Michael's core, with maximum sustained winds of 110 mph, was about 335 miles south of Panama City, Florida, as of 2 p.m. ET Tuesday, the National Hurricane Center said.
Forecasters said Michael's tropical-storm-force winds stretched 370 miles across, with hurricane-strength winds extending up to 35 miles from the center.
Tropical Storm Michael has turned into a hurricane and is expected to strengthen rapidly before hitting Florida this week, bringing with it winds of at least 160 kilometres per hour and life-threatening flash flooding.
After hitting Florida, the storm is forecast to move north-east on Wednesday and Thursday along the Atlantic Coast and batter the Carolinas, which are still recovering from Hurricane Florence last month.
Michael seemed to spin up into a powerful hurricane very quickly because it sprouted over the weekend from a weather system known as a Central American gyre, said Dennis Feltgen of the National Hurricane Center.
From left, Haskel Johnson, Daniel Tippett, Jennifer Tippett and Nobuko Johnson filled sandbags at the Lynn Haven Sports Complex in Lynn Haven, Fla., Monday, to prepare for Hurricane Michael. "We wish all our guests who are traveling back home a safe journey".
Hurricane Michael is headed straight for Florida's Panhandle and is expected to make landfall there sometime on Wednesday.
Nadine, which is no threat to land, is notable as the 14th named storm this hurricane season, which was forecast to have less activity than during a normal year. Its center is located 200 miles south southwest of Apalachicola and 225 miles south of Panama City. By Friday (Oct. 12), the storm will make its way out over the northern Atlantic, NOAA is now predicting.
Hurricane Michael swiftly intensified into a Category 2 over warm Gulf of Mexico waters Tuesday amid fears it would strike Florida on Wednesday as a major hurricane.
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Escambia County Sheriff David Morgan bluntly advised residents choosing to ride it out that first-responders won't be able to reach them while Michael smashes into the coast.
Gov Scott warned in a news conference that Hurricane Michael is a "massive storm that could bring total devastation to parts of our state, especially in the panhandle". Up to four inches (10 centimeters) were expected to fall through Tuesday.
A Storm Surge Warning is in effect for.
Some Panama City residents were seen on the beach enjoying the cool breeze ahead of the storm, while others were loading up cars with luggage and visitors were checking out of hotels.
"I know people are worn out from Florence, but don't let this storm catch you with your guard down", North Carolina Gov. Roy Cooper said, adding, "A number of homes have rooftop tarps that could be damaged or blown away with this wind".
Michael also could dump up to a foot of rain over some Panhandle communities before it sweeps through the Southeast and goes out to sea over the next few days.
"A fast-moving storm also ensures that the storm can maintain some intensity farther inland".
A year ago saw a string of catastrophic storms batter the western Atlantic - including Irma, Maria and Hurricane Harvey - causing a record-equalling $125bn in damage when it flooded the Houston metropolitan area.
The declaration, requested by Scott, encompasses 35 counties and "will provide important resources and assistance from the federal government, including personnel, equipment and supplies, as well as making available funding sources for emergency protective measures", according to a news release from Scott's office.