Florida Gov. Ron DeSantis warned Wednesday morning that Ian could potentially make landfall as a Category 5 hurricane.

“This is a powerful storm that should be treated like you would treat a tornado approaching your home,” he said. “This one has just strengthened and strengthened, and it is the real deal. So, it is going to do a lot of damage, so people should be prepared for that.”

More than 200 shelters are open in South Florida, he said.

Twenty-six states, including New York and New Jersey, have sent support to Florida, he said.

-ABC News’ Mariama Jalloh

Hurricane Ian approached Category 5 status at about 6:30 a.m. ET, with its winds topping out at 155 mph.

Only four hurricanes have ever made landfall in the continental U.S. with winds over 155 mph: Labor Day in 1935 with 185 mph winds; Camille in 1969 with 175 mph winds; Andrew in 1992 with 165 mph; and Michael in 2018 with 160 mph.

Severe Category 5 hurricanes have winds above 157 mph.

“Rapidly intensifying Ian forecast to cause catastrophic storm surge, winds, and flooding in the Florida peninsula,” the National Hurricane Center said at 7 a.m. ET.

-ABC News’ Max Golembo and Samantha Wnek

A whopping 16 feet of storm surge is possible around Fort Myers.

Up to 11 feet of storm surge is forecast for Naples while a maximum of 10 feet is expected for the Sarasota area.

"Our biggest concern as we wait for this storm to make landfall is storm surge," FEMA Administrator Deanne Criswell warned Tuesday. "In 2018, when Hurricane Michael impacted the Florida Panhandle, there were five recorded fatalities as a result of storm surge."

Click here to learn how storm surge works.

Hurricane Ian strengthened to a Category 4 hurricane on Wednesday morning, as its winds climbed to 140 mph, the National Hurricane Center said.

The storm is the first Category 4 hurricane in the Gulf of Mexico since Hurricane Rita in September 2005.

-ABC News’ Max Golembo

Caribbean islands such as Jamaica and Grand Cayman will start to experience the effects from the outer bands of Tropical Storm Ian within the next 24 hours, forecasts show. The islands will experience conditions such as heavy rain, possible flash flooding and storm surge.

The storm system will begin to rapidly intensify overnight into Monday before it closes in on western Cuba on Monday night.

As of 2 p.m., the sustained winds in the tropical storm remained at 50 mph as it moved west-northwest at 12 mph, the center about 265 miles away from Grand Cayman.

Hurricane warnings are in effect for Grand Cayman and western Cuba, while tropical storm warnings and watches are in effect in other portions of both islands.

As a hurricane, Ian is expected to peak at a Category 4 before weakening slightly as it looks to make landfall on the west coast or panhandle of Florida in the coming days.

There is still some uncertainty to the track Ian will take once the system enters the Gulf of Mexico.

The center of the storm and the worst of the impacts could end up heading toward the western coast of Florida's peninsula, including the Tampa area. The other possible scenario has the storm moving more due north and bringing a possible landfall along the Florida peninsula, impacting cities like Panama City and Tallahassee with more direct effects.

The storm will begin to impact the Florida keys and the southern portions of the state by Tuesday night.

-ABC News' Dan Peck