The Tar River early Friday was at 19.7 feet, nearly four feet above flood stage, and could crest at around 20 feet before starting to recede by Saturday morning, National Weather Service forecaster Bob Ussery said.
"We don't know when the thing is really going to go down completely," he said.
Four people were rescued Thursday after floodwater surrounded their homes, authorities said.
The storm dumped 2 to 5 inches of rain as it passed through central and eastern North Carolina on Wednesday, with some areas reporting nearly 8 inches. Alberto flooded roads and low spots, led to scores of traffic accidents and sent rivers rising.
Wes Strickland organized a sandbagging effort to try to protect his business, Club Fuzion, which sits along the Tar River. Among the volunteers were members of a pre-wedding party that was scheduled for Thursday night at the club, he said.
"The bride, groom, best man and the rest of the party were all out here working," Strickland said. "We piled sandbags about three feet high along the perimeter of the club and in front of the doorways. The inside is still dry, but the back patio is completely under water."
Alberto, the first named storm of the Atlantic Hurricane season, came ashore Tuesday in Florida as a tropical storm with 50 mph winds, then headed up the East Coast before moving out to sea on Thursday. One death was blamed on the storm, a 13-year-old boy who drowned after chasing a ball into a culvert during pouring rain near Louisburg.