At its present course and speed, the center of Florence will likely approach the SC or Georgia coastline on Thursday or Friday.
Gov. Ralph Northam said the Virginia National Guard, the Virginia State Police, and other state agencies have already begun preparations.
McMaster's executive order will enable all SC agencies to coordinate resources if the storm hits.
Florence started out as a tropical storm Sunday before strengthening into a Category 1 hurricane.
Swells generated by Florence already were impacting Bermuda and starting to reach the USA coast, with life-threatening surf and rip current conditions likely, according to the NHC. Florence was centered about 985 miles (1,585 kilometers) east-southeast of Bermuda and is moving west at 7 mph (11 kph).
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The South Carolina Emergency Management Division tweeted Sunday that officials there are "preparing for the possibility of a large-scale disaster". It is not yet known if Florence will make landfall.
The National Hurricane Center has issued advisories for the Atlantic on Hurricane Florence, and two tropical storms, Helene and Isaac. Meanwhile, Myrtle Beach Mayor Brenda Bethune urged residents to secure their homes, but said it's too early to know if evacuations will be ordered.
Another "life-threatening impact" from Florence could be freshwater flooding from prolonged heavy rainfall inland, the center said.
The storm crossed the hurricane threshold Sunday morning, with winds climbing just above the 75 miles per hour threshold.
- The Atlantic hurricane season hits its peak on Monday and we now have three named storms to watch - Florence, Helene, and Isaac. Storm surge and freshwater flooding from rainfall could become life-threatening risks, especially from SC through the mid-Atlantic.
A number of tropical storms in the Atlantic Ocean, with the potential to strengthen to hurricanes, are heading towards land.
As of 11 a.m. Sunday, the storm was moving westward at 6 miles per hour. Admiral Christopher Grady said ships can better weather "storms of this magnitude when they are underway".