(CNN) — Hurricane Ian’s “extremely dangerous” eyewall — just shy of Category 5 strength — is moving onshore in southwestern Florida, with the storm poised to inflict “catastrophic” winds, storm surge and flooding across a large portion of the state, forecasters say.

Now at Category 4 with sustained winds of 155 mph, Ian’s center was about 45 miles west-northwest of Naples around 11 a.m. ET and is expected to cross onto land, perhaps north of Fort Myers near the Port Charlotte and Punta Gorda areas, by early Wednesday afternoon, the National Hurricane Center said.

Much of west-central Florida and even places inland face disaster: “Historic” storm surge up to 18 feet is possible and could swallow coastal homes; rain could cause flooding across much of the state; and crushing winds could flatten homes and stop electricity service for days or weeks.

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And for those who didn’t heed evacuation orders along flood-prone coasts, a dire warning: In some places, it’s too late to get out, and people should “move to upper floors to escape rising water if necessary,” the National Weather Service said.

“It’s no longer possible to safely evacuate” from Collier County up to Sarasota County, Gov. Ron DeSantis said around 8 a.m., as key paths out, including the Skyway Bridge from Manatee to Pinellas counties, were closing.

“It’s time to hunker down and prepare for this storm,” he said. “This is a powerful storm that should be treated like you would treat” a tornado approaching your home.

After pummeling Cuba on Tuesday, leaving at least two dead and an islandwide blackout, Ian is taking aim at Florida’s vulnerable Gulf Coast, where residents have been boarding up and leaving in droves on congested highways. More than 2.5 million people were advised to flee, including 1.75 million under mandatory evacuation orders — no small ask in a state with a large elderly population, some of whom have to be moved from long-term care centers.

Ian poses several major dangers:

• Storm surge: Some 12 to 18 feet of seawater pushed onto land is forecast Wednesday for the coastal Fort Myers area, from Englewood to Bonita Beach, forecasters said. Only slightly less is forecast for a stretch from Bonita Beach down to near the Everglades (8 to 12 feet), and from near Bradenton to Englewood (6 to 10 feet), forecasters said.

Lower — but still life-threatening — surge is possible elsewhere, including north of Tampa and along Florida’s northeast coast near Jacksonville.

• Winds: Southwest Florida is facing “catastrophic wind damage.” Winds near the core of Hurricane Ian could exceed 150 mph, with gusts up to 190 mph, the hurricane center said.

• Flooding rain: 12 to 24 inches of rain could fall in central and northeastern Florida — including Tampa, Orlando and Jacksonville. That makes for a top-of-scale risk for flooding rainfall across this area.

After plowing Wednesday into southwest Florida, Ian’s center is expected move over central Florida through Thursday morning. Heavy rain also is possible in South Florida, as well as eastern Georgia and coastal South Carolina.

“Widespread, life-threatening catastrophic flash, urban, and river flooding is expected” across central and southern Florida, the hurricane center said. Considerable flooding also is possible in southern Florida through Wednesday and in northeast Florida, southeastern Georgia and coastal South Carolina through the weekend, the hurricane center said.

“This is a wind storm and a surge storm and a flood storm, all in one. And this is going spread itself out across the entire state. Everybody is going to see something from this,” CNN meteorologist Chad Myers said.

By late Thursday, Ian is due to emerge over the Atlantic Ocean, where it could strengthen again and affect another part of the US.

Parts of far southern Florida by early Wednesday morning had begun feeling the storm’s effects, with tropical storm-force winds and at least two possible tornadoes reported in Broward County, including at North Perry Airport, where planes and hangers were damaged. Major flooding was being reported in Key West due to storm surge, along with power outages.

Schools, supermarkets, theme parks, hospitals and airports had announced closures. The Navy moved its ships, and the Coast Guard has shut down ports.

And more than 265,000 Florida utility customers already were without power as of 11:15 a.m., according to PowerOutage.us.

As winds pick up, gas stations may temporarily run out of fuel, DeSantis said.

A hurricane warning is in effect on Florida’s Gulf Coast from Chokoloskee near the Everglades to the Anclote River north of Tampa, including Tampa Bay, and in the Dry Tortugas. A storm surge warning is in effect for coastal areas, including Tampa Bay.

Life upended as Floridians prepare for landfall

In Tampa, police went door to door Tuesday in a mandatory evacuation zone, making sure residents were ready to flee. Earlier projections had Ian on track to slam Tampa Bay, and even as the hurricane’s path shifted south, mandatory evacuations and preparations continued, Tampa Mayor Jane Castor said.

“No matter where it lands, if it does come in a little south of us, we are not getting out of this unscathed, and there is going to be flooding throughout the Tampa Bay area,” Castor said.

Preparations across the state have been underway for days as residents braced for Ian’s wrath. People lined up to pick up sandbags and flocked to stores to stock up on supplies like water and batteries.

And as the hurricane marched closer, the closures began.

Across Florida, 58 school districts have announced closures due to storm as campuses turned into shelters for evacuees. Disney World is set to close Wednesday and Thursday, as is Kennedy Space Center’s Visitor Complex. And hundreds of Publix grocery stores shut their doors Tuesday evening, expected to remain closed through Thursday.

As millions evacuate, 176 shelters opened statewide and hotels and Airbnbs opened to people leaving evacuation zones, DeSantis said.

Local governments and state agencies also prepared those living in nursing homes and other senior care facilities to evacuate.

Florida has around 6 million residents over the age of 60, according to the state’s Department of Elder Affairs — nearly 30% of its total population. As of Tuesday, all adult day cares, senior community cafes and transportation services in evacuation zones are closed, according to the department.

Authorities also readied services to fan out and respond to calls for rescue and then, in the aftermath of the hurricane, for recovery and repair efforts.

Nearly 400 ambulances, buses and support vehicles were responding to areas where the hurricane was expected to make landfall, according to the governor’s office.

DeSantis activated 5,000 Florida National Guard members for Ian’s response operations, and 2,000 more guardsmen from Tennessee, Georgia and North Carolina were being activated to assist.

Florida urban search and rescue teams also were prepping.

“We have five state teams that are activated with additional five FEMA teams that are in play,” Florida Chief Financial Officer Jimmy Patronis said at a news conference Tuesday night. “We have over 600 resources to bear in addition to these out-of-town teams.”

After landfall, Ian will crawl over Florida

When the hurricane hits, rising water is expected to move inland from the coastline, bringing life-threatening inundation and flooding to coastal areas.

Millions of people are under a storm surge warning, including the Suwannee River southward to Flamingo, Tampa Bay and the Dry Tortugas. The warning is also in place on the state’s east coast from the Flagler-Volusia county line to the mouth of the St. Mary’s River and along the St. Johns River.

After landfall, Ian is expected to crawl across the central part of the state Wednesday into Thursday morning, with damaging winds bringing the threat of tornadoes, according to the National Hurricane Center.

The slow churn over will “dump an enormous amount of rain on the state of Florida,” DeSantis said.

Ian is expected to dump at least two to three months’ worth of rainfall by Friday. Central and Northeast Florida is expected to get 12 to 24 inches of rain, while the Florida Keys and South Florida could get 6 to 12 inches, and eastern Georgia and coastal South Carolina could receive 4 to 12 inches.

Related: Hurricane Ian could be ‘something that we haven’t seen in our lifetime,’ Tampa forecaster says