Tropical Storm Florence: 3-day forecast track. NOAA/NHC graphic

Bermuda issued a hurricane watch before nightfall, alerting the island's 65,000 residents to brace for hurricane conditions in the next 36 hours.

Florence, the sixth tropical storm of the Atlantic hurricane season, was poorly organized and unusually wide.

At 11 pm Friday, its center was about 575 miles south-southeast of Bermuda. Florence was moving northwest at nearly 15 mph but was so large that Bermuda residents would likely feel its outer edges well before the center arrived late on Sunday or early Monday, forecasters at the US National Hurricane Center in Miami said.

Florence's top sustained winds have held steady at about 50 mph for several days but the forecasters said they could reach the 74-mph threshold to become a hurricane as early as Saturday.

Computer tracking models projected the storm would pass over Bermuda and turn northeast, moving away from the United States and into a part of the open Atlantic where shearing winds and cooler waters would sap its strength.

But the storm was expected to kick up heavy surf and dangerous riptides along the US Atlantic coast during the weekend, forecasters said.

In Bermuda, hardware stores were crowded with shoppers buying lanterns and other storm-related supplies, while emergency management officials met to ensure the island was ready for the storm.

"We are definitely in for a prolonged period of tropical storm-force winds. However, I can assure you we are prepared," Public Safety Minister Derrick Burgess said.

The six-month hurricane season that began on June 1 has produced only one hurricane so far. Tropical Storm Ernesto briefly reached hurricane strength near Haiti last month but weakened before drenching the US East Coast.