The storm moved into Georgia overnight on Monday and is expected to go offshore before approaching the South Carolina coast on Thursday, according to the National Hurricane Centre (NHC).
“This is a level four out of four risk for excessive rainfall,” Michael Brennan, director of the NHC, told reporters.
“This is going to result in a prolonged extreme rainfall event with potential for catastrophic flooding across coastal portions of Georgia, South Carolina, even extending up into North Carolina,” he added.
Ron DeSantis, the Florida governor, said that some 250,000 residents in his state were without power.
“Please, be very cautious when you’re going out,” he said, adding that Debby’s winds had not been as damaging as previous hurricanes that have hit Florida.