Editor's note: This is a developing story that is being constantly updated as the storm progresses. Also, follow our live updates here.
After breaking records in Jacksonville, Florida, Tropical Storm Irma continued to move north, flooding parts of coastal Georgia and South Carolina and lashing metro Atlanta with strong winds.
Two storm-related deaths were reported in Georgia.
Charleston, South Carolina, and Savannah, Georgia, two popular tourist destinations on the eastern side of the storm, saw flooding because onshore winds pushed water back into rivers, said CNN meteorologist Taylor Ward.
Irma flooded portions of Savannah's River Street and The Battery, the downtown Charleston neighborhood where the Ashley and Cooper rivers meet.
Waters in Charleston Harbor crested Monday at nearly 10 feet high, the city's third-highest reading, surpassing readings from Hurricane Matthew in 2016, Ward said.
The tropical storm, which stretched about 650 miles east to west, was affecting at least nine states, causing rain from eastern Mississippi to the Carolinas.
"In terms of the heaviest rain, it's covering much of Alabama, Georgia and the Carolinas," Ward said.
The rain and winds were expected to last through the night.
Jacksonville, the largest city geographically in the country, grappled with a record storm surge and immense flooding. The city's Memorial Park has turned into an unrecognizable lake.
"We have very serious, significant river flooding along the banks of the St. Johns River. It's bad now, it's going to continue to get worse," meteorologist Angie Enyedi said. "We've already surpassed historic levels, the levels will continue to rise."
In one Jacksonville yard, Irma snapped a tree near its roots, sending it crashing onto the car of an evacuee from South Florida.
No one was hurt. The tree at the end of Kristine Garcia's driveway, luckily, didn't fall on her house.
"We're definitely shaken up. We were in shock when we went outside," Garcia, 34, told CNN. "If it would have fallen toward our direction, it would have been a .... mess."
The storm was still hurling violent winds, pummeling cities in northeast Florida that had not expected to feel its full wrath.
Emergency workers in Daytona Beach rescued 25 people with a high-water truck after they were suddenly caught in an onslaught of wind and rain.
Another 125 emergency rescues were made in less than an hour in Orange County, home to Orlando -- an inland city to which many coastal residents had evacuated before the hurricane.
Now, the entire Georgia coast is under a storm surge warning as Irma continues its destructive march north.
No electricity or safe water
Irma devastated much of Marco Island, off the southwest coast of Florida, leaving it with no electricity or clean water.
Roommates Zack Forrest and Krock Indigo rode out the storm on the island and likened Irma to a relentless tornado. "The storm was really intense, it was like a tornado that lasted for an hour and a half," Forrest said.
"No way!" Indigo interrupted. "It was like 5 hours."
By the time Irma finished with Marco Island, she left 15 homes that have either lost their roofs or suffered other severe damage, the fire department said.
The latest developments:
-- Irma continued weakening Monday afternoon as it moved into southern Georgia, the National Hurricane Center said. As of 5 p.m. ET, Irma was centered about 10 miles east of Albany, with maximum sustained winds up to 50 mph.
-- Two people died in Georgia: A 62-year-old man who was on his roof was killed in Worth County, which experienced wind gusts of 69 mph, according to Kannetha Clem, a spokeswoman for the Worth County Sheriff's Office. Another man was killed in Sandy Springs, an Atlanta suburb, when a tree fell and "literally cut the home in half," according to Sharon Kraun, communications director for the city.
-- More than 6.7 million electric customers are without power in Florida, according to Floridadisaster.org. FEMA chief Brock Long has said some places won't have electricity for weeks.
-- In South Carolina, more than 177,000 customers were without power, according to utility companies.
-- Storm surge warnings in Florida, Georgia and South Carolina mean "there is a danger of life-threatening inundation," the hurricane center said.
-- Miami streets turned into raging rivers, and the city's airport is closed because of significant water damage.
Extent of devastation not entirely clear
Irma made landfall on the Florida Keys as a Category 4 hurricane Sunday, but the full extent of the damage there is still unknown.
That's because some islands are extremely difficult to access. Southbound US Route 1 -- the only road connecting the Keys -- is closed, Florida Keys spokesman Andy Newman said.
Large debris is blocking access, and a 150-foot stretch "has some buckling," Newman said.
On top of that, "there is currently no cell service, no electricity and no water" in most of the Keys, said Monroe County Commissioner Heather Carruthers.
"We are dealing with satellite phones and those landlines that are working to try to get information out," she said.
'It's the worst storm I've ever seen'
Irma plowed into Georgia and toward other parts of the Deep South -- Alabama, Tennessee and the Carolinas. But that doesn't mean all Floridians should try to go back home.
"We're asking folks to be patient and remain sheltered in place," said St. Augustine Fire Chief Carlos Aviles.
"Stay off the roads, stay off the streets, let us complete our assessment, clear the roads of water, power lines, trees and then you can get out there and determine what happened to your individual property or your neighborhood," said Fort Lauderdale Mayor Jack Seiler.
The massive storm triggered evacuation orders for 5.6 million people before it made two landfalls in the state Sunday.
The first was over the Florida Keys, and the second, over Marco Island, left the island without water and power, authorities said. "It's the worst storm I've ever seen," said Bill South of the National Oceanic and Atmospheric Administration.
More states brace for Irma
Georgia Gov. Nathan Deal already declared a state of emergency for all 159 counties as Irma barreled toward the state. Cities as far inland as Atlanta are under a tropical storm watch Monday and Tuesday.
North Carolina Gov. Roy Cooper also declared a state of emergency for all 100 counties in his state ahead of Irma.
South Carolina Gov. Henry McMaster issued a mandatory evacuation for some barrier islands.
And in Alabama, some city school districts including Birmingham, Huntsville and Auburn planned to close Monday and in some cases Tuesday.
Irma's deadly trail
Before slamming into to the United States, Irma hit Cuba late Friday as a Category 5 hurricane.
On Monday, Cuban state TV announced 10 hurricane related deaths -- meaning Irma killed a total of 36 people in the Caribbean before heading to the US.
This is the first year on record that the continental United States has had two Category 4 hurricane landfalls in the same year.
Last month, Hurricane Harvey devastated much of coastal Texas and killed more than 70 people.
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CNN's Chandrika Narayan, Ed Lavandera, Christina Zdanowiz, Rich Phillips, Deanna Hackney, Dave Hennen, Michael Guy, Andrea Diaz, Joe Sutton and Patrick Oppmann contributed to this report