The counties could also see elevated tide levels and shallow coastal flooding with the evening high tide.
The governor is urging residents to begin preparing now by assembling an emergency kit with food, water, medication, pet supplies and important documents.
The third system - tropical Storm Helene is also strengthening.
It had maximum winds of 70mph with gusts of 85mph.
"If Florence manages to absorb [the tropical area], the increased spin...may expand Florence's size considerably, leading to a larger storm surge and an increased area of wind and rainfall impacts".
Swells generated by Florence could cause unsafe rip currents and coastal flooding on Saturday in areas including coastal DE and New Jersey, the National Weather Service said. The Carolina coast needs to brace for impact Thursday and Friday.
The system is expected to move westward over the coming days, and will likely become a tropical storm later Saturday.
It's track, however, remains uncertain.
Officials say Tropical Storm Helene is approaching islands off the western coast of Africa and is expected to become a hurricane soon.
Some forecast models showed Florence slamming into land by late next week, while others indicated the storm would curve away from the shore.
Governors in Virginia, North Carolina and SC all declared states of emergency over the past two days in anticipation of the hurricane. It is about 1,400 miles east of South Florida. "Being prepared is always the best strategy", McMaster said.
Florence's forecast path as of 5 a.m. on Sunday, Sept. 9. The National Weather Center in Melbourne, Florida, warned of risky rip currents along the state's east-central coast, including Brevard County.
The US East Coast is bracing for Florence as it approaches the coastline.
Tropical Storm Florence is expected to intensify into a hurricane on Sunday as it continues to take aim at the United States' southeast coast.
Florence was southeast of Bermuda, with tropical-storm strength winds of 70 miles per hour, up from 65 miles per hour earlier, the National Hurricane Center said in an 11 p.m. advisory. Currently, winds are sustained at 75 miles per hour.
The latest projection for the path of Tropical Storm Florence is centered on Wilmington, North Carolina, adding more urgency to the State of Emergency declared in the two Carolinas.
McMaster said it was "way too early" for any decisions on evacuations but said now is the time for S.C. residents - across all the state - to prepare.
- Goldman Sachs CFO About Its Bitcoin Trading Desk Roll Back:"Fake News"
- John McCain buried at US Naval Academy
- Galaxy users get advanced access to the much anticipated Fortnite Android beta
- IPhone Xs Max and iPhone 9 cases discovered at Walmart - they're big
- Leak claims cases for new iPhones found in Walmart
- Trump demands The New York Times 'turn over' anonymous source 'at once!'
- Sloane Stephens: Defending US Open Champion Reaches Quarterfinals
- Longtime Dem loses to left-wing challenger in primary upset
- Elon Musk smokes weed on Joe Rogan podcast
- Russian and Syrian jets pound Idlib province after summit