Obama has declared federal states of emergency in Florida, Georgia, and North and South Carolina. Authorities struck almost apocalyptic tones, with increasingly urgent warnings to evacuate coastal and low-lying areas, but many others heeded no intention, hunkering down at home.
Torrential rain and strong winds lashed cities, bringing down trees, causing tall buildings to sway after night fell and turning normally bustling population centers into ghost towns.
At least five people died in Florida -- two women killed by falling trees -- a third woman from a heart attack and a couple killed by carbon monoxide as they ran a generator in their home's garage.
Matthew damaged roofs at the Kennedy Space Center but spared Florida's heavily populated south-central coast a direct hit. "The worst effects are still likely to come," warned Governor Rick Scott, referring to expected flooding.
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In St Augustine -- a one-time Spanish colony that calls itself the nation's oldest city -- roads were deserted, many blocked by downed trees or flooded with ocean water and the city eerily empty under darkly menacing skies.
Mayor Nancy Shaver said up to half the population in vulnerable zones had refused to evacuate. Officials up and down the coast urged people to stay at home once the storm moved in.
Across the state, 1.1 million people were left without power, about 11 percent of customers, officials confirmed.
The center of Matthew passed northeast Florida and coastal Georgia overnight into Saturday before raking the South Carolina coast, weakening but retaining hurricane force until moving out to sea on Sunday, the NHC predicted.
Evacuation orders covered some three million residents, with cities like Jacksonville and Savannah, Georgia in the storm's path.
Cities including Savannah and Charleston in South Carolina ordered dusk to dawn curfews to keep people off the streets and guard against looting.
Matthew was expected to dump up to 14 inches (36 centimeters) of rain along the Georgia coast, where Governor Nathan Deal ordered six counties home to nearly a half-million people to evacuate.
The South Carolina Emergency Management Division tweeted that the situation was "DIRE for barrier islands" most exposed to the ocean.
South Carolina Governor Nikki Haley said the storm was "getting worse." More than 300,000 of the 500,000 people in at-risk areas had been evacuated but that "was not enough."
Some coastal islands, Haley added, "will be under water."