Hurricane Dennis, the second hurricane to threaten the United States in a week, inched closer to the southeastern coast today, picking up power as it advanced. But exactly where and when the slow-moving--and increasingly dangerous--storm would touch shore remained a mystery.

Although, by late afternoon, Dennis was still a minimal hurricane, packing 80 mph winds as it descended on the northwestern Bahamas, there were ominous signs that it was organizing and strengthening. Forecasters said the storm could intensify into a perilous Category 3 hurricane, with winds of 111 to 130 mph, capable of inflicting extensive damage.

"There is every reason to believe Dennis is going to become a very menacing hurricane," said Todd Kimberlain, a meteorologist with the National Hurricane Center in Coral Gables, Fla.

The last Category 3 storm to reach the southeast coast was Hurricane Fran on Sept. 6, 1996, which slammed the Wrightsville Beach-Wilmington area of North Carolina, causing an estimated $2 billion in damage. On Sunday, Hurricane Bret struck Kenedy County, Tex., on the Gulf Coast as a Category 3 but caused little damage because the area was sparsely populated, with cows far outnumbering people.

Up and down the southeastern coast today, residents prepared for a hurricane that could hit practically anywhere from the heavily populated central Florida coast to the fragile North Carolina beaches on Sunday or Monday. Hardware stores began to sell out their supplies of plywood, grocery stores welcomed a rush of customers and--off the coast of Florida and the Outer Banks of North Carolina--adventurous surfers took advantage of the increasingly high waves stirred up by the storm.

"We're prepared, but we get a surprise every time," said Mayor Avery Roberts Jr. of Wrightsville Beach, N.C., population 3,000, which has weathered three hurricanes since 1996.

Hurricane warnings were posted for the northwestern Bahamas, where Dennis was expected to strike tonight. Residents there boarded up windows, opened shelters and stocked up on provisions. Abaco Air, a regional airline, stowed its planes in a hangar, and residents hauled boats out of the water or put down extra moorings. Cruise ships canceled stops in the Bahamian capital of Nassau as a precaution, dealing a blow to Bay Street merchants. But Nassau was in the clear by midday.

A hurricane watch--meaning that hurricane conditions are possible but not necessarily likely within 36 hours--was posted at 11 a.m. today from Sebastian Inlet along the central Florida coast to Fernandina Beach 30 miles north of here and near the Georgia border.

Part of the uncertainty concerning the storm's destination had to do with its extremely slow pace, about 5 mph in a west-northwesterly direction, Kimberlain said. A predicted turn north, which would have steered Dennis away from the Florida coast, had not yet occurred. Late last night, the storm was nearly 300 miles southeast of Cape Canaveral.

As it dallied, Dennis's potential to become more dangerous was growing, forecasters said, for at least two reasons. A wind shear that had prevented the storm from becoming better organized Thursday was gone today, and the hurricane's long stay over warm waters could only increase its energy, Kimberlain said.

In South Carolina today, Gov. Jim Hodges (D) ordered 1,000 National Guard troops and 500 state troopers to prepare for duty, but no decision had been made on any coastal evacuations. Paul Whitten, director of emergency preparedness in Horry County, which includes the Grand Strand of Myrtle Beach, said he is ready to evacuate 100,000 residents and 125,000 vacationers should that become necessary.

"Oh, we're doing a lot of fun things," he said dryly about the preparations.

But at the Home Depot in Charleston, S.C., employee Gary Dalbec was too busy to worry about what was brewing, far too close, in the Atlantic.

"We're moving a hundred sheets of plywood an hour out of here," he said. "And generators? We've sold 60 of them today."

Special correspondent Catharine Skipp in Miami contributed to this report.