Debby is moving at just 3 mph, with its center located about 10 miles east of Savannah, Georgia, as of 5 p.m. ET.

Tornadoes are possible in South Carolina and North Carolina on Tuesday night.

After a day of heavy rainfall, the coastal cities of Savannah and Charleston, South Carolina, could see a few more inches of rain this week. Both cities are under flash flood warnings.

But going forward, the worst of the rain will be focused further north. Myrtle Beach, South Carolina; Wilmington, North Carolina; and Lumberton, North Carolina, are forecast to get hit the hardest over the next 24 to 36 hours.

More than 1,100 flights have been canceled within, into or out of the U.S. on Tuesday as Debby slams the Southeast and thunderstorms strike the Northeast. Ground stops are in place at the New York City area's three airports.

The threat of severe flooding is very high across South Carolina with Tropical Storm Debby "creeping across our state," South Carolina Gov. Henry McMaster warned on Tuesday.

The rain from the slow-moving storm may be particularly heavy from Myrtle Beach to the North Carolina border, the governor said.

Forty-eight state roads are closed due to flooding and 50 high-water vehicles are at the ready to respond, he said.

More than 90,000 customers remain without power in Florida, more than 24 hours after Debby tore through the state.

"Restoration is taking longer in areas with flooding and heavy vegetation, as crews must wait for the water to recede and clear through debris before they can restore power," Florida Power & Light said Tuesday morning. "Rest assured, we’ll continue working until everyone who can safely receive power is restored. Avoid these flooded areas and stay away from downed power lines."