A truck is overturned as Hurricane Irma passed south Florida, in Miami, on Sunday. Thomson Reuters

Hurricane Irma made landfall in the Florida Keys on Sunday morning as a Category 4 storm and battered Florida into Monday morning.

Irma is now a tropical storm, but it's still producing strong winds and storm-surge flooding.

After pummeling the Keys, the storm moved up the west coast of Florida, heading for Georgia, South Carolina, and Alabama.

Irma hit several Caribbean islands as a Category 5 storm last week, killing at least 41 people.

Tropical Storm Irma, which hit Florida as a major hurricane on Sunday, has finally crossed the state line and is now slamming Georgia with powerful winds. The storm is causing record flooding in certain cities because of rainfall and dangerous storm surges.

Irma ravaged the west coast of Florida and brought storm-surge flooding, tornadoes, and forceful wind gusts across the state. On Monday morning, it was classified as a tropical storm, according to the National Hurricane Center. But the NHC's latest update says the storm is still producing 50-mph winds with higher gusts, as well as life-threatening storm surges.

Irma made landfall at Cudjoe Key in the Florida Keys at 9:10 a.m. ET on Sunday, marking the first time in recorded history that two Category 4 storms have made landfall in the US in the same year.

After pummeling the Keys, the storm made landfall on the state's peninsula in Marco Island at 3:35 p.m. ET. The police department there reported a 130-mph wind gust. Irma has since been moving north up Florida's Gulf Coast.

Tropical-storm warnings have been discontinued in Florida, but now areas north of Altamaha Sound in Georgia to the South Santee River in South Carolina are under warnings.

Record flooding is expected to hit Savannah, Georgia, on Monday. Storm-surge warnings are still in effect from north of Fernandina Beach in Florida to the South Santee River, and on Florida's west coast from north of Clearwater Beach to the Aucilla River.

Irma's effects in the South

Irma arrived in Georgia on Sunday after steamrolling several Caribbean islands and leaving at least 41 people dead. In Havana, waves crested 36 feet on Sunday, forcing Cubans and tourists to evacuate inland.

Predictive models initially suggested that the storm was heading straight for Miami, but Irma took a westward turn that helped that city avoid the brunt of the storm. The hurricane was big and powerful enough to affect most of Florida, however, including parts that didn't come near the eye. The storm is almost as large as Texas — wide enough that both Florida coasts experienced hurricane-force winds.

Early reports indicate at least five people died as a result of the storm in Florida. Nearly 6 million homes and businesses had lost power in the state as of Monday afternoon, and another 720,000 were without power in Georgia.

On Sunday morning, water from as far away as Tampa was already starting to be sucked out by the storm — ready to come rushing back in with storm surge.

That storm surge caused severe flooding in Jacksonville, Fort Myers, and other cities. Streets in Miami were also flooded, and high winds caused a massive crane to collapse onto a building there as well.

Irma passed over some of the warmest water it has encountered before making landfall in Florida, which helped it maintain its intensity. The storm regained strength to become a Category 4 hurricane for part of the day Sunday, but its current wind speeds are hovering at around 50 mph.

On Saturday, Irma began spinning funnel clouds near South Florida, prompting at least one tornado to touch ground in the Fort Lauderdale suburb of Oakland Park, according to the National Weather Service. There were reports of several other tornadoes, and a few more are still possible across northeast Florida, parts of Georgia, and South Carolina through Monday night.

Recently planted palm trees are strewn across the road as Hurricane Irma passes by Miami on Sunday. AP Photo/Wilfredo Lee

At a press conference Saturday evening, Gov. Rick Scott of Florida said that more than 6.5 million Floridians had been ordered to evacuate and more than 70,000 had taken refuge in more than 385 shelters. By Sunday morning, that number had reached 114,000 people in 500 shelters statewide.

Scott urged evacuees to be patient and not return to their homes until local officials confirm it is safe to do so.

The NHC's latest forecast for Irma suggests storm will hit Georgia and the Carolinas with heavy rain on Monday and head into Alabama on Tuesday.

NOAA

President Donald Trump on Saturday evening urged everyone in Irma's path to "get out of its way."

"This is a storm of enormous destructive power," Trump said at a Cabinet meeting, later adding: "Property is replaceable, but lives are not. And safety has to come first."

Irma's intensity

Irma is one of the most powerful Atlantic hurricanes in recorded history. At its strongest, the storm's wind speeds hovered around 185 mph, with gusts of more than 215 mph.

Initial damage from the storm was observed on Wednesday in Barbuda, an island east of Puerto Rico. Gaston Browne, the prime minister of Antigua and Barbuda, said Barbuda was "totally demolished," with 90% of its buildings destroyed. Communication with the island was cut off because of the destruction.

Barbuda officials worked to evacuate the entire population of 1,800 on Friday amid the threat of Hurricane Jose, though Jose's eye did not end up hitting the island directly.

Boats at a marina in South Beach as Hurricane Irma arrives in Miami Beach on Sunday. REUTERS/Carlos Barria

Irma also destroyed an estimated 95% of buildings in parts of St. Martin and devastated popular tourist destinations in St. Barts. It slammed the Virgin Islands before passing just north of Puerto Rico. Winds were still strong enough to cut off power to half of Puerto Rico's residents, however, and reports suggest some may not regain electricity for months.

As Irma's wind speeds decrease, officials are cautioning against referring to the storm as "downgraded." As seen with Hurricane Harvey, which made landfall late last month as a Category 4 hurricane but caused most of its damage with heavy rain, the number doesn't always accurately predict a storm's impact.

"I fear that people will say, 'Oh, it is only Category 4 now, we are safe,'" Marshall Shepherd, a meteorologist, wrote early Friday, after the storm lost Category 5 status. "This would be devastating and potentially deadly."

Preparing for a monster storm

Irma formed off the coast of western Africa last week and almost immediately started crossing the Caribbean Sea. It officially became a named storm on August 30 and was classified as a hurricane the next day.

Phil Klotzbach, a meteorologist at Colorado State University specializing in Atlantic hurricanes, told Business Insider that a combination of conditions — including a warm tropical Atlantic, a weak wind shear, and a change from drier to wetter weather — made it easy for Irma to pick up strength.

The moisture of unusually warm waters in the Atlantic Ocean and the Caribbean Sea allowed the storm to maintain its size and wind speeds for many days. Its sustained wind speeds of 185 mph for 37 hours set a record for the longest a cyclone has maintained that intensity.

Here's how Irma compares with other major hurricanes. Reuters

In the days before Irma hit the US, officials urged people in Florida and South Carolina to ready their emergency plans and supplies. In total, 7 million people were told to evacuate, leading to traffic jams and fuel shortages across the states.

Scott last week ordered all 7,000 members of Florida's National Guard to report for duty on Friday morning. Trump declared states of emergency in Florida, Puerto Rico, and the US Virgin Islands on Tuesday evening to free up federal resources to respond to the storm.

"We are running out of time ... You need to go now," Scott said on CNN Friday morning, urging anyone in an evacuation zone to flee immediately. "This is a catastrophic storm that our state has never seen."

But a larger population of elderly residents in the state complicated evacuation efforts, overwhelming shelters.

"We've been all over Florida today, seeking shelter," Jack Shively, 85, told Reuters after he and his wife were turned away from three locations.

A busy hurricane season

Irma, the season's fourth hurricane, put the Atlantic far ahead of the average accumulated cyclone energy — a measure of the energy of tropical cyclone systems — for this time of year. Klotzbach said that half of a season's cyclone energy usually occurs in September. But as of Wednesday, there had already been enough to meet the definition of an average season.

Both Colorado State University and The Weather Company predicted an unusually active hurricane season this year. Irma is the fourth hurricane of 2017, though the average date of the fourth hurricane in a year is September 21. For a couple of days last week, three hurricanes — Irma, Jose, and Katia — swirled in the Atlantic.

Jose, now a Category 2 hurricane, threatened some of the same Caribbean islands that Irma has already devastated, though it has mostly stayed to the north of populated areas. Katia made landfall in Mexico late Friday night.

A hurricane's category is determined by its wind force — here's what the scale means:

This is a developing story. Find all of Business Insider's latest Irma coverage here.