A flash flood emergency was declared in areas north of Orlando as the region was hit by about a foot of rain, the National Weather Service said.

The declaration covered Sanford, Lake Mary and Heathrow, the service said. Nearby Central Orange, Seminole and South Central Volusia were also under flash flood warnings.

“This is a particularly dangerous situation,” the service said. “Seek higher ground now! Life threatening flash flooding of low water crossings, small creeks and streams, urban areas, highways, streets and underpasses.”

Parts of the region have seen more than 16 inches of rain within 24 hours, with more than 9 inches in the last six hours, making the storm a 1,000-year flood event in the Orlando area and to the north.

-ABC News’ Max Golembo and Kenton Gewecke

More than 2.5 million customers were without power in Florida at about 5:30 a.m. on Thursday, the state’s providers said.

Ian’s winds slowed to 65 mph early on Thursday morning, downgrading the system to a tropical storm, the National Hurricane Center said.

“The Hurricane Warnings along the east and west coasts of the Florida peninsula have been changed to Tropical Storm Warnings,” the center said.

-ABC News’ Max Golembo

Florida Governor Ron DeSantis will be holding his next press conference on the latest developments of Hurricane Ian at 8:45 a.m. ET at the State Emergency Operations Center in Tallahassee.

President Biden, meanwhile, will visit FEMA headquarters in Washington, D.C. to receive an update on Hurricane Ian at 12 p.m. ET.

While Hurricane Ian is expected to become a tropical storm on Thursday, the storm has battered southwest Florida and has left more than 2 million people without power so far.

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