At least six people have been killed from Debby, authorities said.

A 13-year-old boy was killed when a tree fell on a mobile home Monday morning in Levy County, Florida, which encompasses Cedar Key, according to the Levy County Sheriff’s Office.

"Our thoughts and prayers are with this family as they deal with this tragedy," the sheriff’s office said in a statement. "We encourage everyone to use extreme caution as they begin to assess and clean up the damage. Downed powerlines and falling trees are among the many hazards."

In Hillsborough County, Florida, which encompasses Tampa, a driver of a semi was killed when the truck went into a canal off Interstate 275 Monday morning, according to the Hillsborough County Sheriff's Office.

Deputies recovered the semi and found the driver dead inside the cab, authorities said.

In Dixie County, Florida, a driver “lost control due to the inclement weather and wet roadway” on Sunday night, crashing into the center median guardrail and then overturning the car, the Florida Highway Patrol said.

The driver, a 38-year-old woman, and a passenger, a 12-year-old boy, were killed, authorities said.

Another passenger, a 14-year-old boy, was seriously injured, the highway patrol said.

In Gulfport, Florida, a 48-year-old man died after he tried to ride out the storm on his sailboat which was anchored off Gulfport’s Veteran’s Park, police said, according to ABC Tampa affiliate WFTS.

In Moultrie, Georgia, a 19-year-old man was struck and killed by a fallen tree while he was inside a home on Monday, according to Moultrie police.

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.