Helene is now 2,065km south-southwest of the Azores in the Atlantic and Met Éireann said the storm would approach the south coast by Tuesday.
Meanwhile, the US National Hurricane Centre said Hurricane Helene may be downgraded to a tropical storm.
Met Office meteorologist Alan Deakin said: "Hurricane Helene may influence things in combination with an area of low pressure over the coming days".
In contrast, no coastal watches or warnings were in effect for Helene, but residents of the Azores were encouraged to monitor the storm.
Florence is unlike Hurricane Hugo, a category 4 storm that struck Charleston in 1989.
The Trinidad and Tobago Meteorological Service said Tropical Storm Isaac is moving away from Lesser Antilles and eastward into the Caribbean Sea.
Florence's new eye appears to be stable enough that the storm is unlikely to undergo another eye-wall-replacement cycle; this means that Florence can continue to build in power, which could spell trouble for the US states that lie in the storm's path, Stewart said in the report. This can cause a temporary weakening of the storm, AOML explained. Warmer ocean temperatures drive greater storm activity. While people along large swathes of the Eastern Seaboard have been dreading the storm for days, you can say one thing: it arrived right on time.
Like Isaac, Tropical Storm Joyce is packing maximum winds of about 65 km/h. "Heavy and long-lasting rainfall could lead to catastrophic flooding in inland parts of North Carolina, South Carolina and Virginia".
Some fluctuation in wind speeds are expected, and weakening forecast Thursday, but it's still expected to be a fierce storm triggering risky flooding.
At 5 a.m., the storm was centered 575 miles (925 km) southeast of Cape Fear, North Carolina, moving at 17 mph (28 kph).
The NHC forecast track shows the storm decreasing in forward speed as it heads west towards Central America. In South Carolina, local officials reported 300,000 had already evacuated the coast as of Wednesday.
The agency is also monitoring a tropical disturbance that has a 60 percent chance of forming a tropical storm.