Isaac is expected to move near or over the central Lesser Antilles on Thursday and move into the eastern Carribbean Sea on Thursday night.
The most likely arrival time for tropical storm force wind is during the day Friday.
Rain projections also increased, with some locations along the North Carolina coast now expected to get up to 40 inches.
It has been a calmer than-normal tropical storm season up until this point, yet things are grabbing big: three hurricanes are beating in the Atlantic, with Hurricane Florence quickly moving toward the US East Coast. Coastal areas in both Carolinas have taken steps to evacuate residents and tourists, including a mandatory evacuation in SC.
Florence's slow storm movement after landfall may bring an extended period of heavy rainfall through the weekend into early next week.
There are no watches or warnings in effect for the Midlands or Central Savannah River Area.
Isaac is forecast to gradually weaken over the next few days and could degenerate into a tropical wave at any time.
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Brock Long, the head of the US Federal Emergency Management Agency (FEMA), warned that Florence could "kill a lot of people". As a result, the agency didn't have almost enough supplies or qualified staff to send to Puerto Rico .
It said that the storm, which was located at 775 miles east of the Lesser Antilles, had maximum sustained winds of 70 miles per hour.
The Met Office issued a weather warning ahead of Storm Helene's arrival, and said "injuries and danger to life from flying debris are possible".
He said as a result of the damage caused by Maria a year ago, the island does not have its full complement of shelters but emergency shelters across the island would be opened by Wednesday.
Subtropical Storm Joyce has maximum sustained winds of near 45 miles per hour (75 km/h) with higher gusts.
Behind Isaac, Hurricane Helene also continued to deliver good news. Regardless of development, heavy rainfall and gusty winds are expected across portions of northeastern Mexico, Texas, and Louisiana late this week. A hurricane hunter plane is scheduled to investigate the system Thursday.
There were two other systems in development Wednesday as well.