Tropical Storm Florence: 5-day forecast track. NOAA/NHC graphic

The sixth tropical storm of the Atlantic hurricane season was forecast to strengthen into a hurricane before its center passes west of Bermuda early next week, forecasters at the US National Hurricane Center in Miami said.

Florence was over the open sea and expected to turn away from the United States. But it was an unusually large storm more than 1,000 miles wide. Forecasters said it would roil the surf along the eastern US shore, make swimming dangerous and erode beaches.

"Since the wind field is so large, it will send a swell out ahead of the system. That looks like it will affect much of the east coast Sunday into Monday," said Mark Willis, a meteorologist at the hurricane center. "It means dangerous surf conditions."

The storm's center was about 930 miles southeast of Bermuda. Florence was moving west-northwest at 14 mph and had top sustained winds of 50 mph.

It would become the season's second hurricane if its top sustained winds hit 74 mph, and was expected to reach that threshold by Saturday.

Computer tracking models projected Florence would hook sharply north and turn away from the United States during the weekend. It was too early to predict the potential impact on Bermuda, but the storm's vast size made it likely the British banking center would at least feel the fringes of the storm by Monday.

"Bermuda is in the cone of uncertainty at this point," Willis said.

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

The 2005 season broke records with 28 tropical storms, of which 15 became hurricanes. The worst was Hurricane Katrina, which devastated New Orleans, killed some 1,500 people along the US Gulf Coast and caused $80 billion in damage.

Forecasters had predicted this hurricane season would be busier than average, but most trimmed their predictions amid early signs of the El Nino weather phenomenon, which impedes hurricane formation in the Atlantic.