Charley, with winds of 145 mph, was expected to make landfall south of Tampa, track along the Florida peninsula, then emerge into water and move along the Georgia and South Carolina coasts on Saturday.
"Based on all the information we've received on this storm, it's pretty clear that a lot of low lying areas along our state's southeastern coast are directly in harm's way," Sanford said.
A hurricane warning means sustained winds of 74 mph or higher are expected within 24 hours.
Flood watches were posted from the Midlands to the sea. The National Weather Service also issued an inland tropical storm wind watch warning winds of between of 30 to 40 mph with gusts of 50 mph could be felt in inland areas east of Columbia as the storm moved by.
Sanford said sustained winds of up to 65 mph - somewhat below hurricane strength of 75 mph - could be expected in Beaufort, Colleton and Jasper counties, as the storm approached Saturday.
A storm surge of 3 to 5 feet was expected although officials said in some places in Beaufort County it could be 10 feet. Sanford said there was ''very little chance of it going to a mandatory evacuation'' unless Charley strengthened considerably before making landfall in Florida.
"This is for low-lying areas. It's for oceanfront. It's for river areas. It's for low-lying properties. It's for people in mobile homes," the governor said.
However, Sanford spoke with reporters at noon when Charley was packing winds of 110 mph. The wind speed quickly increased to 145 mph as it approached the Florida coast and hurricane warnings and watches were issued for the South Carolina coast.
The governor was monitoring the situation and taking in the new information, spokesman Will Folks said.
Schools in Beaufort County closed Friday, officials closed Hunting Island State Park in Beaufort County was closing Friday and Hilton Head Island officials urged visitors and residents alike to stay off area beaches.
At least four Lowcountry counties planned to open shelters for people who wanted to ride out the storm on higher ground.
The storm system was expected to bring more heavy rains in the wake of Tropical Storm Bonnie, the remnants of which moved through on Thursday dumping up to five inches of rain in places.
"The purpose of a voluntary evacuation is for people to evaluate for themselves whether or not they are going to be impacted by a 3 to 5 foot storm surge," Sanford said.
"If the winds degrade as they come up the Florida peninsula, it could be less than that. I think you want to look at worst-case scenarios," he added.
During Bonnie, North Myrtle Beach received more than 4 inches of rain while an inch was recorded in Charleston. Locally heavier amounts of as much as 5 inches of rain occurred in a band from Aiken to Lexington counties, the National Weather Service said. A few streets and low lying areas were flooded.
The National Weather Service reported Thursday tornadoes damaged several homes in Fairfield and Chesterfield counties, and four tornadoes were confirmed in Horry County, said Joe Farmer, a spokesman for the state Emergency Management Division.