as 3-6 hours. A small chance of heavier rain and flooding could pose "an elevated threat to life and cause significant disruptions to transportation, flood underground infrastructure and basements and first floors of residences and businesses," with water rescues needed, the weather service said.

The first round of tropical downpours have begun in parts of the Carolinas. NOAA's Weather Prediction Center has issued a Level 2 out of 4 flash flood risk for parts of the Carolinas, with heavy rain also expected in Georgia.