, one of the most powerful Atlantic storms in a century, drove toward Florida on Friday as it lashed the Caribbean with devastating winds and torrential rain, leaving behind at least 21 deaths and a swath of destruction.
The "extremely dangerous" hurricane was downgraded from a category 5 to a category 4 early on Friday, but it still packed winds as strong as 155 miles per hour, the U.S. National Hurricane Center (NHC) said in an advisory.
The mayor of Miami Beach, Philip Levine, warned anyone who remained in the resort city to comply with a mandatory evacuation order.
"The size of the storm is massive – it's a nuclear hurricane, it has devastating effects," Levine told CNBC's Closing Bell in a phone interview from Mount Sinai Medical Center. He warned that those who stay behind will not have access to emergency services until the storm passes.
"During the storm itself, when the hurricane comes ashore, our first responders will not be going out there," the mayor said. "We're not going to risk their lives. People have been told, they have been mandated, they've been ordered to leave the beach."
Source: U.S. National Hurricane Center
The storm was last spreading westward over portions of Cuba and the central Bahamas, the NHC said.
It was will continue to move near the north coast of Cuba and the central Bahamas on Friday and Saturday before slamming into southern Florida on Sunday.
In Miami, hundreds lined up for bottled water and cars looped around city blocks to get gas on Thursday. Gasoline shortages in the Miami-Fort Lauderdale area worsened on Thursday, with sales up to five times the norm.
In Palm Beach, the waterfront Mar-a-Lago estate owned by U.S. President Donald Trump was ordered evacuated, media said. Trump also owns property on the French side of Saint Martin, an island devastated by the storm.
Dozens of cities and counties in increasingly-northern regions of Florida are issuing mandatory and voluntary evacuations, many of which are already being executed. Some highways have ground to a halt from congestion as evacuees flee the most dangerous hurricane areas.
In an interview with CNBC's Closing Bell, Florida Attorney General Pam Bondi said her office has received 7,000 complaints through its hotline about price gouging on fuel, water and hotel lodging. That's more than 100 complaints per hour, she said.
During a press conference earlier in the day, Bondi said convenience store chain 7-Eleven has been called out for price gouging on water in more than 100 complaints.
"7-Eleven, come on. This isn't the time to make a buck, this is the time to help your fellow citizens," Bondi said.
The attorney general encouraged people who see price gouging for fuel, water and other essentials to take photos of the establishment's name, the price they are charging and file a complaint at my myfloridalegal.com.
"We are out in the field all over this state so people can have the fuel that they need, essential commodities such as water, etc.," Bondi told CNBC.
Gov. Rick Scott, R-FL, seconded Bondi's admonition during a press conference. "It's disgusting if anybody price gouges," he said. "This is the time to help our neighbors. This is not the time to take advantage of our neighbors."
Among the biggest obstacles for evacuation efforts are overcrowded highways and fuel shortages, Scott said. To expedite the process, all tolls have been waived and the use of shoulders has been activated on certain highways.
Scott urged gas stations in evacuation zones to stay open as long as possible, and said that millions of gallons of fuel have been delivered by tanker trucks to areas with shortages.
Hospitality app-maker Airbnb activated a "disaster response program" for hosts to provide temporary housing to evacuees.
"Protecting life is our absolute top priority," the governor said.
St. Petersburg Mayor Rick Kriseman asked Floridians to comply with their area's evacuation orders on Friday.
"If this thing shoots right up the center of the state, everyone in the state of Florida is going to feel the impact of the storm, and so we really need to take this thing very seriously," Kriseman said on CNBC's "Squawk on the Street."
Irma is not expected to linger over the U.S. mainland as long as Hurricane Harvey, a phenomenon that led to record rainfall and storm surge in areas of Texas and Louisiana. But high flood potential still exists, and the NHC expects the storm to bring a deluge of up to 20 inches of isolated rainfall to parts of southeast Florida.
"You're gonna see the flooding, similar to what you saw with Harvey," George Foresman, former undersecretary of the Department of Homeland Security, told "Squawk on the Street" on Friday.
"But the potential for devastation will be much more widespread and much more destructive" given the high winds and the track of the storm, Foresman said.
Irma has ravaged a series of small islands in the northeast Caribbean, including Barbuda, Saint Martin and the British and U.S. Virgin Islands, ripping down trees and flattening homes and hospitals.
U.S. Virgin Islands Governor Kenneth Mapp on Friday confirmed four deaths in that territory.
A Reuters witness described the roof and walls of a well-built house shaking hard as the storm rocked the island of Providenciales and caused a drop in pressure that could be felt in people's chests.
Throughout the islands in its wake, shocked locals tried to comprehend the extent of the devastation — and simultaneously prepared for another major hurricane.
Hurricane Jose, moving west-northwest and last located about 415 miles off the Northern Leeward Islands, was updated to an "extremely dangerous" Category 4 storm and is expected in the northeastern Caribbean on Saturday.
President Trump said "America stands united, and I mean totally united" against the hurricanes in his weekly address.
The death toll from Irma has risen to 21 as emergency services gained access to remote areas.
The storm passed just to the north of the island of Hispaniola, shared by the Dominican Republic and Haiti, damaging roofs and causing flooding and power outages as it approached the Haitian side. One man was reported missing after trying to cross a river in Haiti's Central Plateau region, and the government said some 10,000 people were in emergency shelters.
The storm passed just to the north of the island of Hispaniola, shared by Dominican Republic and Haiti, causing some damage to roofs, flooding and power outages as it approached the impoverished Haitian side, which is particularly vulnerable to hurricanes and rain, although it did not make landfall.
Cuba started evacuating some of the 51,000 tourists visiting the island, particularly 36,000 people at resorts on the northern coast. In Caibarien, a coastal town in the hurricane's predicted path, residents were heading farther inland.
Irma was the strongest hurricane ever recorded in the Atlantic Ocean and one of the five most forceful storms to hit the Atlantic basin in 82 years, according to the NHC.
The storm activity comes after Hurricane Harvey claimed about 60 lives and caused property damage estimated at as much as $180 billion in Texas and Louisiana.
— CNBC's Ted Kemp contributed to this report
— Reuters contributed to this report