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

Hurricane Ian was moving closer to Florida’s west coast, the National Hurricane Center said in its 2 a.m. ET update on the storm's position.

The eye of the Category 3 storm was about 95 miles southwest of Naples, Florida, and was moving north-northeast at about 10 mph, officials said.

“On the forecast track, the center of Ian is expected to approach the west coast of Florida within the hurricane warning area this morning, and move onshore later today,” the update said.

Officials said the storm was expected to pass over central Florida on Wednesday night and Thursday morning, before emerging over the Atlantic Ocean late Thursday.

The hurricane was “expected to cause life-threatening storm surge, catastrophic winds and flooding in the Florida peninsula,” the update said.

Officials in Miami-Dade County suspended transit services at 1 a.m. ET on Wednesday, as Hurricane Ian approached Florida.

Florida’s most populous county halted its Metrobus, Metrorail, Metromover and Special Transportation Services until further notice, officials said in a news release.

-ABC News’ Ahmad Hemingway