Evacuations have been ordered in parts of 10 counties.
Alabama's weather won't change much today; high clouds will increase ahead of Hurricane Michael in the Gulf of Mexico, and a few scattered showers will develop today and tonight, but nothing especially widespread.
The Miami-based center forecast as much as 1 foot (30 cm) of rain in parts of Florida, Georgia and SC.
Residents of the U.S. state of Florida are boarding up homes and seeking evacuation routes away from Hurricane Michael, forecast to slam ashore midweek with life-threatening waves, winds and rains.
Beach dwellers rushed to board up their homes and sandbag their properties against the fast-moving hurricane, which was expected to blow ashore around midday Wednesday.
"Hurricane Michael is a monstrous storm and the forecast keeps getting more unsafe", said Florida Governor Rick Scott, who has activated 2,500 members of the National Guard in response.
President Trump has approved Florida's declaration of emergency.
Hurricane Michael would be the first major hurricane to hit the panhandle since Hurricane Dennis in 2005, which made landfall near Pensacola, according to hurricane centre data. "It looks like another big one", he said. Tropical storm warnings cover a further 3.1 million people in the three states.
Warnings are in effect for more than 300 miles of coastline, the National Weather Service said.
"We're looking at a significant storm with significant impact, possibly greater than I've seen in my 59 years of life", Johnson said of his city on Apalachicola Bay, where about 90 percent of Florida's oysters are harvested.
Scientists have long warned that global warming will make storms more destructive, and some say the evidence for this may already be visible. "Don't think that you can ride this out if you're in a low-lying area", Nelson said on CNN.
Speaking alongside emergency officials in Pasco County, Scott said Monday he's waiving tolls. More than 80 deaths were linked to that storm, according to The Associated Press - including residents of a nursing home. By Friday (Oct. 12), the storm will make its way out over the northern Atlantic, NOAA is now predicting.
For residents who want to leave, local authorities are stressing timely evacuation, too.
"This storm will be life-threatening and extremely risky", Scott said in a news briefing.
Florida State University announced it was closing for the week on Tuesday, along with schools in Leon County, home to the state capital Tallahassee.