Hurricane Irma bears down on Florida Keys, at least 1 dead

Hurricane Irma, which has strengthened again to Category 4, is bearing down on the Florida Keys, bringing damaging winds and heavy rain and leaving at least one person dead.

A man in Monroe County, which encompasses Key West, was killed after he lost control of a truck he has driving that carried a generator as winds whipped at tropical-storm strength, according to the Monroe County Sheriff's Office.

The death toll from the hurricane in the Caribbean meanwhile has risen to 22, bringing the total number of killed to at least 23.

The storm's eye was as of 8 a.m. ET about to move over the Florida Keys, as it was about 20 miles from Key West and 110 miles south of Naples, Florida, on the state's west coast.

Wind gusts up to 82 mph were reported in the Keys and up to 68 mph in Miami, according to the National Hurricane Center, reported ABC meteorologist Dan Manzo.

The National Weather Service earlier Sunday morning urged everyone in the Florida Keys to "hunker down," warning, "the worst winds are yet to come."

A tornado watch is also in effect across the area, with two twisters already reported in South Florida.

In Collier County, emergency vehicles were pulled from the roads Sunday morning as heavy wind gusts began blowing in, the county said.

A weather station near Fort Pierce reported a four-hour rainfall total of 10.53 inches between midnight and 5 a.m. The rainfall rate along the east coast of Florida was 3 inches per hour.

Parts of Tennessee were under a tropical storm watch Sunday morning, which could bring be a significant wind event for Alabama, Georgia, southern Tennessee and South Carolina.

The storm, which had been downgraded to Category 3 after making landfall as a rare Category 5 hurricane in Cuba early Saturday morning, has sent 124,000 people to Red Cross and Red Cross-affiliated shelters, as of Sunday morning. That figure does not include church and ad hoc shelters. More than six million people have been warned to evacuate its path.

Florida Gov. Rick Scott called the storm unprecedented.

"This is a life-threatening situation," Scott said Saturday. "Our state has never seen anything like it."

The governor stressed the dangers of what he called a "deadly, deadly, deadly storm surge."

ABC News meteorologists are forecasting storm surges of 10 feet in Tampa and Sarasota, and 10 to 15 feet from Fort Myers to Naples. Somewhat lower storm surges of 3 to 6 feet may occur from Miami to Key Largo.

Winds were already picking up in Florida early Saturday, with gusts between 40 and 60 mph.

Hurricane-force winds with gusts over 115 mph are possible in the Keys by daybreak Sunday.

Power outages, halted flights and empty ATMs

As of 7 a.m. ET Sunday, 478,340 customers in Florida were without power. In Miami-Dade County alone, 250,740 were without power.

Some 10,000 flights have been cancelled in anticipation of Irma, about 7,000 of them in Florida alone.

Ahead of Irma's arrival in the Sunshine State, the last flights departed Friday night from Miami International Airport and Fort Lauderdale-Hollywood International Airport. Miami's airport officially remains open, while Fort Lauderdale's airport is closed for Saturday and Sunday.

Many ATM machines across southwest Florida were out of cash by late Friday night after people stocked up in case Hurricane Irma causes power outages that make debit and credit card transactions impossible, The Associated Press reported.

Millions evacuate; many others take shelter

Approximately 6.3 million Floridians are under mandatory or voluntary evacuations, the Florida Division of Emergency Management said Saturday. When evacuation orders in South Carolina and Georgia are included, the number climbs to 6.8 million.

Germain Arena, a large shelter between Naples and Fort Myers along Florida's west coast, is already at capacity Saturday as hundreds of people were in line waiting to get in.

Miami-Dade County Mayor Carlos A. Giménez said Saturday morning about 25,000 residents are sheltered in Miami-Dade alone, a number he called "unprecedented in our history."

"We must remain vigilant," Giménez said. "The storm will still strengthen ... and we will be impacted."

A slew of counties and cities in south Florida have issued curfews as the storm nears. Counties include Broward, Flagler, Orange, Palm Beach, Volusia, and Seminole. The cities of Miami, Miami Beach, North Miami Beach, Homestead, Key Biscayne, Melbourne Beach, Coral Springs, and Coral Gables also issued curfews.

ABC News' Chris Donato, Matt Foster, Daniel Peck and Max Golembo contributed to this report.