Debby updates: Flash flooding, tornadoes target DC, Mid-Atlantic, Northeast

Tropical Storm Debby -- currently sitting off shore of Myrtle Beach, South Carolina -- is forecast to make a second landfall late Wednesday into early Thursday near Myrtle Beach, bringing more heavy rain.

The storm is bringing significant to catastrophic flooding to portions of the Southeast, with flood watches and warnings covering much of the Carolinas. Rain totals could reach 20 to 25 inches in South Carolina and 15 inches in North Carolina.

A "high risk" for excessive flooding has been issued from North Carolina into southern Virginia for Thursday. Click here to read more.

Tropical Storm Debby’s winds slightly strengthened early Wednesday, rising to about 45 mph, as the storm moved over warm water in the Atlantic about 65 miles from the coast of Charleston, South Carolina.

Debby is expected to remain a tropical storm into Thursday, when it’s forecast to make a second landfall in South Carolina.

The National Hurricane Center said it expects the storm to not strengthen much more, maybe rising up to about 50 mph ahead of that landfall.

The storm is then forecast to continue inland overnight into Friday, with its winds slowing as it travels through North Carolina and into Virginia.

-ABC News’ Max Golembo

Tropical Storm Debby is slowly moving eastward offshore the coasts of Georgia and South Carolina.

Debby is moving east at 5 mph while max sustained winds remain at 40mph, with its center currently about 50 miles east of Savannah, Georgia, as of 11 p.m. ET.

Tornado Watch remains in effect across eastern South Carolina and North Carolina, from Myrtle Beach to Morehead City until 1 a.m. ET.

Flood Watches remain in effect from Georgia to Virginia as bands of heavy rain will continue through the night.

Debby will be back over open water tonight and could gain a bit more strength over the next 24 hours, but is forecast to remain at tropical storm intensity.

A second landfall is expected in South Carolina Wednesday evening or early Thursday morning.

ABC News' Melissa Griffin

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.