Joe Biden warned Americans Hurricane Milton was “a matter of life and death” as the “catastrophic” storm barrelled towards Florida.
In an address on Wednesday evening, Mr Biden said the hurricane is “looking like the storm of the century".
Mr Biden acknowledged that some areas have been left "without a moment to catch their breath", with Milton coming hot on the heels of Hurricane Helene, which wrought chaos just a fortnight ago.
"I know it's really tough, all your belongings, everything you own," he told Americans.
"But I urge everyone in Hurricane Milton's path to follow all safety instructions as we head to the next 24 hours. It's literally a matter of life and death."
As it headed towards the west coast of Florida on Wednesday with wind speeds of up to 130mph, Hurricane Milton hurled rain and gusty winds at the Tampa Bay area, and officials sounded urgent warnings for residents to evacuate ahead of a life-threatening storm surge.
Florida traffic cameras caught a large tornado on the ground in Broward County.
The storm was on a collision course for the Tampa Bay metropolitan area, home to more than three million people, though forecasters said the path could vary before the storm makes landfall late Wednesday night.
The National Hurricane Centre said the storm's path might "wobble", but that the entire Tampa Bay region and points south were at grave risk.
Millions of people along a stretch of more than 300 miles (483 km) of coastline were under evacuation orders, and authorities issued increasingly dire warnings on Wednesday morning as the storm approached.
The famous Sunshine Skyway Bridge, which spans the mouth of Tampa Bay, closed around midday on Wednesday. Other major bridges closed, as well.
"This is it, folks," said Cathie Perkins, emergency management director in Pinellas County, which sits on the peninsula that forms Tampa Bay.
"Those of you who were punched during Hurricane Helene, this is going to be a knockout. You need to get out, and you need to get out now."
Sarasota County Emergency Management Chief Sandra Tapfumaneyi told CNN that people who remain on the barrier islands in her county just south of Tampa likely would not survive the projected 10- to 15-foot storm surge.
"If you choose to stay, make sure you have a life preserver handy," she said.
The storm was on a collision course for the Tampa Bay metropolitan area, home to more than 3 million people, though forecasters said the path could vary before the storm makes landfall late Wednesday.
British nationals in Florida expressed their “anxiety” as they prepared for Hurricane Milton to hit.
At 8am local time (2pm in the UK), the eye of the storm was 250 miles (405km) southwest of Tampa.
Milton was downgraded to a Category 3 hurricane - the third-highest level - late on Wednesday but remained a major storm and a grave threat as it closed in on Florida's west coast, where officials sounded urgent warnings for residents to flee inland or face grim odds of surviving the storm's surge.
Milton has fluctuated in strength as it approached, but regardless of the distinction in wind speeds, the National Hurricane Center said it would be a major and extremely dangerous storm when its center makes landfall late on Wednesday local time.
The hurricane was centered about 100 miles (155km) southwest of Tampa with maximum sustained winds of 125mph (205kph), the centre reported.
It was expected to maintain hurricane strength as it crossed the Florida peninsula, posing storm-surge danger on the state's Atlantic Coast as well.
While wind speeds may drop further, the size of the storm was growing, endangering more coastal areas.
Officials including Biden and Tampa Mayor Jane Castor warned people in evacuation zones to get out or risk death.
"There is high confidence that this hurricane is going to pack a major, major punch and do an awful lot of damage," Governor Ron DeSantis said on Wednesday.
Trucks have been running 24 hours a day to clear mounds of debris left behind by Helene before Milton potentially turns them into dangerous projectiles, Mr DeSantis said.
Deanne Criswell, administrator of the Federal Emergency Management Agency, said she would travel to Florida on Wednesday and remain there after the storm to help coordinate recovery efforts.
FEMA has moved millions of liters of water, millions of meals and other supplies and personnel into the area. None of the additional aid will detract from recovery efforts for Hurricane Helene, she said.
"I want people to hear from me directly, FEMA is ready," she said.
About 2.8 per cent of U.S. gross domestic product is in the direct path of Milton, said Ryan Sweet, chief U.S. economist at Oxford Economics. Airlines and energy firms were among the companies beginning to halt their Florida operations as they braced for disruptions.
Major Florida theme parks shuttered ahead of the storm, with Disney World, Universal Studios and SeaWorld all planning to close their doors later on Wednesday.
Mobile homes, nursing homes and assisted-living facilities faced mandatory evacuation.
