Hurricane Matthew has been downgraded to a Category 1 storm as it heads towards the US after killing nearly 900 people in Haiti.
Northern Florida, Georgia and South Carolina remain on high alert as the weather system sweeps along the east coast at 12mph with winds reaching 85 mph.
The storm has toppled trees and knocked out power to more than half a million people, but southern Florida has avoided severe damage, despite four people reportedly losing their lives.
"We have been very fortunate that Matthew's strongest winds have remained a short distance offshore of the Florida and Georgia coasts thus far, but this should not be a reason to let down our guard," the Hurricane Centre said.
Matthew has now moved to South Carolina after claiming the lives of at least 887 people in Haiti , and it is expected to hit North Carolina later.Earlier, the eye of the storm hung just offshore as it moved up the coastline, sparing communities the full force of its winds.
President Obama recalled the lessons of Hurricane Sandy, which slammed into New Jersey and New York City in 2012, leaving 200 dead and causing tens of billions of dollars in damage.
"Initially people thought 'This doesn't look as bad as we thought', and then suddenly you get a massive storm surge and a lot of people were severely affected."
Mr Obama warned that the potential for storm surge, flooding, loss of life and severe property damage "continues to exist" and that people need to be prepared in the days to come.
On Thursday, two million people living in low-lying areas and on the barrier islands in Florida were told to "evacuate, evacuate, evacuate" by Governor Rick Scott, who warned: "This storm will kill you."
The National Weather Service in Jacksonville, Florida, said tide levels in Charleston in South Carolina and Fort Pulaski in Georgia could approach or even surpass those seen during major floods in 2015.
In St Augustine, south of Jacksonville, many of the 14,000 residents refused to leave despite warnings of a 2.4 metre storm surge.
Airlines have cancelled at least 4,500 flights, including many in and out of Orlando, where all three of its world-famous theme parks - Walt Disney World, Universal Studios and SeaWorld - have closed.
Airports in southern Florida have started to return to normal, however, with American Airlines seeing its first arrival at Miami at 9.05am local time.
At the Kennedy Space Center in central Florida, NASA and private company SpaceX were taking precautions to protect capsules and rockets.
