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

Florence is the sixth tropical storm of the Atlantic hurricane season and would become the second hurricane if its top sustained winds hit 74 mph.

The storm was about 1,240 miles southeast of Bermuda and moving west-northwest at 9 mph, the Miami-based National Hurricane Center said.

The swirling mass of thunderstorms had top sustained winds of 50 mph, up slightly from Tuesday. It was slowly strengthening in the open Atlantic and could become a hurricane on Thursday, the hurricane center said.

Computer tracking models projected Florence would hook sharply north over the weekend. That would spare the Caribbean islands and keep Florence away from the US coast but could imperil the mid-Atlantic island of Bermuda early next week.

The six-month hurricane season that began on June 1 has only produced one brief hurricane so far.

Tropical Storm Ernesto reached hurricane strength near Haiti late last month. It later made landfall twice in the United States as a tropical storm, in Florida where it was barely noticeable, and then over the mid-Atlantic coast, where it poured torrential rain on several states.

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

Forecasters originally predicted this hurricane season would be busier than average but most have now cut their predictions amid early signs of the El Nino weather phenomenon. By warming Pacific waters, El Nino affects wind patterns, helping to squelch hurricane formation in the Atlantic.