(CNN) -- Irma was downgraded from a hurricane to tropical storm Monday, but that did little to lessen the threat of more destruction in the coming hours.
The storm was still hurling 70 mph winds Monday morning and 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.
"It wasn't supposed to be like this," CNN correspondent Sara Sidner said from Daytona Beach. "It's been strong enough to knock us over."
Another 125 emergency rescues were made in less than an hour Monday in Orange County, home to Orlando -- an inland city where many coastal residents had evacuated to before the hurricane.
Jacksonville -- the largest city geographically in the country -- is grappling with a record storm surge and will get slammed with severe flash flooding before Irma continues her destructive northward march to Georgia and beyond.
Extent of devastation still not clear
Irma made landfall on the Florida Keys as a Category 4 hurricane Sunday, but officials still have no idea how bad the damage there is.
That's because many of the islands are now inaccessible.
"There's no electricity throughout the Keys. No cell service in at least the lower and middle Keys," said Bill South of the National Oceanic and Atmospheric Administration.
"US 1, the only road that comes in and out of the Florida Keys, there's three choke points -- there are three places that are completely inaccessible."
Authorities plan to fly over the Keys on Monday, officials said.
The latest developments:
-- 5.8 million customers are without power across Florida, state emergency officials said. FEMA chief Brock Long said some places won't have electricity for weeks.
-- One person was found dead in Orange County in a single-car accident linked to the storm, police said without providing details.
-- Strong winds and flash flooding are still a threat as Irma spins into north Florida and toward Georgia over the next 24 hours.
-- Alabama, Tennessee, South Carolina and North Carolina will also feel effects of the storm.
-- More than 17,000 customers have already lost power in Savannah, Georgia.
-- In Venice, Florida, the water plant was shut down after it was damaged by the storm.
-- Miami streets turned into raging rivers, and the city's airport is closed because of significant water damage.
'It's the worst storm I've ever seen'
The storm is plowing into Georgia and others parts of the Deep South -- Alabama, Tennessee and the Carolinas -- bringing the danger of life-threatening storm surge and hazardous winds.
But that doesn't mean 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 one was over the Florida Keys, which Irma hit as a Category 4 hurricane. The second one, in Marco Island, left the island without water and power, authorities said.
"It's the worst storm I've ever seen," said South, the NOAA meteorologist.
More states brace for Irma
Georgia Gov. Nathan Deal has already declared a state emergency for all 159 counties as Irma barrels toward the state. Cities as far inland as Atlanta are under a tropical storm watch Monday and Tuesday.
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 of destruction
Before slamming into to the United States, Irma hit Cuba late Friday as a Category 5 hurricane.
It killed at least 26 people before leaving the Caribbean for Florida.
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.