Storm is getting very close to Savannah and that area is at high tide, so flooding from Savannah to Hilton Head and in much of the SC low country is likely to be severe. Michael Lowry, who is on at TWC right now (a hurricane/storm surge expert), is talking about Matthew making landfall earlier than expected, between Hilton Head and Charleston in the next few hours (NHC forecast has it coming closest to landfall around Charleston).
The only saving grace for Charleston would be that the storm will be closest to that city around 7-8 am, which will be near low tide, which makes a huge difference (5' in that area). Also winds should be down to about 90-95 mph by then (now at 105 mph, as weakening is underway), which is still a serious storm, especially if it's actually making landfall a few hours from now, SW of Charleston, as that would put Charleston in the strongest quadrant of the storm.
NHC just downgraded the winds to 85 mph, as the storm nears Charleston (it's 20 miles SSW of there as of 8 am). It actually did not "officially" come ashore overnight, but the NW part of the eyewall, with the worst winds, did come ashore from Hilton Head to Charleston; the center of circulation needs to be over land for there to be an official landfall.
Storm surge is still a major issue, although fortunately, it's near low tide in the Charleston area, so the 5-8' surge will not result in major flooding for most. Rainfall amounts have been off the charts for the entire SE coast and inland, with most places having gotten 10-15" of rain, so river and urban flooding is a major issue right now - even I-95 in SC is closed.
As the system is now moving NE at 12 mph, it's still paralleling the coast, as it has been doing since Thursday night as it approached Florida, and it's expected to continue paralleling the coast until it passes Wilmington, NC, after which it will more more to the ENE and then E, heading out to sea. It's not clear if it will officially make landfall anywhere, which would be remarkable, given how close it has been to the FL/GA/SC coasts for the last ~500 miles or so. Best chance is the beaches just south of Charleston, like Folly Beach.
Beyond the NC coast, Matthew will likely do the loop down to the Bahamas, weakening to a tropical depression along the way and it is unlikely to restrengthen again. However, some models show the weakened tropical storm not quite going to the Bahamas, but heading towards Bermuda and then NE into the Atlantic.