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.

The Federal Emergency Management Agency's search and rescue teams have been out in the field since 4 a.m. Thursday, FEMA Administrator Deanne Criswell told "Good Morning America." It will prioritize saving lives and helping people impacted by the storm, who may be trapped.

Criswell said FEMA has been hearing reports of people calling 911 through the night and will use information it has gathered in those hours to prioritize rescues in harder-hit areas.

"This has been just a catastrophic storm and it's left significant damage in its path," Criswell said.

Criswell said FEMA will be able to conduct rescues by land, air and sea.

The most significant impacts have been happening in Lee County, where people are without power and water. Criswell also expects impacts across the western coast of Florida.

"Water is dangerous. Even though the storm has passed, the water that is there is still dangerous. There's debris, there's chemicals, there could be downed power lines. People need to be careful, they need to stay vigilant," Criswell said.