The system has weakened and is now packing winds near 45 miles per hour.
Hurricane conditions are expected to reach the Carolinas on Friday, says the National Hurricane Center, but tropical force winds are expected to show up somewhere along the coast late Wednesday or Thursday morning.
Federal states of emergency have been declared in all three states, and collectively about 1.5 million people have fled the coast under mandatory evacuation orders.
Helene is one of a number of tropical storms brewing in the region, with experts warning it poses a "risk to life". "This storm is extremely unsafe".
It added that maximum sustained winds were near 75 miles per hour with higher gusts.
There is the potential for damaging winds with gusts up to 70mph possible, with sustained winds of 30 to 40mph.
At 500 a.m. AST (0900 UTC), the center of Subtropical Storm Joyce was located near latitude 33.8 north, longitude 43.1 west.
Meteorologists are still trying to work out exactly which way the weather system could track, she said, adding: "But at the moment it does look as though it's tracking towards the western side of the United Kingdom and will cross the southern half of the United Kingdom". A faraway system, about 600 miles southwest of the Azores, also looks likely to form some time Wednesday, head to the southwest over the next two days, then turn to the northeast. following Helene's path.
"Heavy and long-lasting rainfall could lead to catastrophic flooding in inland parts of North Carolina, South Carolina and Virginia".
Tropical storm warnings were in effect for Martinique, Dominica and Guadeloupe, while watches were in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius. Regardless of development, heavy rainfall and gusty winds are expected across portions of northeastern Mexico, Texas, and Louisiana late this week.